Calculations Involving Masses Practice Questions

Question 1

3 marks

Calcium nitrate, $\mathrm{Ca(NO_3)_2}$ , is used as a fertiliser because it supplies plants with nitrogen.

(relative atomic masses: N = 14, O = 16, Ca = 40)

(a)

Calculate the relative formula mass, $M_r$ , of calcium nitrate, $\mathrm{Ca(NO_3)_2}$ .

$M_r$ = ..............................................................

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(b)

Calculate the percentage by mass of nitrogen in calcium nitrate, $\mathrm{Ca(NO_3)_2}$ .

percentage by mass of nitrogen = .............................................................. %

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Question 2

3 marks

Sodium chloride, $\mathrm{NaCl}$ , is obtained from sea water.

(relative atomic masses: Na = 23, Cl = 35.5)

(a)

State what is meant by one mole of a substance.

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(b)

Calculate the number of moles of sodium chloride in $11.7\ \mathrm{g}$ of sodium chloride.

number of moles = .............................................................. mol

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Question 3

4 marks

A student investigates mass changes in two reactions.

(a)

In the first reaction the student mixes lead nitrate solution and potassium iodide solution in a stoppered conical flask. A yellow precipitate of lead iodide forms.

The total mass of the flask and its contents before the reaction is $128.65\ \mathrm{g}$ .

Give the total mass of the flask and its contents after the reaction and explain your answer.

mass = .............................................................. g

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(b)

In the second reaction the student adds excess hydrochloric acid to marble chips (calcium carbonate) in an open flask standing on a balance. Carbon dioxide gas is given off.

The total mass falls from $95.40\ \mathrm{g}$ to $93.20\ \mathrm{g}$ .

Calculate the mass of carbon dioxide given off. Give your answer to 3 significant figures.

mass of carbon dioxide = .............................................................. g

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Question 0

5 marks

A student dissolves $17.5\ \mathrm{g}$ of potassium nitrate in water. The solution is made up to a total volume of $400\ \mathrm{cm^3}$ .

(a)

Calculate the concentration of this solution, in $\mathrm{g\ dm^{-3}}$ . Give your answer to 3 significant figures.

concentration = .............................................................. $\mathrm{g\ dm^{-3}}$

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(b)

The student pours out $50\ \mathrm{cm^3}$ of this solution.

Calculate the mass of potassium nitrate in this $50\ \mathrm{cm^3}$ .

mass = .............................................................. g

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Question 0

6 marks

Empirical formulae can be worked out from reacting masses or from percentage composition by mass.

(a)

Magnesium burns in nitrogen to form magnesium nitride.

In an experiment, $0.72\ \mathrm{g}$ of magnesium combined with $0.28\ \mathrm{g}$ of nitrogen.

Calculate the empirical formula of magnesium nitride.

(relative atomic masses: N = 14, Mg = 24)

You must show your working.

empirical formula = ..............................................................

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(b)

An oxide of sulfur contains 40% sulfur and 60% oxygen by mass.

Calculate the empirical formula of this oxide of sulfur.

(relative atomic masses: O = 16, S = 32)

You must show your working.

empirical formula = ..............................................................

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Question 0

4 marks

Chemists use both molecular formulae and empirical formulae.

(a)

Ethanoic acid has the molecular formula $\mathrm{C_2H_4O_2}$ .

Deduce the empirical formula of ethanoic acid.

empirical formula = ..............................................................

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(b)

Which of these compounds has the same empirical formula as butene, $\mathrm{C_4H_8}$ ?

Put a cross ( ☒ ) in the box next to your answer.

☐ A $\mathrm{C_2H_6}$

☐ B $\mathrm{C_3H_6}$

☐ C $\mathrm{C_4H_{10}}$

☐ D $\mathrm{C_6H_6}$

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(c)

Compound Z has the empirical formula $\mathrm{CH_2O}$ and a relative molecular mass of 180.

Deduce the molecular formula of compound Z.

(relative atomic masses: H = 1, C = 12, O = 16)

molecular formula = ..............................................................

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Question 0

4 marks

In a lime kiln, calcium carbonate is heated and decomposes to form calcium oxide and carbon dioxide.

$\mathrm{CaCO_3 \rightarrow CaO + CO_2}$

The gas escapes from the kiln.

(relative atomic masses: C = 12, O = 16, Ca = 40)

(a)

Calculate the maximum mass of calcium oxide that can be obtained from $500$ tonnes of calcium carbonate.

mass of calcium oxide = .............................................................. tonnes

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(b)

Use the law of conservation of mass to calculate the mass of carbon dioxide released when $500$ tonnes of calcium carbonate decomposes completely.

mass of carbon dioxide = .............................................................. tonnes

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Question 0

5 marks

A dietary supplement tablet contains $380\ \mathrm{mg}$ of magnesium chloride, $\mathrm{MgCl_2}$ .

(relative atomic masses: Mg = 24, Cl = 35.5; Avogadro constant = $6.02 \times 10^{23}$ )

(a)

Calculate the number of moles of magnesium chloride in one tablet.

number of moles = .............................................................. mol

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(b)

Calculate the total number of ions in the magnesium chloride in one tablet.

number of ions = ..............................................................

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Question 0

6 marks

A student is given a $10\ \mathrm{cm}$ length of magnesium ribbon, a crucible with a lid, tongs, a pipeclay triangle, a tripod, a Bunsen burner and a balance reading to $0.01\ \mathrm{g}$ .

Devise a method the student could use to find the empirical formula of magnesium oxide.

In your answer you should include:

the measurements the student should make
how the student can be sure that all the magnesium has reacted
how the results are used to find the empirical formula.

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Question 0

4 marks

Zinc reacts with dilute hydrochloric acid.

$\mathrm{Zn + 2HCl \rightarrow ZnCl_2 + H_2}$

In an experiment, $0.10\ \mathrm{mol}$ of zinc is added to a solution containing $0.15\ \mathrm{mol}$ of hydrochloric acid.

(a)

Use the equation to show that the zinc is in excess in this experiment.

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(b)

Explain why the mass of zinc chloride formed in this experiment is controlled by the amount of hydrochloric acid used.

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Question 0

3 marks

When lead is heated in air it forms an oxide.

Two possible equations for this reaction are

A $\ \mathrm{2Pb + O_2 \rightarrow 2PbO}$

B $\ \mathrm{Pb + O_2 \rightarrow PbO_2}$

It was found that $4.14\ \mathrm{g}$ of lead was heated in excess air and formed $4.46\ \mathrm{g}$ of the oxide.

Carry out a calculation to decide which equation, A or B, represents the reaction taking place.

(relative atomic masses: O = 16, Pb = 207)

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Question 0

5 marks

Compound Q is a hydrocarbon. It contains carbon and hydrogen only.

When $2.90\ \mathrm{g}$ of Q is burned completely, $8.80\ \mathrm{g}$ of carbon dioxide and $4.50\ \mathrm{g}$ of water are formed.

(relative atomic masses: H = 1, C = 12, O = 16)

(a)

Calculate the empirical formula of compound Q.

You must show your working.

empirical formula = ..............................................................

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(b)

The relative molecular mass of compound Q is 58.

Deduce the molecular formula of compound Q.

molecular formula = ..............................................................

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Question 0

5 marks

Hydrated magnesium sulfate has the formula $\mathrm{MgSO_4{\cdot}7H_2O}$ .

When it is heated, all of the water of crystallisation is driven off:

$\mathrm{MgSO_4{\cdot}7H_2O \rightarrow MgSO_4 + 7H_2O}$

(relative atomic masses: H = 1, O = 16, Mg = 24, S = 32)

(a)

Show that the relative formula mass of $\mathrm{MgSO_4{\cdot}7H_2O}$ is 246.

/ 1

(b)

A sample of $24.6\ \mathrm{g}$ of $\mathrm{MgSO_4{\cdot}7H_2O}$ is heated in an open crucible until its mass stays constant.

Calculate the mass of water driven off.

mass of water = .............................................................. g

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(c)

Calculate the mass of anhydrous magnesium sulfate, $\mathrm{MgSO_4}$ , left in the crucible.

mass of $\mathrm{MgSO_4}$ = .............................................................. g

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Question 0

4 marks

$250\ \mathrm{cm^3}$ of sodium carbonate solution, of concentration $10.6\ \mathrm{g\ dm^{-3}}$ , is added to an excess of calcium chloride solution. A precipitate of calcium carbonate forms.

$\mathrm{Na_2CO_3 + CaCl_2 \rightarrow CaCO_3 + 2NaCl}$

Calculate the maximum mass of calcium carbonate that can be formed.

(relative atomic masses: C = 12, O = 16, Na = 23, Ca = 40)

mass of calcium carbonate = .............................................................. g

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Question 0

5 marks

A sample of gas G contains $1.505 \times 10^{23}$ molecules and has a mass of $7.00\ \mathrm{g}$ .

(relative atomic masses: H = 1, C = 12; Avogadro constant = $6.02 \times 10^{23}$ )

(a)

Calculate the relative molecular mass of gas G.

relative molecular mass = ..............................................................

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(b)

Gas G has the empirical formula $\mathrm{CH_2}$ .

Deduce the molecular formula of gas G.

molecular formula = ..............................................................

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Key Concepts in Chemistry

States of Matter & Mixtures

Chemical Changes

Extracting Metals & Equilibria

Groups in the Periodic Table

Rates of Reaction & Energy Changes

Fuels & Earth Science

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