LAD G.2 (pg 1 of 4)
Determination of the Molar Mass of a Solid Acid
Name__________________Per___
Introduction
Volumetric analysis by titration can be used to determine the molar mass of a solid acid.
Procedure Overview: There are two parts to the procedure.
Part A:
Part A is a repeat from LAD G.1, Part A. If we have standardized basr remaining from LAD G.1, we will not need to do this part
of this LAD G.2. It is preparation for Part B in which you will standardize your sodium hydroxide solution. Recall from the LAD
B.2 − Redox Titration, that the first step in that LAD was to determine the concentration of the purple permanganate solution.
This process is called standardization. In this Lab we will use KHP (potassium hydrogen phthalate), a very stable monoprotic
solid acid, KHC8H4O4. Because it is stable, when you mass a particular amount of it, you can be confident that it is delivering the
moles of acid that you calculate from the mass you measure. When you titrate to the equivalence point, you can conclude that the
moles of acid is equal to the moles of base. Knowing the moles of base and its volume, allows for calculation of concentration.
Part B:
Part B is the purpose of the lab as stated in the title. By titrating the standardized sodium hydroxide solution with a massed sample
of a solid acid, you can determine the molar mass of the solid acid.
PreLAD:
Data Table − On a separate piece of paper, not on this LAD sheet.
Read the Procedure Overview, Procedure and Processing the Data and set up a data table. Put your data entries in rows and your
various trials will be the columns. No need to record the volume of water used to dissolve the solid acid. No need to record the
number of drops of phenolphthalein, however, be sure and make three rows for NaOH − volume at the start, volume after, and a
column for the volume used (a simple subtraction).
Problems − Show your calculations in the space below.
1. When an acid ionizes, H+ is formed. H+ is essentially just a proton. Thus an acid with one ionizable hydrogen ion is
called monoprotic, two ionizable hydrogen ions, diprotic and three would be triprotic. To which acid; mono, di or
triprotic would you need to add more base for neutralization?
2.
Suppose you measured out 0.20 g of tartaric acid, a diprotic acid, H2C4H4O6 , determine the volume of 0.15 M sodium
hydroxide required to neutralize this acid.
3.
In this LAD, you won’t know the name, and formula of the solid acid, and thus you can not calculate the molar mass
from that information, instead we must calculate the molar mass from the lab data. So if 0.118 g of some triprotic acid is
titrated to the end-point with 21.5 ml of 0.089 M sodium hydroxide, calculate the molar mass of this acid.
LAD G.2 (pg 2 of 4)
Determination of the Molar Mass of a Solid Acid
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Materials at each lab station for two groups to use
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2x ring stands with buret for NaOH solution
100 ml beaker for filling NaOH burets (blue label)
1x vial of KHP (~ 2 g total)
1x scoop − CAUTION: Do NOT cross-contaminate
scoops, be sure and rinse and wipe dry before using it
with different solids.
1x phenolphthalein
2x stirring bar
2x stirring plate
2x flasks
1x squirt bottle with deionized water
vials of solid acids (~2 g total) − Be sure and record
the letter on the vial so you can check your results
with me.
Acids
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malonic acid, 104 g/mol CH2(COOH)2
maleic acid, 116 g/mol H2C4H2O4
oxalic acid, 126 g/mol H2C2O4
adipic acid, 146 g/mol HO2C(CH2)4CO2H
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citric acid, 192 g/mol H3C6H5O7
ascorbic acid 176 g/mol HC6H7O6
potassium hydrogen phthalate 204 g/mol KHC8H4O4
Procedure − Protective eyeware is not optional.
Part A − Standardization of NaOH All waste solutions can go down the sink. Hang on to the stirring bar.
This Part is only necessary if we do not have any standardized base remaining from LAD G.1
A. Follow good lab procedures by preparing the base buret by rinsing out the burets with a small amount of the solution that will
be going in them, being sure to allow it to run through the tip. Make sure there are no air bubbles hiding in the top of your
buret. Fill the buret with the NaOH solution. Each lab group should have their own buret with base.
B. Measure out between 0.2 − 0.3 g of KHP into a flask. Dissolve the KHP in a small amount of water (enough to just barely
cover the stirring bar) using the water in the squirt bottle. Add 2 or 3 drops of phenolphthalein and insert a stirring bar. Set the
solution on the stirrer.
C. Place a small white paper over your stirring plate so you have a white background against which to view the color change of
the indicator. Read the starting volume of NaOH in the buret, and titrate the KHP solution with the NaOH solution to the
endpoint. The endpoint is signaled by the solution turning from colorless to pale pink. If the solution is dark pink, it means
you have gone too far and added too much base.
D. Record the final volume of base.
E. Repeat the titration and perform the Part A calculation to see if the molarity of the NaOH solution is consistent. Check your
results with you lab bench mates. Repeat a third trial if necessary.
Part B − Analysis of the Solid Acid. All waste solutions can go down the sink. Hang on to the stirring bar.
Read the entire procedure before starting.
F. Mass approximately 0.2 g of solid acid (Be sure and record the exact mass) into a clean tared 125 ml flask. Dissolve the acid
in enough water to cover the stirring bar. Do not be concerned if it does not completely dissolve. As the titration proceeds it
will continue to dissolve.
G. Drop in the stirring bar, and add 3−4 drops of phenolphthalein. Be sure and read the initial volume of the base in the buret.
You should start all trials with a filled buret.
H. Place a small white paper over your stirring plate so you have a white background against which to view the color change of
the indicator. Allow the base to run into the dissolved solid acid until the indicator is close to changing. Slow the flow of the
base when getting close to the endpoint so that you will stop the flow of base as close to the endpoint as possible. When one
drop is added, and the faint pink color does not disappear, you have reached the endpoint (You can assume that this is also
close to the equivalence point – when the moles of acid is neutralized by the moles of base.) Some of the acids are not so
soluble in water and they will not dissolve until yo start adding base − this will cause some lag time in the neutralization.
Thus pink will show up, but then may disappear as the base forces the equilibrium to the right and the weak acid to dissolve
as it reacts with the base.
I.
Read the final volume in the buret, and record it in your data table.
J.
Repeat this trial for the same solid acid. Repeat for a second solid acid (with a different number of ionizable protons), and if
time allows, try a third.
LAD G.2 (pg 3 of 4)
Determination of the Molar Mass of a Solid Acid
Processing the data:
Part A − May not be necessary if standardized base remaining from LAD G.1
1. Calculate the moles of KHP.
2. Calculate the molarity of the NaOH solution.
• KHP is monoprotic and sodium hydroxide is “mono-basic”.
• Thus the moles of solid KHP acid equals the moles of NaOH in solution.
3. Repeat the calculation for all three trials.
Part B
Calculate all trials separately.
1.
Knowing the molarity (that you determined in Part A or in LAD G.1) and volume of the base used to neutralize the acid,
calculate the number of moles of base used to neutralize the acid.
2.
Assume that the endpoint is the equivalence point thus the moles of base = moles of acid.
3.
If the acid is polyprotic, the moles of the solid acid will be one-half or one-third of the number of moles of acid
neutralized.
4.
Knowing the mass of solid acid, and now the number of moles of solid acid, calculate its molar mass.
• Remember that molar mass is a ratio: mass / # moles
Disposal and Clean Up
• Discard any small amount of excess acid and base down the sink with plenty of rinse water.
• Be sure all containers are covered.
• Be sure hands are washed before leaving the laboratory.
Post LAD Questions
1.
Why was it not necessary to measure the volume of water added to the solid acid?
2.
When doing any titration, why are at least three (perhaps more) trials appropriate?
3.
What is meant by the term equivalence point? What is true about the relationship between moles of solid acid and moles
of sodium hydroxide base at the equivalence point of a titration? Explain how this different for a polyprotic acid?
LAD G.2 (pg 4 of 4)
4.
Determination of the Molar Mass of a Solid Acid
Error Analysis
a. A student failed to notice that there were air bubbles in the tip of their base buret before titrating the solid acid. After
the titration to the endpoint, the air bubbles were cleared. Would the student’s neglect of clearing air bubbles before
the titration affect the final molar mass calculation cause their calculated molar mass be bigger or smaller than the
theoretical?
Justify by explaining which measurements and resulting calculations would change (higher or lower).
b.
The student did a second trial and put their solid acid into a wet beaker and then went on to tare that beaker to weigh
their solid acid, would their calculated molar mass be bigger or smaller than the theoretical?
Justify by explaining which measurements and resulting calculations would change (higher or lower).
c.
While weighing their solid acid a careless student failed to notice that they had spilled some of the solid acid onto
the balance pan, would their calculated molar mass be bigger or smaller than the theoretical?
Justify by explaining which measurements and resulting calculations would change (higher or lower).
d.
If a student failed to notice that they were using a diprotic acid instead of a monoprotic acid, would their calculated
molar mass be bigger or smaller than the theoretical? Explain