Conservation of mass and relative formula mass: practice

4 marksFoundation and Higher5.3.1.3, 5.3.1.1

A student heats 2.40 g of magnesium ribbon in an open crucible. It all reacts to form 4.00 g of magnesium oxide.

\mathrm{2Mg + O_{2} \rightarrow 2MgO}

.1

Calculate the mass of oxygen that reacted with the magnesium.

[1 mark]

.2

The student says: "The mass went up, so mass was not conserved." Explain why the student is wrong.

[2 marks]

.3

The student repeats the experiment in a sealed container full of air and weighs the whole container. Predict what happens to the total mass.

[1 mark]

6 marksHigher5.3.1.4, 5.3.1.2

A student heats four samples of copper carbonate and measures the mass of copper oxide left each time. 4.03 g, 4.07 g, 3.59 g, 4.05 g

.1

Identify the anomalous result and give a reason.

[1 mark]

.2

Calculate the mean mass. Do not include the anomalous result.

[2 marks]

.3

Calculate the uncertainty in the mean, using the range of the three results you used.

[1 mark]

.4

Calculate the percentage by mass of copper in copper oxide, CuO. (

A_r

: Cu 63.5, O 16)

[2 marks]

4 marksFoundation and Higher5.3.1.2, 5.3.1.1

Ammonium nitrate,

\mathrm{NH_{4}NO_{3}}

, is used as a fertiliser.

A_r

: H = 1, N = 14, O = 16

.1

Calculate the

M_r

of ammonium nitrate.

[2 marks]

.2

Calculate the percentage by mass of nitrogen in ammonium nitrate.

[2 marks]

6 marksFoundation and Higher5.3.1.2

Calcium hydroxide has formula

\mathrm{Ca(OH)_2}

. Relative atomic masses: Ca = 40, O = 16, H = 1.

.1

Calculate its relative formula mass.

[3 marks]

.2

Calculate the percentage by mass of oxygen in calcium hydroxide.

[3 marks]

7 marksHigher5.3.1.4

A student measures a beaker as 42.36 g empty and 47.82 g with a dry solid. Each balance reading has uncertainty ±0.01 g. Add absolute uncertainties for a mass found by subtraction.

.1

Calculate the mass of solid.

[2 marks]

.2

Calculate the percentage uncertainty in the solid mass.

[3 marks]

.3

Suggest how a larger mass of the same solid affects percentage uncertainty with this balance.

[2 marks]

5 marksFoundation and Higher5.3.1.1

Propane burns completely in oxygen to form carbon dioxide and water. A sample uses 8.0 g oxygen and produces 6.6 g carbon dioxide and 3.6 g water. No products escape before weighing.

.1

Write the balanced equation for complete combustion of propane, C3H8.

[2 marks]

.2

Calculate the mass of propane that reacted.

[3 marks]