Review A-Level Edexcel Chemistry Core Practical 3 through accurate titration technique with rough titres, dropwise acid addition and indicators.
A student is required to determine the concentration of hydrochloric acid using a solution of sodium carbonate of concentration 0.105 moldm−3.
Outline procedure
Step 1 Fill a clean burette with the hydrochloric acid.
Step 2 Pipette 25.0 cm3 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 cm3 (any value in this range) before the rough titre
Allow to a value in the range 19.4−22.4 cm3
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)
Clip with
(b)