Determination of the molar mass of a weak acid by titration
Unit 2 Module 2
Skills tested: A&I
Aim: To determine the molar mass of a tribasic (triprotic) acid by titration
Introduction
You will determine the molar mass of an unknown acid by titration with standardised sodium hydroxide solution. Your
unknown acid is, H3A. The solid was dissolved in water and phenolphthalein was added to it and the mixture titrated
against standard NaOH solution. The reaction between the acid and NaOH is as follows: H3A + 3NaOH Na3A + 3H2O
Procedure
1. Weigh approximately 1 g of the solid acid and dissolve it in 25 cm3 of water
2. Rinse the burette with the standard 0.45 mol dm-3 NaOH solution and fill to the zero mark.
3. Add 2 drops of phenolphthalein solution and swirl flask.
4. Titrate until the first permanent pale pink colour is seen.
5. Record your burette readings.
6. Repeat the entire experiment two more times using a fresh accurately known mass of your acid.
7. Do not write a full lab report.
Results
Mass of acid /g
Volume of NaOH used /cm3
Trial 1
1.013
35.00
Trial 2
1.02
35.20
Trial 3
1.00
34.90
Treatment of results
1. 250 ml of the 0.45 mol dm-3 NaOH solution was made. What mass of solid sodium hydroxide would have to used to
prepare 0.45. mol dm-3 NaOH solution?
2. Perform calculations on each of the following trials
- the # of moles of NaOH used
- the # of moles of acid present in the reaction mixture
- the molar mass of the acid
Present your calculations in tabular form (a suggested format for your table is given below)
Trial #
Mass of acid used/g
Volume of NaOH used/ cm3
# mol NaOH used
# mol acid present
Molar mass of acid /g mol-1
1
2
3. Based on its average molar mass, suggest the identity of this triprotic acid.
3
4. If the acid was a diprotic acid instead of triprotic, show how this would affect the calculations by showing the working
using the values from TRIAL 1 ONLY.
5. A polyprotic acid like a triprotic acid has three Ka values (Ka1, Ka2 and Ka3) and each successive value is smaller than
the previous one. Write equations to show the successive donation of hydrogen ions using H3A as the formula of the
original acid and suggest why the Ka values decrease after each ionisation.