O LevelChemistryCambridgeTopic Questions3. Stoichiometry3.3 The Mole & the Avogadro Constant

The Mole & the Avogadro Constant (Cambridge O Level Chemistry)

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11 mark

Which row correctly describes the mole and the value of Avogadro’s constant?

 

one mole of a substance is equal to

 one mole of a substance contains
A the substances relative atomic or molecular mass in grams 6.02 x 1023 atoms, molecules or formula units
B the substances atomic number in grams 12.02 x 1023 atoms, molecules or formula units
C the substances relative atomic or molecular mass in grams 6.02 x 1022 atoms, molecules or formula units
D the substances atomic number in grams 12.02 x 1023 atoms, molecules or formula units

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21 mark

The complete combustion of methane produces carbon dioxide and steam.

CH4(g) + 2 O2(g) → 2 H2O(g) + CO2(g)

Which statements are about the reaction correct?

1 The empirical formula of methane is CH4
2 The number of atoms in 1 mole of methane is 4 x Avogadro’s constant
3 1 mole of methane produces 72 dm3 of gaseous products at r.t.p
4 1 mole of methane occupies a volume of 12 dm3 at r.t.p

  • 1, 2 and 3

  • 1 and 2

  • 1 and 3

  • 2 and 4

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31 mark

Magnesium carbonate and hydrochloric acid react to produce salt, water and carbon dioxide.

MgCO3 (s) + 2HCl (aq) → MgCl2 (aq) + H2O (l) + CO2 (g) 

What is the volume of CO2 produced when 21 g of magnesium carbonate (Mr = 84) reacts completely with excess hydrochloric acid?

One mole of gas occupies 24.0 dm3 at room temperature and pressure.

  • 4 dm3

  • 8 dm3

  • 6 dm3

  • 2 dm3

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41 mark

Calcium carbonate undergoes thermal decomposition at high temperatures to form calcium oxide and carbon dioxide.

CaCO3 → CaO + CO2

What mass of calcium oxide (Mr = 56) is formed when 60 g of calcium carbonate (Mr = 100) is completely decomposed?

  • 28 g

  • 18.5 g

  • 60 g

  • 33.6 g

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11 mark

Calcium carbide and water react to produce ethyne and calcium hydroxide.

CaC2 (s) + 2H2O (l) → C2H2 (g) + Ca(OH)2 (s)

What is the volume of C2H2 produced, at standard temperature and pressure, when 45 g of water reacts completely with calcium carbide?

  • 72 dm3

  • 30 dm3

  • 24 dm3

  • 6 dm3

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21 mark

Aluminium and oxygen react to produce aluminium oxide.

4 Al + 3 O2 → 2 Al2O3

How many kilograms of aluminium oxide are formed when 5.4 kg of aluminium react with excess oxygen?

  • 5.4 kg

  • 88 kg

  • 10.2 kg

  • 33.6 kg

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31 mark

Magnesium nitride and water react to produce a precipitate of magnesium hydroxide and ammonia gas.

Mg3N2 + 6 H2O → 3 Mg(OH)2 + 2 NH3

How much magnesium nitride is needed to produce 87 g of the precipitate in excess water?

  • 24 g

  • 50 g

  • 30 g

  • 12 g

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41 mark

A student was asked to find the concentration in g / dm3 of a solution of sulfuric acid. She transferred 25 cm3 of the acid to a 250 cm3 volumetric flask and made it up to the mark. She titrated 25 cm3 of the diluted acid against a standard solution of sodium hydroxide of 0.1 mol dm-3. The average titre of sodium hydroxide was 18 cm3.

What is the concentration of the original acid in grams per decimetre?

  • 9.85 g dm-3

  • 12.25 g dm-3

  • 98 g dm-3

  • 35.28 g dm-3

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51 mark

A chemist reacted 0.6 g of magnesium with 0.6 dm3 of oxygen in a sealed container to produce magnesium oxide.

2Mg + O2 → 2MgO

Which row correctly describes the limiting reactant?

  limiting reactant reason
A Mg in excess
B Mg is used up first
C O2 is used up first
D O2 in excess

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11 mark

What is the concentration in mol dm-3 of a solution of sodium hydroxide that contains 16 g of NaOH in 200 cm3 of distilled water?

  • 2 mol dm-3

  • 2.5 mol cm-3

  • 0.2 mol dm-3

  • 1 mol cm-3

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21 mark

A student was given a sample of hydrochloric acid and asked to find its concentration in mol / dm3. She titrated 25 cm3 of the acid against a standard solution of sodium hydroxide of 0.1 mol dm-3. The average titre of sodium hydroxide was 30 cm3.

What is the concentration of the acid?

  • 0.50 mol dm-3

  • 0.12 mol dm-3

  • 1.25 mol dm-3

  • 1.0 mol dm-3

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31 mark

A student thermally decomposes 24.8 g of copper carbonate and obtains 11.2 g of copper(II) oxide and carbon dioxide gas which is released.

CuCO3 → CuO + CO2

What is the percentage yield of CuO?

  • 55 %

  • 70 %

  • 82 %

  • 90 %

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41 mark

A sample of chalk (limestone) with a mass of 1.70 g was reacted with excess HCl and the amount of carbon dioxide given off was measured using a gas syringe. 0.36 dm3 of CO2 gas was collected at r.t.p.

CaCO3 + 2HCl → CaCl2 + CO2 + H2O

What is the percentage purity of the chalk?

  • 75 %

  • 88 %

  • 90 %

  • 98 %

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51 mark

A chemist wants to make calcium nitrate. They start with 8.00 g of pure calcium oxide and an excess of dilute nitric acid. They produce 12.65 g of pure, dry anhydrous calcium nitrate crystals.

What is the percentage yield of calcium nitrate?

[relative atomic masses, Ar: Ca, 40; N, 14; H, 1; O, 16]

  • 54.0

  • 63.2

  • 67.1

  • 86.8

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61 mark

The relative formula masses of four compounds are given.

A student has a 1.0 g sample of each compound.

Which sample contains the highest number of moles of oxygen atoms?

 

compound

relative formula mass

A

Al2O3

102

B

CuO

80

C

H2SO4

98

D

HNO3

63

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71 mark

A compound contains 70% by mass of iron and 30% by mass of oxygen.

What is its empirical formula? [Ar: O, 16; Fe, 56]

  • FeO

  • Fe2O3

  • Fe3O2

  • Fe3O4

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81 mark

25.0 g of hydrated copper(II) sulfate crystals are heated to produce anhydrous copper(II) sulfate and water.

CuSO4•5H2O(s) → CuSO4(s) + 5H2O(g)

What is the mass of anhydrous copper(II) sulfate formed?

[Mr: CuSO4, 160; H2O, 18]

  • 9.0 g

  • 16.0 g

  • 22.5 g

  • 25.0 g

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91 mark

The relative formula masses of four compounds are given.

A student has a 1.0 g sample of each compound.

Which sample contains the highest number of moles of oxygen atoms?

 

 

compound

relative formula mass

A

Al2O3

102

B

CuO

80

C

H2SO4

98

D

HNO3

63

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101 mark

50.0 cm3 of 0.10 mol/dm3 silver nitrate, AgNO3, is added to 150.0 cm3 of 0.05 mol/dm3 sodium iodide, NaI, in a beaker.

After the reaction, solid silver iodide is present in the beaker.

What else is present?

  • aqueous silver nitrate and aqueous sodium nitrate

  • aqueous sodium iodide and aqueous sodium nitrate

  • aqueous sodium iodide only

  • aqueous sodium nitrate only

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111 mark

When 0.1 mol of the hydrocarbon, C5H12, is completely combusted it produces carbon dioxide, CO2, and water, H2O.

C5H12(l) + 8O2(g) → 5CO2(g) + 6H2O(g)

What is the volume of carbon dioxide produced when measured at room temperature and pressure?

  • 0.5 dm3

  • 2.4 dm3

  • 5.0 dm3

  • 12 dm3

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121 mark

Which compound has the lowest percentage by mass of nitrogen?

  • (NH2)2CO [Mr: 60]

  • (NH4)2SO4 [Mr: 132]

  • (NH4)3PO4 [Mr: 149]

  • NH4NO3 [Mr: 80]

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