Edexcel IAL Chemistry AS Ratios and Percentages

Edexcel IAL Chemistry AS Ratios and Percentages
Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry, Issue 1, September 2017Chemistry XCH11/YCH11

Practise Edexcel IAL Chemistry ratios, fractions and percentages through percentage composition, yield, error, volume fractions and mole-ratio calculations.

How this is tested

  • Calculate percentage by mass from formulae or repeat units, checking every carbonyl, chlorine or OH contribution.
  • Use percentage yield or composition to scale masses, volumes and land area with unit conversions.
  • Derive mole ratios from percentage composition and relative atomic masses, then simplify to an empirical ratio.

Question 4(b)(i)

[Maximum number: 1]

This question is about calcium and calcium hydroxide, Ca(OH)2\mathrm{Ca}(\mathrm{OH})_{2}.
A student reacted calcium with water to determine a value for the molar volume of hydrogen at room temperature and pressure.

Procedure

Step 1200 cm31200 \mathrm{~cm}^{3} of distilled water (an excess) was transferred to a conical flask.
Step 2 A small piece of calcium metal was placed in a pre-weighed weighing boat. The boat was then reweighed.

Step 3 The calcium was dropped into the conical flask and a bung connected to a gas syringe was inserted.

Step 4 The volume of hydrogen collected was recorded.

Figure for Question 4(b)(i) — Edexcel A-Level Chemistry AS

Results

Table for Question 4(b)(i) — Edexcel A-Level Chemistry AS

The equation for the reaction in the conical flask is shown.

Ca+2H2O→Ca(OH)2+H2\mathrm{Ca}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_{2}+\mathrm{H}_{2}

A second student using this method obtained a value of 21.8dm3 mol−121.8 \mathrm{dm}^{3} \mathrm{~mol}^{-1} for the molar volume of hydrogen.

Calculate the percentage error in this student's value.

The data book value for the molar volume of hydrogen under these conditions is 23.9dm3 mol−123.9 \mathrm{dm}^{3} \mathrm{~mol}^{-1}.