Titration
4 marks · NormalA student wants to find out how much sodium hydroxide solution is needed to neutralise 25.0 cm³ of dilute hydrochloric acid using phenolphthalein as the indicator. Describe the main steps to carry out this titration accurately.
State the first step to prepare the acid sample for titration.
Describe how the student should handle the burette and indicator.
Explain how to know when the endpoint is reached and what to do as it approaches.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Measure 25.0 cm³ of acid with a pipette and transfer to conical flask.
(b) Fill burette with sodium hydroxide, record initial volume, and add phenolphthalein indicator to flask.
(c) As the endpoint (pale pink colour) nears, add sodium hydroxide slowly, swirling, and stop when the colour stays.
Also accept: colours for phenolphthalein
Common mistakes: using measuring cylinder; ignoring swirling or reading burette incorrectly
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Measures acid with pipette and transfers to conical flask.
B2 (b) · 1 mark · Fills burette with sodium hydroxide, reads volume, adds indicator to flask.
B3 (c) · 1 mark · Describes adding sodium hydroxide slowly near endpoint with swirling.
B4 (c) · 1 mark · Recognises endpoint by persistent pale pink colour.
Worked solution
a) Use a pipette to measure 25.0 cm³ of hydrochloric acid and place in a conical flask.
b) Fill a burette with sodium hydroxide and note its volume. Add a few drops of phenolphthalein indicator to the flask.
c) Add sodium hydroxide from burette while swirling the flask. As the pale pink end point approaches, add alkali dropwise until a permanent pale pink appears.