Pre Lab Reports Required for Each of the Attached Lab Reports

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47

MOLAR MASS OF AN UNKNOWN ACID

Before beginning this experiment, be sure to read this procedure and the one from the previous experiment, Standardization of an Aqueous NaOH Solution first.

I. PURPOSE Using the standard solution from the previous lab and ap- plying the techniques chat you learned , yo u will perform m ore titrations co accurately determine the molar m ass of an unknown acid.

II. BACKGROUN D The sample of acid, which you will receive as an unknown is a pure, m onoprocic weak acid. The product of any re- action of an aqueous acid an d an aqueous base is water, usually written in the equation as H

2 0W. We can write

th e net ionic equation for the reaction of chis aci d (HA) with NaOH as fo llows.

HA(aq) + OH (aq)- H 2 0(1) + A-(aq) (1)

The anion A - (aq) is the unproronaced fo rm, called the conjugate base of the weak acid HA (aq). We can measure

che molecul ar m ass of chis monoprocic acid by titration

with th e standardized NaO H solution prepared in t he

previo us experiment.

Given that the acid bei ng titrated is monoprotic, this means tha t every mole of acid can react with only one mole of hydroxide. T herefore, if we know the concen-

tration of o ur NaO H titrant solution, and we titrate a

known amount of the acid accurately, we can determine

how m any mol es of the acid are present in the flask char we have just titrated .

And if we know the mass of rhe solid sa mple that was used

co prepare that unknown acid solution, we can use that

48 EXPERIMENT SA

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mass and the number of moles of the acid chat we determined ro be in the flask co

direccly determine the molar mass (g/mol) of the acid.

Ill. SAFETY PRECAUTIONS

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Safety glasses are always required as long as anyone in the lab is still performing

laboratory work!

An apron or lab coat and closed-toe/closed-heel shoes are required.

No food or drinks in the lab at any time .

The NaOH solution should be handled with great care co avoid contact with your

skin and eyes.

If you come in contact with ic, wash it off with water immediately .

All chemical waste is to be collected-none to be poured into the sink!

IV. MATERIA LS AND REAGENTS

Equipment (communal)

50 ml Buret

1 Pipet bulb

2 Erlenmeyer flask (from your locker)

25 ml Pipet

Reagents

Your standard solution of Sodium Hydroxide

Unknown (check from your TA)

Phenolphthalein

~300 ml WASTE

V. EXPERIMENTAL PROCEDURE You will work individually on chis experiment to determine the molar mass of yo ur unknown. You will share rhe standardized base (NaOH ) char you prepared last week.

Do not check out the unknown sample until your TA has checked the quality of your NaOH standardization data (no exceptions).

I . Set up a support stand with a burer clamp before you bring the burer over to your bench.

2. Pick our a 50 mL burer and check to make certain it is clean. Ir if it looks dirty, clean it with diluted soapy water and rinse thoroughly with water. Dry the out-

side. Pour in a small beaker about 50 mL ofNaOH and pour about 5 mL into rhe burer to rinse any excess water. Do this twice and remember to rotate the stopcock several rimes co ensure you are rinsing it correcrly. Finally, fill it up close to but below the zero mark with yo ur base.

3. If yo u did not clean your Erlenmeyer flasks , clean them with diluted soapy water

and rinse thoroughly several times with tap water, then , for the final rinse, with DI water. Shake out any excess water from the inside and dry the outside of the flask (you do nor need to d ry the inside) .

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MOLAR MASS OF AN UNKNOWN ACID

4. The unknown sam ple bottle co ntai ns approximately 2.5 grams of acid. You wi ll ,,·Jnt to do three replicate ritrations with the sample, so each titration should be done with - 0.8 grdms of the solid.

Note: \r1t,igh out the unknown sample and divide it up into three, for the three trials.

5. Clean a weighing tray with tap water, and then dry it out thoroughly with a paper towel. It will still be slighdy wee, so air dry it for a few minutes or slowly wave it back and forth fanlike for about one minute. If you do not do this the weight is going to be wrong and so consequendy are your calculations.

6. Place it on the balance and care it, then transfer approximately 0.8 g of the un- known to the tray using a spatula. Record the final sample weight to the nearest 0.00 I gram in your laboratory notebook.

7. You need to quantitatively transfer 1/3 of the weighed solid unknown into the Erlenmeyer fl ask. Hold the tray in one hand with one corner over the mouth of the Erlenmeyer fl ask. Using your wash bocrle in your ocher hand, carefully rinse the entire sample into rhe fl ask.

8. Add about 25 mL of distilled water and gendy swirl to dissolve all of the crystals. If any crystals are stuck to the sides of the flask, rinse rhem into the solution with your wash boccie.

9. Add three drops of phenolphthalei n indicator solution to your flask and then rirrare ir with the sta ndardized base you prepared in the previous experiment; the titration proced ure and endpoint are the same, so refer to the previous experiment Pare C steps 4-9 as needed.

I 0. Repeat steps 7-9 of chis procedu re twice more, each time with approximately 0.8 grams of the solid (recorded to 0.001 g) .

11 . Be sure to rinse out your Erlenmeyer flask thoroughly between titrations. Calculate che molar mass for all three sets of dara. Be sure ro report your results to che cor- rect number of significant figures and units.

12. If your three trials do nor agree well (ro within 20 g/mole) , consult with your TA about how co proceed. Your standard deviation calcularion can help you decide wh at to report.

SPECIAL NOTE Fill your 1 L plastic bottle with tap water and store it in your locker.

This will help you for next week's experiment.

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VI. DI SCUS SION / QUES TION S I. Discussion: Discuss the accuracy, precision, systematic errors, and random errors

of your data.

2. Calculate the percent error in the calculated molar mass of the unknown acid that would be caused by titrating one drop past the end point. Assume that the mass of the acid is 1.000 g and that 30.00 mL of 0.1 M NaOH was used in the titration.

3. Consider the effect of exposing the NaOH solution to rhe air: Hint: Refer back to Experiment 4.

a. Would the calculated molar mass be roo high or roo low if your standardized NaOH were exposed to the air for a long period of rime after the standardiza- tion ? Explain how you reached that co nclusion.

b. Would the calculated molar mass be roo high or roo low if your NaOH solu- tion were exposed to the air for a long period of rime before the standardiza- tion? Explain how you reached that conclusion.

4. What if yo u had dissolved the acid in 50 mL of water instead of 25 mL? Would the accuracy of the molar mass be significantly affected by that change in proce- dure? Explain how you reached that conclusion.

5. Wo uld the calculated molar mass be too high or coo low if the burer had been rinsed with deionized water and then filled wi th NaOH solution without having been rinsed with the NaOH solution first? Explain how you reached that conclu- ston.

REFERENCES Chemistry and Biochemistry Deparrmenr. "Manual for CHEM 200/202 ." Laborarory

Manual. San Diego State University. San Diego. 2013. Prine.