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

Soluble salts can be made using a base and an acid.

Complete this method of preparing dry crystals of the soluble salt cobalt(II) chloride-6-water from the insoluble base cobalt(II) carbonate.

   step 1
   Add an excess of cobalt(II) carbonate to hot dilute hydrochloric acid.

  step 2

   step 3

   step 4

CoCO₃ + 2HCl → CoCl₂ + CO₂ + H₂O

CoCl₂ + 6H₂O → CoCl₂.6H₂O

Crystals of sodium sulphate-10-water, Na₂SO₄.10H₂O, are prepared by titration.

25.0 cm³ of aqueous sodium hydroxide is pipetted into a conical flask.

A few drops of an indicator are added. Using a burette, dilute sulphuric acid is slowly added until the indicator just changes colour. The volume of acid needed to neutralise the alkali is noted.

Suggest how you would continue the experiment to obtain pure, dry crystals of sodium sulphate-10-water.

Using 25.0 cm³ of aqueous sodium hydroxide, 2.24 mol / dm³, 3.86 g of crystals were obtained. Calculate the percentage yield.

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

Na₂SO₄ + 10H₂O → Na₂SO₄.10H₂O

Number of moles of NaOH used =

Maximum number of moles of Na₂SO₄.10H₂O that could be formed =

Mass of one mole of Na₂SO₄.10H₂O = 322g

Maximum yield of sodium sulphate-10-water, in g =

Percentage yield = 

Calcium and other minerals are essential for healthy teeth and bones. Tablets can be taken to provide these minerals.

Healthy Bones

Each tablet contains
calcium
magnesium
zinc
copper
boron

Boron is a non-metal with a macromolecular structure.

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Describe the reactions, if any, of zinc and copper(II) ions with an excess of aqueous sodium hydroxide.

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Each tablet contains the same number of moles of CaCO₃ and MgCO₃. One tablet reacted with excess hydrochloric acid to produce 0.24 dm³ of carbon dioxide at r.t.p.

CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O
MgCO₃ + 2HCl → MgCl₂ + CO₂ + H₂O

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The soluble salt hydrated lithium sulfate is made by titration from the soluble base lithium hydroxide.

The sulfuric acid is added slowly from the burette until the indicator just changes colour. The volume of sulfuric acid  needed to just neutralise the lithium hydroxide is noted.

Describe how you would continue the experiment to obtain pure dry crystals of hydrated lithium sulfate.

Separate: Chemistry and Extended Only

Using 25.0 cm³ of aqueous lithium hydroxide, concentration 2.48 mol / dm³, 2.20 g of hydrated lithium sulfate was obtained.

Calculate the percentage yield, giving your answer to one decimal place.

2LiOH + H₂SO₄ → Li₂SO₄ + 2H₂O
Li₂SO₄ + H₂O → Li₂SO₄.H₂O

   Number of moles of LiOH used = .......................
   Number of moles of Li₂SO₄.H₂O which could be formed = .......................
   Mass of one mole of Li₂SO₄.H₂O = 128g
   Maximum yield of Li₂SO₄.H₂O = ....................... g
   Percentage yield = .......................%

An experiment was carried out to show that the formula of the hydrated salt is Li₂SO₄.H₂O. A sample of the hydrated salt was weighed and its mass recorded. It was then heated and the anhydrous salt was weighed. This procedure was repeated until two consecutive masses were the same. This procedure is called ‘heating to constant mass’.

The elements in Period 3 and some of their common oxidation states are shown below.

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Aluminium sulfide contains two elements. Predict its formula.

Choose a different element from Period 3 that matches each description.

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Draw a diagram that shows the arrangement of the outer electrons in the ionic compound sodium phosphide.

Use o to represent an electron from sodium.
Use x to represent an electron from phosphorus.

Sodium reacts with sulphur to form sodium sulfide.

2Na + S → Na₂S

An 11.5 g sample of sodium is reacted with 10 g of sulfur. All of the sodium reacted but there was an excess of sulfur.

Calculate the mass of sulfur left unreacted.

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Sulfuric acid is made by the Contact process.

2SO₂ + O2 2SO₃

This is carried out in the presence of a catalyst at 450°C and 2 atmospheres pressure.

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Sulfuric acid was first made in the Middle East by heating the mineral, green vitriol, FeSO₄.7H₂O. The gases formed were cooled.

FeSO₄.7H₂O (s) → FeSO₄ (s) + 7H₂O (g)
green crystals      yellow powder

2FeSO₄ (s) → Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)

On cooling
SO₃ + H₂O → H₂SO₄ sulfuric acid
SO₂ + H₂O → H₂SO₃ sulfurous acid

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9.12 g of anhydrous iron(II) sulfate was heated. Calculate the mass of iron(III) oxide formed and the volume of sulfur trioxide, at r.t.p., formed. 

2FeSO₄ (s) → Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)

mass of one mole of FeSO₄ = 152 g
number of moles of FeSO₄ used = _____________
number of moles of Fe₂O₃ formed = _____________ 
mass of one mole of Fe₂O₃ = _____________ g
mass of iron(III) oxide formed = _____________ g
number of moles of SO₃ formed = _____________ 
volume of sulfur trioxide formed = _____________ dm³

Sulfur dioxide, SO₂, and sulfur trioxide, SO₃, are the two oxides of sulfur.

Sulfur trioxide can be made from sulfur dioxide.

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Sulphur dioxide is easily oxidised in the presence of water.

SO₂ + 2H₂O – 2e^– → SO₄^2– + 4H⁺

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Sulphur dioxide reacts with chlorine in an addition reaction to form sulphuryl chloride.

SO₂ + Cl₂ → SO₂Cl₂

8.0 g of sulphur dioxide was mixed with 14.2 g of chlorine. The mass of one mole of SO₂Cl₂ is 135 g.

Calculate the mass of sulphuryl chloride formed by this mixture.

Calculate the number of moles of SO₂ in the mixture = ..................

Calculate the number of moles of Cl₂ in the mixture = ..................

Which reagent was not in excess? ...............................

How many moles of SO₂Cl₂ were formed = ...................

Calculate the mass of sulphuryl chloride formed = ............. g

Two salts can be made from potassium hydroxide and sulfuric acid. They are potassium sulfate, K₂SO₄, and the acid salt potassium hydrogen sulfate, KHSO₄. They are both made by titration.

25.0 cm³ of potassium hydroxide, concentration of 2.53 mol / dm³, was neutralised by 28.2 cm³ of dilute sulfuric acid.

2KOH (aq) + H₂SO₄ (aq) → K₂SO₄ (aq) + 2H₂O (l)

Calculate the concentration of the sulfuric acid.

number of moles of KOH used = ............................

number of moles of H₂SO₄ needed to neutralise the KOH = ............................

concentration of dilute sulfuric acid = ............................ mol / dm³

In the conical flask there is a neutral solution of potassium sulfate which still contains the indicator used in the titration.

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KOH (aq) + H₂SO₄ (aq) → KHSO₄ (aq) + H₂O (l)

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Describe a test which would distinguish between aqueous solutions of potassium sulfate and sulfuric acid.

   Test:

   Result:

Define the following

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Which two of the following contain the same number of molecules?
Show how you arrived at your answer.

   2.0 g of methane, CH₄
   8.0 g of oxygen, O₂
   2.0 g of ozone, O₃
   8.0 g of sulfur dioxide, SO₂

4.8 g of calcium is added to 3.6 g of water. The following reaction occurs.

Ca + 2H₂O → Ca(OH)₂ + H₂

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Chemists use the concept of the mole to calculate the amounts of chemicals involved in a reaction.

Define mole.

3.0 g of magnesium was added to 12.0 g of ethanoic acid.

Mg + 2CH₃COOH → (CH₃COO)₂Mg + H₂

The mass of one mole of Mg is 24 g.

The mass of one mole of CH₃COOH is 60 g.

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In an experiment, 25.0 cm³ of aqueous sodium hydroxide, 0.4 mol / dm³, was neutralised by 20.0 cm³ of aqueous oxalic acid, H₂C₂O₄.

2NaOH + H₂C₂O₄ → Na₂C₂O₄ +2H₂O

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Fe₂(SO₄)₃ (s)→ Fe₂O₃ (s) + 3SO₃ (g)

Number of moles of Fe₂(SO₄)₃ =

Number of moles of Fe₂O₃ formed = 

Mass of iron(III) oxide formed in g =

Number of moles of SO₃ produced = 

Volume of sulphur trioxide at r.t.p. in dm³ =

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Sulphur trioxide can be made from sulphur dioxide.

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Sulphur dioxide is easily oxidised in the presence of water.

SO₂ + 2H₂O – 2e^– → SO₄^2- + 4H⁺

What colour change would be observed when an excess of aqueous sulphur dioxide is added to an acidic solution of potassium manganate(VII)?

Sulphur dioxide reacts with chlorine in an addition reaction to form sulphuryl chloride.

SO₂ + Cl₂ → SO₂Cl₂

8.0 g of sulphur dioxide was mixed with 14.2 g of chlorine. The mass of one mole of SO₂Cl₂ is 135 g.

Calculate the mass of sulphuryl chloride formed by this mixture.

Calculate the number of moles of SO₂ in the mixture = 

Calculate the number of moles of Cl₂ in the mixture = 

Which reagent was not in excess? 

How many moles of SO₂Cl₂ were formed =

Calculate the mass of sulphuryl chloride formed in g =

The food additive E220 is sulfur dioxide. It is a preservative for a variety of foods and drinks.

Sulfur dioxide is a reductant (reducing agent). Describe what you would see when aqueous sulfur dioxide is added to acidified potassium manganate(VII).

Sulfur dioxide can also be made by the reaction between a sulfite and an acid.

Na₂SO₃ + 2HCl → 2NaCl + SO₂ + H₂O

Excess hydrochloric acid was added to 3.15 g of sodium sulfite. Calculate the maximum volume, measured at r.t.p., of sulfur dioxide which could be formed.
The mass of one mole of Na₂SO₃ is 126 g.

The alkanes are a family of saturated hydrocarbons. Their reactions include combustion, cracking and substitution. 

The complete combustion of hydrocarbons produces carbon dioxide and water only.

Cracking is used to obtain short-chain alkanes, alkenes and hydrogen from long-chain alkanes.

Chlorine reacts with propane in a substitution reaction to form 1-chloropropane.

 CH₃–CH₂–CH₃ + Cl₂ → CH₃–CH₂–CH₂–Cl + HCl

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

State the colour of methyl orange in aqueous sodium hydroxide.

Determine the concentration of the dilute sulfuric acid in g/dm³ using the following steps.

• Calculate the number of moles of aqueous sodium hydroxide added to the conical flask.

..................... mol

• Calculate the number of moles of dilute sulfuric acid added from the burette.

...................... mol

• Calculate the concentration of the dilute sulfuric acid in mol/dm³.

............ mol/dm³

• Calculate the concentration of the dilute sulfuric acid in g/dm³.

............... g/dm³

Ethanoic acid is a weak acid and hydrochloric acid is a strong acid.
Both ethanoic acid and hydrochloric acid dissociate in aqueous solution.

Hydrochloric acid produces salts called chlorides.
Magnesium carbonate reacts with hydrochloric acid to produce magnesium chloride.

MgCO₃ + 2HCl→ MgCl₂ + H₂O + CO₂

A student used 50.00 cm³ of 2.00 mol/dm³ hydrochloric acid in an experiment to produce magnesium chloride.

Calculate the mass, in g, of magnesium carbonate needed to react exactly with 50.00 cm³ of 2.00 mol/dm³ hydrochloric acid using the following steps.

• Calculate the number of moles of HCl present in 50.00 cm³ of 2.00 mol/dm³ HCl.

.............................. mol

• Determine the number of moles of MgCO₃ which would react with 50.00 cm³ of 2.00 mol/dm³ HCl.

.............................. mol

• Calculate the relative formula mass, M_r, of MgCO₃.

M_r of MgCO₃ = ..............................

• Calculate the mass of MgCO₃ needed to react exactly with 50.00 cm³ of 2.00 mol/dm³ HCl.

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

When copper(II) oxide is heated at 800 °C it undergoes the reaction shown by the equation.

4CuO → 2Cu₂O + O₂

Calculate the volume of oxygen, measured at r.t.p., which is formed when 1.60 g of CuO reacts as shown in the equation.

Oxygen is produced by the decomposition of hydrogen peroxide. Manganese(IV) oxide is the catalyst for this reaction.

The equation for the decomposition of hydrogen peroxide is shown.

2H₂O₂ (aq) → 2H₂O (l) + O₂ (g)

25.0 cm³ of aqueous hydrogen peroxide forms 48.0 cm³ of oxygen at room temperature and pressure (r.t.p.).

Calculate the concentration of aqueous hydrogen peroxide at the start of the experiment using the following steps.

• Calculate the number of moles of oxygen formed.
• Deduce the number of moles of hydrogen peroxide that decomposed.
• Calculate the concentration of hydrogen peroxide in mol/dm³.

This question is about elements X, Y and Z.

What is the name of the amount of any substance that contains 6.02 × 10²³ particles?

The constant 6.02 × 10²³ has a name.

What is the name of this constant?

Sulfur dioxide, SO₂, is used in the manufacture of sulfuric acid.

The next stage of the process is a reaction which can reach equilibrium.

The equation for this stage is shown.

2SO₂ (g) + O₂ (g)  2SO₃ (g)

Calculate the percentage by mass of sulfur in sulfur trioxide, SO₃.

percentage = ..............................%

This question is about reactions of bases and acids.

A student wanted to find the concentration of some dilute sulfuric acid by titration. The student found that 25.0 cm³ of 0.0400 mol/dm³ NaOH (aq) reacted exactly with 20.0 cm³ of H₂SO₄ (aq).

Calculate the concentration of the H₂SO₄(aq) in mol/dm³ using the following steps.

• Calculate the number of moles of NaOH in 25.0 cm³.
  
  

• Deduce the number of moles of H₂SO₄ that reacted with the 25.0 cm³ of NaOH (aq).
  
  

• Calculate the concentration of H₂SO₄ (aq) in mol/dm³.
  
  

A student wanted to find the concentration of some dilute sulfuric acid by titration. The student found that 25.0 cm³ of 0.0400 mol/dm³ NaOH (aq) reacted exactly with 20.0 cm³ of H₂SO₄ (aq).

Calculate the concentration of the 0.0400 mol/dm³ NaOH (aq) in g/dm³.

Ammonia, NH₃, is used to produce nitric acid, HNO₃. This happens in a three-stage process.

Stage 1 is a redox reaction.

4NH₃ + 5O₂ → 4NO + 6H₂O

In this reaction the predicted yield of NO is 512 g. The actual yield is 384 g. Calculate the percentage yield of NO in this reaction.

The equation for the reaction in stage 3 is shown.

4NO₂ + 2H₂O + O₂ → 4HNO₃

Calculate the volume of O₂ gas, at room temperature and pressure (r.t.p.), needed to produce 1260 g of HNO₃.

Use the following steps.

• Calculate the number of moles of HNO₃.

moles of HNO₃ = ..............................

• Deduce the number of moles of O₂ that reacted.

moles of O₂ = ..............................

• Calculate the volume of O₂ gas that reacts at room temperature and pressure (r.t.p.).

volume of O₂ gas = .............................. dm³

The structure of propenyl ethanoate is shown.

Use the structure to explain why propenyl ethanoate is unsaturated.

Describe a chemical test to show that propenyl ethanoate is unsaturated.

test:.........................................................

observation:............................................

Propenyl ethanoate is prepared by the reaction between a carboxylic acid and an alcohol, as shown.

In an experiment 11.6 g of the alcohol is reacted with an excess of the carboxylic acid. The experimental yield of propenyl ethanoate is 6.72 g.

 [The relative formula mass of propenyl ethanoate is 100.]

Salts can be prepared by the reaction of acids with bases or alkalis and also by precipitation reactions.

State the ionic equation for the reaction between an acid and an alkali.

Sodium sulfate is a soluble salt prepared by a titration method using an acid and an alkali. Identify the acid and the alkali used to prepare sodium sulfate.

Aqueous sodium sulfate is used to prepare barium sulfate in a precipitation reaction.

Ba²⁺ (aq) + SO₄²⁻ (aq) → BaSO₄ (s)

In an experiment 20.0 cm³ of 0.550 mol / dm³ of barium nitrate is added to an excess of sodium sulfate.

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[M_r : BaSO₄, 233]

Show your working.

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