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6. The titration of a weak acid e.g. ethanoic acid with standard sodium hydroxide solution

[Author © Dr Phil Brown GRIC, PhD: Doc Brown's chemistry exam revision notes on ethanoic acid-sodium hydroxide titration suitable for students of UK GCSE level and Advanced A-level chemistry courses, ~US grades 9-12 chemistry notes  [page updated RE-EDIT]

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Titration index  *  Full chemistry calculations index

Quiz 12 The basics of acid-alkali titration calculations Good exam practice questions for GCSE level

apparatus method diagram for titrating weak ethanoic acid (vinegar) with standardised sodium hydoxide solution, titration calculation

6. The titration of a weak a strong acid with sodium hydroxide solution

e.g. titrating ethanoic acid (vinegar) with standardised sodium hydroxide solution (of known concentration in burette) using phenolphthalein indicator

The apparatus, chemicals and indicator colours are illustrated in the diagram on the right.

Initially the burette is clamped carefully in position and using a funnel, it is filled with standard sodium hydroxide solution (e.g. 0.10 to 1.0 mol/dm3, but accurately known, preferably to 4 sig. figs.).

Wearing goggles, pour in  using a funnel and work below eye level.

The sodium hydroxide solution is run through until the reading below the meniscus is 0.00 cm3 (the reading in the diagram is 7.00 cm3, which could represent a titration value). The burette is usually calibrated on the scale to 50.00 cm3 in 0.10 cm3 increments (only 10.00 cm3 in diagram - couldn't fit rest of scale on!)

The acid solution is accurately measured out into the conical flask with e.g. a 25 cm3 pipette and suction bulb (see diagram further down).

Add a few drops of phenolphthalein indicator to the acid solution and it should turn colourless for an acid. Carefully place the conical flask under the tip of the burette so drops don't go astray!

The titration: You carefully add small portions of the sodium hydroxide, swirling after each addition and checking the colour of the indicator (not shown in the diagram, but its good to stand the flask on white tile).

At the start of the titration the phenolphthalein indicator is colourless.

As you add the alkali you get 'splurges' of pink colour until the mixture is swirled in the conical flask.

The swirling of the flask contents is important, it ensures all the added sodium hydroxide reacts with the acid in the flask.

Try to add dropwise when you seem to be near the faint pink colour of the endpoint.

The end-point is the first faint, but permanent pink colour, that is when all the acid is neutralised by the sodium hydroxide.

You taking the reading of the volume of the sodium hydroxide on the scale line the meniscus lies on (see lower part of diagram above on the right).

If you 'overshoot' the titration with excess alkali, the phenolphthalein indicator becomes an even deeper pinkish-red and the result is invalid.

The titration should be repeated several times with other 25 cm3 portions and the average (mean) titration value calculated to use in any subsequent calculations.

The theory of which indicator to use is explained on the Changes in pH in a neutralisation, choice and use of indicators page.

There is also a paragraph on errors and reasons for repeating a titration several times.
  • Titration Calculation Example 12.6 analysing vinegar
    • Given the equation: CH3COOH(aq) + NaOH(aq) ==> CH3COONa(aq) + H2O(l)
      • This is read as 1 mol  +  1 mol  reactants  ===> 1 mol  +  1 mol  products
    • 25.00 cm3 portions of vinegar (ethanoic acid, CH3COOH, 'acetic acid' solution) from a local supermarket were pipetted into a conical flask and titrated with a standardised solution of sodium hydroxide, of concentration 0.2000 mol/dm3 using phenolphthalein indicator. In APPENDIX 2 this is titration is described in procedure 2.
    • (i) If the average titration value was 14.70 cm3 of the sodium hydroxide solution, what is the molarity of the ethanoic acid in the vinegar?
      • moles NaOH in titration = molarity x volume in dm3
      • mole NaOH = 0.200 x (14.70/1000) = 0.00294 moles
      • from the equation moles NaOH = moles CH3COOH = 0.00294
      • molarity CH3COOH = mol CH3COOH/volume CH3COOH in dm3
      • molarity CH3COOH = 0.00294 / (25.00/1000) =  0.1176 mol/dm3
    • (ii) What is the concentration of the ethanoic acid in g/dm3?
      • mass = moles x formula mass
      • formula mass of CH3COOH = 12 + 3 + 12 + 16 + 16 + 1 = 60
      • mass CH3COOH = 0.1176 x 60 = 7.056g (in 1 dm3, 1000 cm3)
      • concentration = 7.06 g/dm3 (2dp, 3sf)
        • for other mass/volume values ...
        • e.g. if you were asked for the concentration in g/100cm3,
          • divide the g/dm3 by 10 giving 0.706g/100cm3
        • or if you were asked for the concentration in g/cm3,
          • divide the g/dm3 by 1000 giving 0.00706 g/100cm3
      • You can also work out the vinegar concentration from the original moles and volume of the titration e.g.
      • mass = moles x formula mass
      • mass = 0.00294 x 60 = 0.1764g in the 25 cm3 sample titrated.
      • Since 25cm3 = 25/1000 = 0.025 dm3
      • concentration = 0.1764 / 0.025 = 7.06g/dm3
    • (iii) If a bottle of vinegar from the super-market contained 250 cm3 of liquid, how many grams of ethanoic acid are in the solution?
      • concentration CH3COOH = formula mass CH3COOH x molarity of CH3COOH
      • formula mass CH3COOH = 12 + (3x1) + 12 +(2x32) + 1 = 60
      • concentration CH3COOH = 60 x 0.1176 = 7.056 g/dm3
      • Since 1 dm3 = 1000 cm3, there must be proportionately
      • 7.056 x (250/1000) = 1.76 g of CH3COOH in the bottle of vinegar (3 sf)

Practise exam questions on acid-alkali titrations for Advanced A-level chemistry students

(From my original A-level acid-alkali titration questions SET 1 and SET 2)

After each question there is a link to the fully worked out answers.

Q17 The % purity of a weak organic acid can be determined by the procedure outlined in the answer to Q15(a).

0.236g of benzoic acid required 19.25 cm3 of 0.100 mol dm-3 sodium hydroxide for complete neutralisation.

Calculate ...

(a) moles of sodium hydroxide used in titration,

(b) moles and mass of benzoic acid titrated [at. masses: C = 12, H = 1 and O = 16]

(c) % purity of benzoic acid from this assay titration.

ANSWERS to Q17 SET 1. questions

Q18 Using the method described in the answer to Q15(a), sodium hydroxide solution can be standardised. 0.250 g of very pure benzoic acid (C6H5COOH) was titrated with a solution of sodium hydroxide of unknown molarity.

If 22.5 cm3 of the alkali was required for neutralisation, calculate ...

(a) moles of acid titrated [at. masses: C = 12, H = 1 and O = 16],

(b) mol alkali used in titration,

(c) the molarity of the alkali.

ANSWERS to Q18 SET 1. questions

Q19 The solubility of calcium hydroxide in water can be measured reasonably accurately to 3sf by titrating the saturated solution with standard hydrochloric acid.

(a) If the standard hydrochloric acid is made by diluting '2M' bench acid, what volume of the '2M' acid is required to make up 250 or 500 cm3 of approximately 0.1 mol dm-3 hydrochloric acid and how might you do it?

(b) Why must the 2M acid be diluted and why must the diluted acid be standardised?

In the calculation below assume the molarity of the standardised hydrochloric acid is 0.1005 mol dm-3.

(for standardisation method see Q13)

At 25oC, a few grams of solid calcium hydroxide was shaken with about 400 cm3 of deionised water, and then filtered. 50.0 cm3 samples of the 'limewater' gave an average titration of 15.22 cm3 of 0.1005 mol dm-3 hydrochloric acid using phenolphthalein indicator.

(c) If the acid is in the burette, how would you measure out the calcium hydroxide solution? and why is phenolphthalein indicator used?

(d) Give the equation for calcium hydroxide reacting with hydrochloric acid.

(e) What is the reacting mole ratio of Ca(OH)2 : HCl and hence calculate the moles of them involved in the titration.

(f) Calculate the molarity of the solution in terms of mol Ca(OH)2 dm-3.

(g) What is the approximate solubility of calcium hydroxide in g Ca(OH)2 per 100g water?

ANSWERS to Q19 SET 1. questions

Titration index  *  Full chemistry calculations index

Quiz 12 The basics of acid-alkali titration calculations Good exam practice questions for GCSE level

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