Acids, Alkalis and Titrations
6 marks · NormalA student titrates 25.0 cm³ of sodium carbonate solution with hydrochloric acid. The burette readings below are recorded.
| Reading | Volume (cm³) |
|---|---|
| Initial | 3.10 |
| Final | 25.60 |
Calculate the volume of hydrochloric acid added. Give your answer to two decimal places.
Write a balanced equation for the reaction between sodium carbonate solution and hydrochloric acid.
State the ion produced in sodium carbonate solution that is responsible for its alkaline pH, and explain its formation using an ionic equation.
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.
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(a) 22.50 cm³ cm^3
(b) Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)
(c) OH^-; carbonate ions react with water: CO3^{2-}(aq) + H2O(l) ⇌ OH^-(aq) + HCO3^-(aq)
Common mistakes: wrong subtraction order; incorrect formulae in equation; naming Na+ as alkaline ion
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Subtracts initial from final burette reading (25.60 - 3.10)
B1 (a) · 0 marks · Gives answer to two decimal places (22.50 cm³) (follow-through)
B2 (b) · 1 mark · Writes correct reactant formulae
A1 (b) · 1 mark · Correct balanced equation and states
B3 (c) · 1 mark · Identifies hydroxide ion, OH^-, as responsible for alkalinity
M2 (c) · 1 mark · Writes correct ionic equation for formation of OH^-
A2 (c) · 1 mark · Explains that carbonate ions react with water to make OH^- ions
Worked solution
(a) Volume of acid added = Final − Initial = 25.60 − 3.10 = 22.50 cm³
(b) The equation is:
(c) The ion responsible for the alkaline pH is the hydroxide ion (OH^-). Carbonate ions react with water:
Thus, some carbonate ions produce OH^- ions in water, making the solution alkaline.