Acids, Alkalis and Titrations

6 marks · Normal

A student investigates different alkaline solutions and later performs a titration.

(a)

State the ion responsible for the alkalinity of sodium carbonate solution.

[1 mark]

(b)

Write the ionic equation for how carbonate ions in sodium carbonate solution produce this ion, using water.

[2 marks]

(c)

The student uses sodium carbonate solution in a titration with hydrochloric acid. Name the piece of apparatus they should use to measure 25.0 cm³ of sodium carbonate solution accurately into a conical flask.

[1 mark]

(d)

State the indicator that should be used for this titration and give the colour change expected at the endpoint.

[2 marks]

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) OH^-

(b) CO_3^{2-}(aq) + H_2O(l) \leftrightarrow OH^-(aq) + HCO_3^-(aq)

(d) methyl orange; red to orange/yellow

Common mistakes: stating Na+ as responsible for alkalinity; incorrect ionic equation; using measuring cylinder instead of pipette; stating universal indicator; incorrect indicator colour change

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · States OH^- as the ion responsible for alkalinity

M1 (b) · 1 mark · Correctly writes formula for carbonate reacting with water

A1 (b) · 1 mark · Products are OH^- and HCO_3^-, correct equation including states

B2 (c) · 1 mark · Names pipette (or volumetric pipette) as correct apparatus

M2 (d) · 1 mark · Names methyl orange as suitable indicator

A2 (d) · 1 mark · States correct colour change at endpoint (red to orange or yellow)

Worked solution

(a) Sodium carbonate solution is alkaline due to the presence of hydroxide ions, OH^-.

(b) The equation shows that carbonate ions react with water to produce OH^- ions and hydrogen carbonate ions.

(d) Methyl orange should be used for this titration; the colour change at the endpoint is from red (acid) to orange or yellow (alkali).