Edexcel A-Level Chemistry AS 8.21 Core Practical 3 Finding the Concentration of a Solution of Hydrochloric Acid Questions

Practise Edexcel IAL Chemistry 8.21 by using a rough titre, adding acid dropwise near the endpoint, swirling continuously and stopping when the indicator colour changes.

Exam points

Describe accurate titration technique using rough titres, dropwise acid and swirling.

[Maximum number: 3]

A student is required to determine the concentration of hydrochloric acid using a solution of sodium carbonate of concentration $0.105 \mathrm{~mol} \mathrm{dm}^{-3}$ .

Outline procedure

Step 1 Fill a clean burette with the hydrochloric acid.
Step 2 Pipette $25.0 \mathrm{~cm}^{3}$ of the sodium carbonate solution into a conical flask and add a few drops of methyl orange indicator.

Step 3 Carry out a rough titration.
Step 4 Carry out accurate titrations until concordant results are obtained.

The student refilled the burette and prepared a second conical flask by adding sodium carbonate solution and methyl orange indicator.

Describe how an accurate titration should then be carried out.

A description that makes reference to the following points:

- add the acid (quickly) until just short of the rough end-point

- add the acid drop-by-drop

- with swirling

and

until the indicator colour (just) changes

Accept within $1-4 \mathrm{~cm}^{3}$ (any value in this range) before the rough titre

Allow to a value in the range $19.4-22.4 \mathrm{~cm}^{3}$

Allow 'until close to the rough value' Ignore 'carbonate added'

Do not award 'until the rough value is reached'

Standalone mark (award even if M1 not given)

Allow 'dropwise'

Allow 'a few drops at a time'

Ignore 'add very slowly'

(3)

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Edexcel A-Level Chemistry AS 8.21 Core Practical 3 Finding the Concentration of a Solution of Hydrochloric Acid Questions question 1