chemistry prelabs

[Kingtal0n]

here are the prelabs for 45% of the chemistry lab its basic chemistry, entry level stuff but you can google most of it

Determination of Enthapy of neutralization
Purpose: To determine the enthalpy of reaction (AH) for the neutralization of an acid and a base.
Summary: A caloriemeter is basically an insulated device for measuring temperature of a solution- as it being changed. It does affect the change by absorbing energy so not only does the experimenter need to calculate the change in temp of the solution inside, but he would also have to know the “calorimeter constant”, a number that represents how much energy the given calorimeter would be absorbing per temp change. To determine this constant, first water is used in an experiment. Hot water and cold water, in separate cups, both of which have a well recorded temperature (charted with change vs time) are mixed in the calorimeter and the temp changes of them as they mix is recorded on same chart. The calories gained by the calorimeter is (calories lost by hot water – calories gained by cold water). The calorimeter constant is just that divided by the temp change of the cold water, since the calorimeter started out with that temperature to begin with, since the cold water was in it from the start. (the hot water is poured into the calorimeter which contains the cold).
After the constant is recorded, the same experiment is done with an acid and a base. The base is in slight excess to ensure all of the acid reacts. The two temps start out similar, but change after they form a solution, since the neutralization process is exothermic, heat energy will be released and absorbed by both the resulting solution and the calorimeter. Using a chart, again, the change in temp is recorded in time intervals and the calorie constant from before is used to determine how much energy is absorbed by the calorimeter and how many calories TOTAL was evolved by the reaction.

oxidation reduction titration
Purpose: To standardize a solution of KMnO34 and use it to determine the concentration of an unknown solution of H2C2O4
755 words (1 written page is about 180 words, 3x written pages would be 540 words)
Summary:
In an oxidation reduction reaction one substance loses electrons and one gains electrons. The permanganate ion (MnO4) is -1, because Mn has +7 and O is -2(4)=8, so the overall charge is -1. In an acidic solution the Mn becomes +2, which means 5 electrons are gained. That makes Normality = Molarity X 5, since normality = m x # electrons moved.
The other substance, oxalic acid (H2C2O4) does not ionize like KMnO4, instead, the entire dissolved molecule is broken (convalent bonds break) when the reaction takes place. Since the only element moving electrons is Carbon, we have Carbon going from +3 to +4 (when it bonds to Oxygen) so it only loses 1 electron. But 1 electron is all it takes, that makes oxalic acid the reducing agent, since it is causing Mn to be reduced in oxidation state (+7 -> +2)

To proceed with the reaction, additional H+ protons are required to make H2O, so we need an acid such as Sulfuric, which will break up into H+ and SO4- ions in solution (ionize in water). The H+ ions will be used to form water and push along the Mn+7 -> Mn+2 oxidation reduction reaction. The whole point of this reaction is to determine the exacting concentration of an unknown solution of H2C2O4. That is, the students will know the solution is H2C2O4, but they will not be told what the concentration of it is. The first step will be to standardize a solution of KMnO4. Standardize means to know EXACTLY the concentration, so it can be used in titration. Technically the students could start with a standardized solution from the beginning, but that would be very difficult for the lab to prepare, so they prepare their own.

This is done with a known concentration of reducing agent AND oxidizing agent. The concentrations must be known to standardize one or the other. The students will start by taking an amount of KMnO4 solution with a known concentration and diluting it to ABOUT .1N. Since N=Mx5 for this reaction they will have to do the math to figure out how much KMnO4 they need. Once a .1N~ KMnO4 solution is prepared, it is loaded into a buret. An exactly known concentration of H2C2O4 is then titrated until the color begins to change, indicating the absence of the H2C2O4 molecule with which to form Mn+2 ions, so the solution will begin to turn pink-purple indicating the presence of intact MnO4 ions (permanganate ions). Once this happens a buret reading it taken, and the VoxNox=VredNred calculation is used to determine the concentration of the KMnO4 solution in the buret EXACTLY.

This step is repeated to ensure accuracy, the rest of the KMnO4 solution is SAVED EXACTLY AS IT SITS In the buret, for future use in titration. The known H2C2O4 solution in the OTHER buret is discarded and the buret is rinsed with distilled water… followed by the rinsing with the new solution of H2C2O4 with unknown concentration. The unknown H2C2O4 is then loaded into the buret and dispensed into a flask for the actual titration procedure. Each flask requires not only the H2C2O4, but also some distilled water, and some H2SO4 to provide H+ ions for the reaction to take place. If the reaction halts unexpectedly perhaps there was no enough H+ ions to proceed. And if KMnO4 is added to swiftly to the heated solution prepared then it will also fail, so care must be taken and the reaction should proceed SLOWLY lest it be ruined.

Once the new concentration unknown H2C2O4 is loaded into the buret and 10mL is added to the flask along with the acid and water, the titration can proceed. KMnO4 is dripped into the flask until the color change becomes evident, indicating the presence of the MnO4 ion intact, which will tell the student that all of the H2C2O4 has reacted (hopefully). The buret reading is taken and once again the VoxNox=VredNred equation is used to determine the concentration of the H2C2O4 that was previously unknown. Of course this reaction must be repeated at least 2 or 3 times to ensure proper results. The buret and all used equipment that came in contact must be rinsed with H2O2 and H2SO4 solution to ensure all of the MnO4 ions are removed, as they are quite tenacious, and stick to glassware and contaminate future experiments easily.


Electrolysis and Avogadros number
Purpose: To calculate Avogadro’s number, N, through the electrolysis of a sulfuric acid solution.

Summary:
Electrolysis is a chemical reaction that occurs when you pass an electrical current through an electrolyte solution. Electrolytes (dissolved ions capable of passing current) are necessary for electrons to pass through the solution. Reduction occurs at the cathode, and oxidation occurs at the anode. The reaction only takes place while electrons are flowing from one to the other, while the electrical current is maintained.

A direct current will be passed through a solution with the intent of oxidizing AND reducing water, the oxidation will cause O2(g) to form and the reduction will cause H2(g) to form. The gasses will be collected individually via a special apparatus designed specifically for this purpose. Gas law calculations will be used to determine the moles of each gas that forms. Also needed is the “amount of electricity” a rather ambiguous number calculated from the amperage and time, which in my opinion, is the largest fudge factor of the entire experiment, since the numbers are inexact approximations, and they effect the outcome of the experiment. Luckily, avogadro’s is well known, 6.022x10^23, so any minor error in calculations can be error %, should be minimal. The quantity of amperage and time will be expressed in in coulombs, or total electrical charge… by using 1.6x10^-19 coulomb (charge of 1 single electron) the number of moles of electrons flowing in the system can be calculated… and from the number of moles of gas, with moles of electrons used- the total moles of electrons per mole of gas can be determined. Dividing this number by the number of electrons in the half-reaction (of each respective gas) should yield the number of electrons in a mole, avogadros number. For instance if we form 1 molecule of O2(g), we also release 4 electrons. Or 1 molecule of H2(g) forms, we used 2 electrons. So 1 mole of H2 gas means we used 2 moles of electrons…

The apparatus used to collect gas is specialized so that the pressure of the gas collected moves a column of water. The height difference is used to calculate moles, the calculation involves conversion to a pressure against mercury (13.6 times denser than water). The vapor pressure of water is used to calculate how many molecules of H2O are bouncing around above the column of water. This is important because the H2 or O2 gas takes up a specific space above the column of water… but the H2O molecules also take up some of that space. So the vapor pressure is how much pressure is due to H2O molecules, once those are ruled out, the pressure due to the other gasses (O2 or H2) can be calculated.








Procedure / calculations include sulfuric acid for the solution, and a direct current power supply with the amps and seconds being used to determine coulombs used. The amount of the time in seconds must be known as exact as possible to use this calculation. Two experiments should be done, each using different volumes of gasses, to provide two different pathways to calculate avogadro’s number. This is typical of any experiment of this nature. After the power is shut off there is a delay from which the remaining gases will form, so time must be taken carefully afterwards before recording volumes and determining moles of gasses formed. A meter stick (barbaric) is used to measure heights of columns of each gas. Then the calculations start- first we convert the pressures to torr. Then the vapor pressure of the H2O is removed from the calculation. Then number of moles is calculated using ideal gas law (PV=nrT). The same is done for both H2 and O2. then coulombs is calculated (if not done already), followed by number of electrons used, then finally the number of electrons per mole of H2 produced… which (should) lead us to avogadro’s wonderful number of 6.022x10^23. okay, we have a percentage error, when dealing with the infinitely small, calculate your error and build a bridge and get over it.



Chemical Kinetics
Purpose: Determine the reaction rates of several different concentrations of solutions…

Summary
Kinetics deals with reaction rates. Some reactions take split seconds and some take hundreds of years. Kinetics is a study that allows the interpreter to idealize how long a particular reaction will take- how much will be yielded when.
This experiment uses a color ion, I2+starch, to identify when the reaction has been completed. The reason this works, is because the I2 ion forms when IO3 forms without the presence of free H2SO3 in solution. The process of combining H2SO3 with IO3- ions can be slow depending on the concentrations used and the temperatures of the solutions when mixed. As long as H2SO3 is in the mix, I2 can not form, and therefore the blue color associated with iodine will not be displayed.

The reaction is oxidation-reduction, meaning electrons are being transferred, a solute is losing some and another solute is gaining. Reduced means gained electrons, a reduction in oxidation state is what it means, in this experiment the reducing agent is H2SO3 at first, but when it is all used up, the Iodine ion becomes the reducing agent, pulling IO3- apart forming I2 which forms blue with starch (which is present in the H2SO3 that was added in this experiment). Once the blue forms, the reaction is done, because the acid was used up. Keeping the exact amount of acid for each experiment is critical.

A chart is formed, monitoring concentration vs rate, when the chart is made the observation made will be that this reaction is n=1, meaning the reaction should be sensitive to the concentration used. Higher concentrations mean longer waiting periods before I2 can form, since more moles of KIO3 mean more H2SO3 needs to react before the blue complex can form. I don’t think the reaction is worded correctly in the lab manual because it says exactly “ All of the H2SO3 has to react before the second step can begin” I might argue that it would be the IO3 ion that needs to be used up, since a higher concentration of IO3 means a shorter waiting period before I2 forms… the H2SO3 should be used up faster by an increased concentration of KIO3, and this clearly the case… on the other hand, lower concentrations of KIO3 mean longer waiting period, which means that the H2SO3 is lingering longer, which does coincide with what the manual says, maybe I am just being picky with the wording.














Procedure
2 burets are prepared, one with the acid solution H2SO3 and the other has the KIO3.
Equal amounts of acid are placed into 5 breakers. Different concentrations of KIO3 are used per beaker.
The two solutions are combined and a timer is used to determine how long the reaction takes to finish. When the blue forms, it means all the H2SO3 was used up, and the timer stops. Both solutions are initially mixed at identical temperatures, to ensure accuracy between them all, since temp affects rate.
All the beakers are finished and a chart is made to compare data.
Then a graph is constructed. The graph should be used to help interpret data. It also helps to determine the concentration of an unknown KIO3 solution… which is determined next.

An unknown concentration is prepared and put into a buret.
the same amount of acid is used for the reaction, so timing should be consistent.
The acid and unknown are mixed and the reaction is timed. Formation of blue indicates the reaction is over and calculations yield concentration data.
Finally, a reaction is timed using COLD and HOT preparations of the unknown (which should be known now) and the data resolved is used to determine the effect of temperature on reaction rates. And as expected, hotter solutions usually result in faster rate, and colder preparations are slower to react.

Chemical equilibrium and Lechatelier’s principle

Background: When reactants are combined and a chemical reaction begins to take place, products begin to form…. .. .. ..

Summary

This experiment will be identifying and writing and learning reversible reactions. Sometimes a reaction forms an ion or a product escapes as a gas or somehow a product is “used up” and a reverse cannot occur. Sometimes, some of the products can intereact with each other and form reactants again. This is a reversible reaction. At some point in a reversible reaction, an equilibrium is established, and the reaction is proceeding at the same pace that it is reversing. The reaction appears to have stopped. A fraction is used to describe this situation, using products, reactants, and concentrations. The value of this equation is called the equilibrium constant. The value only changes when temperature changes from that point. Equilibrium can be disturbed by pressure, or concentration changes in solution… Lechatelier’s principle is the idea that the system will counteract the change if equilibrium is disturbed.

Heat introduced into the system should generally favor endothermic reaction, since heat is being used up. This will affect concentration of whichever side of the reaction is endothermic. The reverse is true for exothermic reaction. In this experiment chemical systems will be produced which have an equilibrium and are allowed to reach it. Then, a change is made, equilibrium is disturbed, and the principle aforementioned will be observed. In some cases the change is ph, which is reflected of acidic nature of the solution… in other cases a color change may be evident as certain products or reactants might posses an indicator color… and even precipitates might be formed that can be determined or weighted.


Determining the percentage of aspirin in aspirin tablets

Purpose: to determine the percentage by weight of aspirin in commercial aspirin tablets.

Summary: 613 words
Visible light is also known as electromagnetic radiation. It is energy taking the form of waves, propagating through space, different wavelengths have different distances between maximum intensities of the magnetic fields. Light travels in packets called photons. When light is white it contains the entire visible spectrum of wavelengths. When light is broken up into separate lengths, the colors sort out. If a solution contains molecules that absorb a certain color, then the complementary color will appear to pass through. For instance if a solution absorbs green, then purple will appear to pass through.

Beer-Lambert law takes the absorbance, path length, and concentration into account to generate a formula which can be used to plot a graph. The graph will list concentration vs absorbance… the results should be linear, meaning an increase in concentration should yield a perfect increase in absorbance of the color of light spectrum being identified. This graph once plotted with knowns will be used to determine the concentration of an unknown (aspirin in a tablet) hopefully without looking at the back of the box the experiment should yield results.


In this experiment, aspirin will be reacted with NaOH and then acid in the presence of the Fe3+ ion to form a solute that absorbs purple light. The amount of purple light absorbed will tell how much aspirin was reacted- and assuming all of the aspirin reacts available in solution, it should also tell how much actual aspirin there was to begin with… this is where the device SPECTROPHOTOMETER comes into play. The human eye, although well tuned to the visible wavelengths of light, is unable to perceive minute differences and even large differences are nearly impossible to determine % difference between resulting solution absorption rates… so this device was constructed with this sort of experiment in mind. The device will tell the % transmittance, and using this number you can calculate absorbance (A = 2 – log %T).

Once the absorption is determined, it is matched to the concentration and plotted to said graph. There must be 5 solutions used to design the graph correctly and prevent errors. Stock solution is created and maintained for each example. The final concentration (C2) is what is plotted on the graph. The concentration of the aspirin solution before it was diluted is used for C1. in a 250mL solution, moles / .25L = Molarity.








Procedures
125mL flask, added to it is reagent grade acetyle salicylic acid, .2 grams… weight of flask and solid must be accurate.
5mL 1M NaOH is added and the solution is heated. This forms the -2 ion.
Transfer ALL of the solution (quantitatively) to a clean 250mL flask.
Dilute the contents with distilled water. This is the stock solution of the known.

Pipet 1mL of the stock solution (SS) into 50mL flask, dilute to the mark with the Fe3+ ions (the solution is buffered to prevent ph wander) the dilution must be exact for correct results.
Do the same thing with the SS for #2-5, using # mL of stock solution. This increases the concentration of that flask by 100% for each sample, giving a linear increase in absorbance.
Measure the % transmittance and calculate absorbance. Do this away from the machine…

Then take the aspirin (weighted) and repeat the experiment… this time the concentration of unknown, but the absorbance will be easy to tell if you follow the experiment correctly, since the machine will do that for you. The dot is plotted and the concentration should be easy to tell once graphed.


Determination of Ka for an unknown weak acid

Goal: by plotting PH vs Volume of base required to overcome the buffer, determination of Ka becomes possible thanks to a graph indicating the half-way point for the transition.

Descriptions:
Weak acids are acids that only ionize partially in water. A weak monoprotic acid will maintain some of its concentration of H+ still attached to its anion counterpart even though it is in water. Its still soluable, floating about, its just not separated (ionized), and therefore the H+ is locked up.
K is the equilibrium constant. Ka is going to equal the total ions divided by the locked up complete molecules of H(An) where An = the anion bound to the H+ ion.
In distilled water, a weak acid will free as many H+ ions as it does anions, if the acid is monoprotic. Therefore, the H+ and An- concentrations would be equal.

In this experiment, H+ and An- are NOT equal. There is an additional anion, provided by a salt. For instance in NaCl, which ionizes in water, Na dissociates from Cl. The Cl is an anion, a negative ion in solution, (related to anode, for negative charge coorolation, at an anode is where oxidation occurs, releasing electrons. Its sort of backwards since anions are negative, meaning they have extra electron(s), yet anode is where electrons leave atoms and create cations… at the anode) which means the H+ is going to be lower concentration than the total number of anions in solution. This is where the Henderson-Hasselbach equation comes into play. This equation will relate the salt to the acid, and allow for the pH to be determining the pKa.

Using a known volume of unknown acid, a solution of NaOH will be added in the presence of a PH meter. This will be a titration. A graph is created of pH vs mL added so a titration curve will be represented. At the equivalence point, a sharp vertical rise will occur, then it will taper off again. The endpoint does not mean equivalence point, the halfpoint between the vertical rise is the equivalence point, and that means the acid is all salt. But the endpoint would be indicated by a color change if an indicator was present, which is hardly the same thing, since the color might change when the ph changes but there is still some acid present.

Procedures:
the PH meter is set and calibrated using a 7PH solution to find neutral. It must be kept wet, and it must be recalibrated often thanks to its age and relative sensitivity.
10ml of acid is used, a rough amount of water, with it to ionize in, and the NaOH is added with the volume being recorded and plotted to a graph vs potential hydrogen.
As the ph is recorded the first time around, the relative crossover point is determined.
Following this, a new graph is constructed, one with higher resolution, to determine a more exact crossover point for the equivalence point. Half of a tenth of NaOH- if possible should be added and plotted slowly, monitoring pH right at that special point. A high resolution graph has a more detailed equivalence, and therefore, a better point for pKa.
There arnt many calculations, the hardest part is finding the exact equivalence.
Rinse everything in distilled water and unplug the ancient ph meter and your done!
Determination of water hardness!

Purpose: Determine the hardness, expressed as ppm CaCO3 in a sample of water, and also, to determine the hardness due to magnesium ions and calcium ions separately.

Descriptions:
Water that isn’t distilled (and even some that is) contains “dissolved minerals” generally you find Calcium and Magnesium ions floating around in most water, especially tap water in florida… this is called water HARDNESS for some strange reason. It can interact with other ions to form solid precipitates that settle in places where the water flows. It can also affect the taste and appearance of water. Some places have water softeners that delete these minerals. Calcium for instance interacts with the carbonate ion to form calcium carbonate, basically, tums antacid powder, except without the flavoring. Harmess yet annoying, this experiment determines how many ppm of calcium and magnesium is present in a sample of purposefully hard water.

An indicator is used to indicate the presence of the calcium or magnesium ions. The indicator absorbs blue and looks red in the presence of the ions. It loves the ions, but not as much as EDTA does. EDTA is added to the solution via buret. As the EDTA finds the calcium and magnesium, it has a much greater affinity for these ions than the indicator does, and therefore it takes them away, and locks them up in a little shell so the indicator cant touch them anymore. Without something for the indicator to hold onto, the color will no longer be red. This is the indication needed, when all of the red colour is gone, the indicator indicates that there is no more Ca or Mg to intereact with, therefore, the EDTA has used all of the Ca and Mg up. By determining how much EDTA is takes to use up all the Ca and Mg, determination of how much Ca and Mg is possible. One edta molecule takes one Ca or on Mg. So by knowing the concentration of the edta, and the volume used, the moles of hardness is determinable.

The second part of the experiment is as simple as the first. Instead of determining overall hardness due to Mg and Ca, a hard water sample has a Calcium precipitate added, meaning it causes whatever calcium is in the solution to precipitate out. Remember that the main reason water hardness is annoying is because the calcium and magnesium tend to precipitate out and cause problems. So in this second half, we are precipitating the calcium out of the solution to leave behind ONLY the magnesium, so that out next titration or determination of moles of hardness is only due to the concentration of magnesium in solution. The calcium precipitate is caught in a paper filter, that’s why a funnel is required, and the instructor may even show you how to flute the funnel to increase its surface area… if you have ever plopped a flat filter paper into a funnel and waited an hour for it to drip dry you know why surface area is important.







Procedures:
The calcium precipitate is added to a solution of hard water and allowed to sit so the reaction can take place.
While this is happening you put 25mL of water together with some buffer solution (the buffer is required to maintain the pH. If the pH wanders, the EDTA will not work correctly, among other things)
*Warning: do not smell the buffer, as it is a very strong ammonia-like solution and will trigger nociceptors in your olfactory tract causing pain and disorientation, ask me how I know…. im surprised this isn’t written in the book…
After the buffer and water are together, you drip some indicator and it should turn wine-red, then you titrate with the EDTA right away.
bit by bit the EDTA goes in, the color will start to notable change only during the last few mL to be added. Once you get that nice blue color you make your graph record your data and do it again a few more times. Make sure your results are similar.

After about an hour, the calcium precipitate should be ready. Flute your filter paper and pop it in a funnel and pour your sample through the filter into a clean 100mL flask, and dilute with distilled to the line, also wash the funnel real well make sure you get all the magnesium rich water through the paper and make sure none of the calcium slips by.
use this solution now to titrate once again, add some buffer and some indicator and record how much EDTA it takes. The difference is your magnesium hardness… following the calculations in the book it should be pretty simple to determine… and then draw a conclusion based on your result…

My result: there seemed to be a large amount of magnesium in the water. I bet MgSO4 or something similar and cheap was used to buff the Mg concentration up real high. But that’s acidic… but that’s ok we have pH 10 buffer.


The synthesis of Potassium Trioxalatochromate (III) trihydrate

Purpose: To synth a coordination compound and to determine the %yield

Descriptions:
A complex ion is a particle that contains a central metal ion, covalently bonded to ions or molecules that donate pairs of electrons to empty orbitals on the central metal ion. Most complex ions receive 2, 4 or 6 pairs of electrons from ligands. The total number of electron pairs donated is the coordination number… thus the name, coordination compound, a compound with a complex ion. Tricky eh
The compound is very stable since the covalent bonds are tight. The compound might dissolve in water but it wont ionize apart.

In this particular compound, oxalate ions are the ligands. They donate the electrons to the central metal ion to form covalent bonds. Each oxalate has 2 electrons to donate. There is a total of 6. they arrange symmetrically and form a salt with K+ (potassium), it takes 3 K+ ions to form a neutral salt. Water molecules are incorporated as water of hydration. Since they add molecular weight they must be accounted for. The final compound is synthed via procedure and the weights are used to determine %yield.

Procedures:
9.5g solid oxalic acid is loaded to a beaker
15mL of distilled water is added
heat the beaker until the acid dissolves, bring to a boil, then remove right away.
3g of potassium dichromate with exact weight recorded is set aside.
SLOWLY the dichromate I added with a spatula. This is where the CO2 forms, so it must be added bit by bit. The addition should take 5-10min no longer no shorter. Don’t lose product due to splatter.
Prepare an ice water bath. In a test tube, 25mL of methyl alcohol, cool it in the ice bath. In another tube, cool 25mL of 50/50 methanol/distilled water. Label these, they are for washing later.

When your dichromate is done being vigorous with its CO2 production, heat to boiling again and remove from heat.
3.9g of potassium oxalate is set aside, record the weight exactly.
Slowly add the potassium oxalate stirring and heat gently. Heat and stir until everything is dissolved. Do not let it boil dry, add more water if necessary.
Set it aside and let it cool to about room temp. once its close to room temp, add about 4mL of room temp methanol, then put it in the ice bath. Cool with stirring and initialize crystallization for 15minutes.
Setup a vacuum filtration apparatus.
Once the mixture is cooled and crystallized, put the crystals into the buchner funnel, remove as many crystals as possible from the beaker to the funnel.
Use the vacuum to remove the liquid, add small portions of the cool methanol/water to rinse out the beaker and poured over the funnel. Further wash the crystals with small portions of the cool methanol. Draw each portion through the funnel before adding more. Continue to dry the solid by pulling air through the funnel for a few minutes. Disconnect vacuum, remove the filter paper and xfer the crystals to a clean watch glass. Oven dry for 5~minutes. Determine the mass of a clean vial, and put the dry crystals into the vial and weigh using the difference. Label the vial and turn it in…

What actually happens:
First, the Cr+6 is reduced to Cr+3 by reacting H2C2O4 with K2Cr2O7.
This step generates CO2 and may splatter.

Second the K2C2O4 provide ligands that bind to the Cr+3 forming the complex ion, the oxalic acid also supplies some of the ligands.

Finally as the solution cools, crystals form indicating the combination of the K+ ions to form the neutral salt, also don’t forget the water is incorporated in this step as hydration.
The oxalic acid does two things: it reduces the Cr and then it supplies some oxalate ions that are ligands.
The theoretical yield is calculated after the sample is filtered and dried completely.