L08 · 3.1.2.2, 3.1.2.5

Solutions and volumetric analysis: RP1

Exam conditions

Want proof rather than practice? Try 3 unseen questions alone, one mixed in from an earlier unit, then fix each mark you lose. About 15 minutes.

Start the exam questions The lesson for this unit

5 marksCore chemistryPractical: RP013.1.2.5, 3.1.2.2

A student titrates 25.0

\mathrm{cm^3}

portions of 0.100

\mathrm{mol\,dm^{-3}}

sodium hydroxide with dilute sulfuric acid:

\mathrm{H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O}

. The titres are shown in the table.

Titration results
Titration	Rough	1	2	3
Titre / $\mathrm{cm^3}$	23.10	22.60	22.50	22.55

(a)

Calculate the mean titre, using only concordant results.

[1 mark]

(b)

Calculate the concentration of the sulfuric acid in

\mathrm{mol\,dm^{-3}}

[3 marks]

(c)

Each burette reading has an uncertainty of ±0.05

\mathrm{cm^3}

. Calculate the percentage uncertainty in a titre of 22.55

\mathrm{cm^3}

[1 mark]

4 marksUnfamiliar contextPractical: RP013.1.2.5, 3.1.2.2

To find the calcium carbonate content of eggshell, a student adds 0.450 g of powdered shell to 50.0

\mathrm{cm^3}

of 0.200

\mathrm{mol\,dm^{-3}}

hydrochloric acid (an excess). When fizzing stops, the leftover acid needs 22.40

\mathrm{cm^3}

of 0.100

\mathrm{mol\,dm^{-3}}

sodium hydroxide.

\mathrm{CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2}

(a)

Calculate the amount of HCl that reacted with the eggshell.

[2 marks]

(b)

Calculate the percentage by mass of calcium carbonate in the eggshell (

M_r

\mathrm{CaCO_3}

= 100.1).

[2 marks]

4 marksCore chemistryPractical: RP013.1.2.5, 3.1.2.2

A student must make 250

\mathrm{cm^3}

of a standard solution of sodium carbonate from the solid.

Describe how the student should make the solution so that its concentration is known accurately.

[4 marks]

3 marksCore chemistry3.1.2.5, 3.1.2.2

A technician dilutes 10.0

\mathrm{cm^3}

of 0.500

\mathrm{mol\,dm^{-3}}

sodium carbonate solution to 250

\mathrm{cm^3}

with distilled water (

M_r

\mathrm{Na_2CO_3}

= 106.0).

(a)

Calculate the concentration of the diluted solution in

\mathrm{mol\,dm^{-3}}

[2 marks]

(b)

Calculate the concentration of the diluted solution in

\mathrm{g\,dm^{-3}}

[1 mark]

5 marksUnfamiliar contextPractical: RP013.1.2.5, 3.1.2.2

A student finds the solubility of calcium hydroxide at 20 °C. Excess solid is stirred with water for an hour and the mixture filtered. A 25.0

\mathrm{cm^3}

portion of the filtrate needs 18.60

\mathrm{cm^3}

of 0.0500

\mathrm{mol\,dm^{-3}}

hydrochloric acid:

\mathrm{Ca(OH)_2 + 2HCl \rightarrow CaCl_2 + 2H_2O}

(a)

Explain why the student uses excess solid and then filters the mixture.

[2 marks]

(b)

Calculate the solubility of calcium hydroxide in

\mathrm{g\,dm^{-3}}

[3 marks]