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7. The titration of a weak base-alkali like ammonia with a strong acid using standardised hydrochloric acid

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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 base ammonia with standardised hydrochloric acid, titration calculation

7. The titration of a weak base-alkali with a strong acid

e.g titrating ammonia (pipetted) with standardised hydrochloric acid (of known concentration in the burette) using methyl orange indicator

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

Initially the burette is clamped carefully in position and filled with standard hydrochloric acid (e.g. 0.10 to 1.0 mol/dm3, but accurately known, preferably to 4 sig. figs.).

You wear safety glasses and work below eye level when filling the burette safely using a funnel until a bit above the 0.00 cm3 mark.

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

You suck the ammonia solution up to a few cm above the calibration mark and then let it run down until the meniscus rests on the calibration mark (top of right diagram).

Add a few drops of methyl orange indicator to the ammonia solution and it should turn yellow for an alkali. 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 acid, 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 methyl orange indicator is yellow.

As you add the acid you get 'splurges' of reddish-orange colour until the mixture is swirled in the conical flask.

The swirling of the flask contents is important, it ensures all the added hydrochloric acid reacts with the ammonia solution.

Try to add dropwise when you seem to be near the orange colour at the endpoint.

The end-point is an orange colour indicating when all the ammonia is neutralised by which ever acid you are using.

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

If you 'overshoot' the titration with excess acid, the methyl orange indicator turns 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.

There is also a section on errors and reasons for repeating a titration several times.

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

10. Quiz Chemical, calculations QUIZ 12 on the basics of acid-alkali titrations Good practice questions for GCSE level students
  • Titration Calculation Example 12.5 analysis of household ammonia solution
    • Given the equation: NH3(aq) + HCl(aq) ==> NH4Cl(aq)
      • This is read as 1 mol  +  1 mol  reactants  ===> 1 mol  products
    • 5.00 cm3 portions of household ammonia were titrated with a standard hydrochloric acid solution of 1.00 mol/dm3. In the appendix this is titration procedure 2.
    • If the average titration, using methyl orange indicator, was 22.5 cm3 of hydrochloric acid, calculate ...
    • (i) the molarity of the ammonia solution, and its concentration in (ii) g/dm3 and (iii) g/cm3.
      • (i) mol HCl in titration = molarity HCl x volume of HCl in dm3
      • mol HCl = 1.00 x (22.50/1000) = 0.0225 mol HCl
      • From the equation, 1 mole of NH3 reacts with 1 mole of HCl
      • Therefore mol HCl  = mol NH3 = 0.0225
      • molarity of NH3 = mol NH3 / volume NH3in dm3
      • molarity of NH3 = 0.0225 / (5.00/1000) = 4.50 mol/dm3
      • (ii) concentration = formula mass x molarity
      • formula mass NH3 = 14 + (3x1) = 17
      • concentration of NH3 = 17 x 4.50 = 76.5 g/dm3
      • (iii) since there are 1000 cm3 in 1 dm3
      • concentration of NH3 = 76.5/1000 = 0.0765 g/cm3

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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