Edexcel IAL Chemistry 8.20 Solution concentration and titration

Select concordant titres and calculate their mean, convert volumes to dm³, follow the balanced-equation ratio and report concentration with justified precision.

Exam points

Calculate concentration from a mean titre, pipetted volume and reaction mole ratio.
Convert between mol dm⁻³ and g dm⁻³ using molar mass and suitable significant figures.
Choose methyl orange or phenolphthalein and state its exact endpoint colour change.

[Maximum number: 6]

A student carried out an experiment to identify the metal M in the hydrated carbonate $\mathrm{M}_{2} \mathrm{CO}_{3} \cdot 10 \mathrm{H}_{2} \mathrm{O}$ . A solution was made by dissolving 3.56 g of the hydrated metal carbonate in distilled water and making the volume up to $250.0 \mathrm{~cm}^{3}$ in a volumetric flask. $25.0 \mathrm{~cm}^{3}$ of this solution was placed in a conical flask and titrated with $0.100 \mathrm{~mol} \mathrm{dm}^{-3}$ of hydrochloric acid. The equation for the reaction is

\mathrm{M}_{2} \mathrm{CO}_{3}(\mathrm{aq})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow 2 \mathrm{MCl}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{CO}_{2}(\mathrm{~g})

(a)

Methyl orange indicator was used in this titration. Give the colour change in the conical flask at the end-point. Colour change from to

[ 2 ]

(b)

The results of the titration are shown.

Table for Question (b) — Edexcel A-Level Chemistry AS

[ 4 ]

(i)

Using your answer to (c)(ii), calculate the number of moles of HCl in the mean titre.

[ 1 ]

(ii)

Calculate the number of moles of $\mathrm{M}_{2} \mathrm{CO}_{3}$ in $25.0 \mathrm{~cm}^{3}$ of the solution. Hence calculate the number of moles of $\mathrm{M}_{2} \mathrm{CO}_{3}$ in the $250.0 \mathrm{~cm}^{3}$ volumetric flask.

[ 2 ]

(iii)

Using your answer in (c)(iv) and the mass of $\mathrm{M}_{2} \mathrm{CO}_{3} \cdot 10 \mathrm{H}_{2} \mathrm{O}$ in the $250 \mathrm{~cm}^{3}$ of solution, calculate the molar mass of $\mathrm{M}_{2} \mathrm{CO}_{3} \cdot 10 \mathrm{H}_{2} \mathrm{O}$ .

[ 1 ]

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8.20—Solution concentrations, in mol dm-3 and g dm-3 question 1

Question (a)

Question (b)

Question (i)

Question (ii)

Question (iii)