AMHS AP Chemistry – Review Material: Acid/Base/Buffer
Question Format: Long
Category of Question: Lab
Lemon juice is acidic because of the presence of citric acid, a weak triprotic acid
with the formula C6H8O7, (molar mass 192.12 g) and structure as shown to the
right. The acidity of a lemon varies with both the variety and the growing
conditions with the average being approximately 5% citric acid. This percentage
is represented as grams of citric acid per 100 mL of lemon juice.
Available equipment and supplies (Not all will necessarily be used)
50.00 mL buret
buret clamp and stand
250 mL Erlenmeyer flask
250 mL of standardized NaOH solution
50 mL of juice from a lemon (freshly squeezed and filtered)
pH meter
phenolphthalein
wash bottle
freshly distilled water
10.00 mL volumetric pipet
100 mL graduated cylinder
Questions
(a) Circle the hydrogen atoms that are able to ionize in the citric acid structure.
(b) Write a complete balanced molecular equation to show the neutralization of the citric acid by sodium hydroxide.
(c) When the NaOH solution was standardized, 1.2582 g of potassium hydrogen phthalate, a monoprotic weak acid (molar
mass = 204.22 g) neutralized a 22.43 mL sample of the sodium hydroxide solution. Determine the molarity of the sodium
hydroxide solution.
(d) Design an experiment to determine the citric acid content of a sample of lemon juice. List the steps of the lab
procedure.
(e) Describe all calculations that would be necessary in this experiment.
(f) Many commercial lemon-flavored drink mixes include both sodium citrate and citric acid. Suggest a reason that the
sodium citrate is included.
(g) In another experiment, a 17.6-g sample of solid ascorbic acid (H2C6H6O6) is dissolved into enough water to make
1.00L of solution. This sample is then titrated with a 0.150 M solution of potassium hydroxide to its final endpoint where
both labile protons have been removed.
[The Ka1 and Ka2 values for this acid are 7.9 x 10-5 and 1.6 x 10-12 respectively.]
Determine the pH of the solution at the following points during the titration:
(i)
The initial pH of the solution prior to beginning the titration.
(ii)
After 33.3 mL of base has been added.
(iii)
After 66.6 mL of base has been added.
(iv)
After 99.9 mL of base has been added
(v)
Determine the volume of base needed to be added to just reach the endpoint and the pH at the endpoint.
(h) Draw a graph plotting pH vs vol of base added using details from your calculations from ‘g’ above.