Conservation of mass and relative formula mass: practice

4 marksFoundation5.3.1.2

Relative atomic masses (

A_r

): H = 1, C = 12, O = 16, Mg = 24, Cl = 35.5

.1

Calculate the relative formula mass (

M_r

) of water,

\mathrm{H_{2}O}

.

[1 mark]

.2

Calculate the

M_r

of carbon dioxide,

\mathrm{CO_{2}}

.

[1 mark]

.3

Calculate the

M_r

of magnesium chloride,

\mathrm{MgCl_{2}}

.

[2 marks]

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]

3 marksFoundation5.3.1.1

Balanced equations show that atoms are not made or destroyed in a reaction.

.1

Balance the equation:

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

[1 mark]

.2

Balance the equation:

\mathrm{N_{2} + H_{2} \rightarrow NH_{3}}

[1 mark]

.3

Explain why you must not change the small numbers in a formula when balancing an equation.

[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]

4 marksFoundation5.3.1.1, 5.3.1.3

A sealed flask and its contents have mass 156.84 g before an acid reacts with a carbonate. Carbon dioxide forms but cannot escape. The flask is then opened and 1.32 g of gas escapes.

.1

Calculate the mass of the flask and contents after the gas has escaped.

[2 marks]

.2

Explain the mass before opening and the apparent loss afterwards.

[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]

7 marksFoundation5.3.1.4, 5.3.1.3

Equal samples of a metal are heated in air. Final oxide masses from five trials are 3.26, 3.28, 3.27, 3.25 and 2.91 g. In the fifth trial, some solid visibly spilled.

.1

Calculate the mean final mass from the four trials without a spill.

[3 marks]

.2

Calculate the range of the four valid results.

[2 marks]

.3

Explain why a precise mean does not prove that all the metal reacted.

[2 marks]