QUESTIONS

Question 1: Multiple choice
Choose the correct answer. Only write the letter of the answer you select.
1.1 In the compound SO₂, the ratio of the mass of sulphur to oxygen is always …

1. 1:2.
2. 2:1.
3. 8:16.
4. 1:1. (3)

1.2 The formula 3 Al₂O₃ represents …

1. 3 atoms of aluminium and 6 atoms of oxygen.
2. 3 atoms of aluminium and 2 atoms of oxygen.
3. 3 formula units of Al₂O₃.
4. 6 atoms of aluminium and 3 atoms of oxygen. (3)

1.3 You are given the unbalanced reaction Mg(OH)₂ + HCl → MgCl₂ + H₂O The balanced reaction would be:

1. 2 Mg(OH)₂ + 2 HCl → 2 MgCl +2 H₂O.
2. Mg(OH) + 2 HCl → MgCl + 2 H₂O.
3. Mg(OH)₂ + 2 HCl → MgCl₂ + 2 H₂O.
4. 2 Mg(OH)₂ + 4 HCl → 2 MgCl₂ + 4 H₂O. (3)

1.4 In a physical change, which one of the following statements is true?

1. The mass is conserved but energy is used up.
2. The mass is not conserved but the energy is conserved.
3. The number of molecules remains constant during the reaction.
4. Only the number of atoms remains constant during the reaction. (3)

1.5 Which of the following will NOT cause a precipitate if added to a solution of silver nitrate?

1. sodium chloride
2. copper chloride
3. magnesium sulphate
4. magnesium nitrate (3)

1.6 The reaction that could be used to prepare a bromide salt is:

1. copper oxide and sulphuric acid.
2. silver nitrate and potassium bromide.
3. ammonium chloride and potassium bromide.
4. ammonium nitrate and potassium carbonate. (3)

1.7 An unknown salt forms a creamy yellow precipitate with silver nitrate. The precipitate does not dissolve in ammonium hydroxide solution. The salt probably is:

1. potassium iodide.
2. potassium nitrate.
3. potassium bromide.
4. potassium chloride. (3)

1.8 The ion-exchange reaction HNO₃(aq) + NaOH(aq) → NaNO₃(aq) + 2H₂O(l) is an example of:

1.  an acid-base reaction.
2. a gas-forming reaction.
3. a redox reaction.
4. a precipitation reaction. (3)

1.9 One mole of SO₂ contains:

1. 1 molecule.
2. 6,02 × 10²³ molecules.
3. 3 atoms.
4. 18,06 × 10²³ molecules. (3)

1.10 The formula mass of C₂O₄H₂.2H₂O is:

1. 110.
2. 126.
3. 90.
4. 108. (3)

1.11 Which of the following has the greatest mass?

1. 1 mole of Fe
2. 0,5 mol FeCl₂
3. 0,1 mol FeSO₄
4. 0,2 mol Fe₂O₃ (3)

1.12 The number of moles of water molecules in 126 g of ice is:

1. 18.
2. 7.
3. 7 × 6,02 × 10²³.
4. 6,02 × 10²³. (3)

1.13 If you had 20 g each of the following compounds, which sample would have the largest number of moles?

1. H₂O
2. NO₂
3. N₂
4. NH₃ (3)

1.14 What is the number of atoms in 8 g of helium?

1. 2 × 6,02 × 10²³
2. B 6,02 × 10²³
3. 4 × 6,02 × 10²³
4. 8 ÷ 6,02 × 10²³ (3)

1.15 Which of the following does NOT represent one mole of hydrogen gas at STP?

1. 2 g
2. 22,4 dm³
3. 6,02 × 10²³ molecules
4. 2 × 6,02 × 10²³ molecules (3)

Question 2: True/false
Indicate whether the following statements are true or false. If the statement is false, write down the correct statement.
2.1 The melting of ice is an example of a physical change. (2)
2.2 The mass ratio of carbon to oxygen in a molecule of CO₂ will always be 1:2. (2)
2.3 All nitrates are soluble. (2)
2.4 The SI unit for measuring the quantity of a substance is kilogram. (2)
2.5 The number that is equal to one gram of a substance is called Avogadro’s number. (2)

Question 3: One-word answers
Provide the missing word or term in the sentences below. Write only the word or term next to the question number.
3.1 A positive ion is called a(n) … (1)
3.2 If solutions of barium chloride and magnesium sulphate are mixed, the solid that will form is … (1)
3.3 HO is the … formula of hydrogen peroxide. (1)
3.4 The state symbol for mercury at room temperature is … (1)
3.5 The souring of milk is an example of a … change. (1)

Question 4: Matching pairs
Choose an item from column B that matches the description in column A. Write only the letter of your choice (A–J) next to the question number.

[10]

Question 5: Long questions
5.1 Calculate the empirical formula of a substance that consists of 30,4% nitrogen, and 69,6% oxygen by mass. (7)
5.2 If the formula mass of the substance in 5.1 is 92, what is the molecular formula of the substance? (3)             [10]

Question 6: Long questions
6.1. If a gas is collected at STP, under what conditions of temperature and pressure is it collected? (2)
6.2 A sample of calcium carbonate (CaCO₃) was reacted with HCl in a closed contain- er according to the following equation:
CaCO₃ + 2 HCl → CaCl₂ + CO₂ + H₂O
The carbon dioxide gas produced was collected at STP and occupied a volume of 33,6 dm³. Calculate the mass of calcium carbonate that reacted with the HCl.(7)            [9]

Question 7: Long questions
7.1 Calculate the mass of potassium sulphate produced by 10 g of potassium hydrox- ide according to the equation:
2 KOH + H₂SO₄ → K₂SO₄ + 2 H₂O (7)
7.2 What volume of hydrogen gas, measured at STP, would be produced when 4 g of magnesium reacts completely with excess H2SO4 according to the equation:
Mg + H₂SO₄ → MgSO₄ + H₂ (6)            [13]

Question 8: Long questions
Aluminium oxide (Al₂O₃) is the starting compound for making high-temperature- resistant ceramic products.
8.1 What is the percentage of aluminium in aluminium oxide? (4)
8.2 How much aluminium sulphate Al₂(SO₄)₃ is required to make up 250 ml of a 0,1 mol.dm^-3 solution of this compound? (6) [10]
Question 9: Long questions
9.1 What is a precipitate? (2)
9.2 Complete the following ion-exchange reactions:

9.2.1 KCl(aq) + NaNO₃(aq) → (2)
9.2.2 CaSO₄(aq) + MgCO₃(aq) → (2)
9.2.3 BaCl₂(aq) + Na₂SO₄(aq) → (2)
9.2.4 K₂SO₄(aq) + Pb(NO₃)₂(aq) → (2)

9.3 For each reaction in 9.2, state if any precipitate is formed. If a precipitate is formed, give its formula. (4)           [14]

Question 10: Long questions
10.1 You are given three unlabelled test tubes. One contains sodium bromide, one contains potassium iodide and one contains magnesium carbonate. List the steps you would take to identify what compound was in each sample. (8)
10.2 Give a balanced equation for the reaction that helped you to identify which sam- ple contained magnesium carbonate. (3)     [11]

Question 11: Long questions
11.1 If 4,5 g of calcium burns in oxygen to form calcium oxide (CaO), what mass of CaO will form from that amount of calcium? (5)
11.2 Balance the following chemical reactions:
11.2.1 CH₄ + O₂ → CO₂ + H₂O (2)
11.2.2 Al + Cl₂ → AlCl₃ (2)
11.2.3 CO₂ + H₂O → C₆H₁₂O₆ + O₂ (2)             [6]

Question 12: Long questions
The substance adenosine triphosphate (ATP) is important because it provides a way of transferring energy in living cells.
12.1 A sample of 1,6270 g of ATP was analysed and found to contain 0,3853 g carbon, 0,0518 g hydrogen, 0,2247 g nitrogen and 0,2981 g phosphorus. The rest was oxygen. Calculate the number of moles of each element present in the sample of ATP. (Express answers correct to 4 decimal places.) (7)
12.2 The empirical formula of ATP is C₁₀H₁₆O₁₃N₅P₃. If the relative molecular mass of ATP is 507, what is its molecular formula? (3) [10]

Question 13: Long question
When solutions of potassium chloride (KCl) and silver nitrate (AgNO₃) are mixed together, a precipitate of silver chloride is formed.
13.1 Write down a balanced equation for this reaction. (3)
13.2 In a certain experiment, 24 cm³ of 0,05 mol.dm^–3 silver nitrate solution exactly reacts with 15 cm³ of potassium chloride solution. How many moles of KCl would react with the given amount of AgNO₃? (5)
13.3 Calculate the concentration of the potassium chloride solution. (3)  [11]

Question 14: Long questions
You are given three test tubes, labelled A, B and C, each containing an unknown sodium salt. The following observations were made during a practical investigation to identify the salt in each test tube.

1. When the salt in C was dissolved in water and silver nitrate (AgNO₃) was added, a yellow precipitate formed. This precipitate did not dissolve when an ammonium hydroxide solution was added to it.
2. When the salt in A was dissolved in water and barium chloride (BaCl₂) was added, a white precipitate formed.
3. The salt in B dissolved in HCl and a gas was released.

14.1 Use the above information to identify the salt in each of the test tubes A, B and C. In each case explain how you made your choice. (7)
14.2 Write a balanced chemical equation for the reaction that took place in test tube A when barium chloride was added. (2)
14.3 Write a balanced chemical equation for the reaction that took place in test tube B when HCl was added. (2)      [11]

Answers

1.1 D 
1.2 C 
1.3 C 
1.4 C 
1.5 D 
1.6 B 
1.7 A 
1.8 A 
1.9 B 
1.10 B 
1.11 B 
1.12 B 
1.13 D 
1.14 A 
1.15 D 

2.1 True 
2.2 False. The mass ratio of carbon to oxygen in a molecule of CO2 will always be 3:8.
2.3 True 
2.4 False. The SI unit for measuring quantity of substance is mole.  OR The SI unit for measuring mass is kilogram.
2.5 False. The number that is equal to one mole of a substance is Avogadro’s num- ber. 

3.1 cation 
3.2 barium sulphate
3.3 empirical
3.4 (l) 
3.5 chemical  

Question 4: Matching pairs
4.1 F 
4.2 H 
4.3 J 
4.4 I 
4.5 D
4.6 A 
4.7 E 
4.8 B 
4.9 G 
4.10 C 

Question 5: Long questions
5.1

• N 30,4%             O 69,6%
  In 100 g of compound; 30,4 g is N and 69,6 g is O. Therefore, the mole ratio in 100 g is:
  N 30,4  = 2,17         O 69,9 = 4,35
       14                            16
  Reduced to whole numbers:
  N 2,17 = 1        O 4,35 = 2
     2,17                  2,17
  Ratio N:O = 1:2
  Empirical formula is NO₂. 

5.2

• Formula mass of NO₂
  = (14 + 16 + 16) = 46 
  92 = 2 
  46
  
  Therefore, molecular formula = N₂O₄. 

Question 6: Long questions
6.1 Temperature 0 °C (or 273 K)            Pressure 101, 3 kPa = 1,013 × 10⁵ Pa
6.2

• n_CO2   = Vol = 33,6 = 1,5 moles
                 22,4  22,4
  According to the equation, 2 mol HCl reacts with 1 mol CaCO₃ 
  Therefore, 1,5 mol HCl reacts with x mol Ca CO₃
  x = 1,5 = 0,75 mol CaCO₃ 
         2
  Mass of CaCO₃ = n × M = 0,75 × (40 + 12 + 48)
  Mass CaCO₃ = 75 g 

Question 7: Long questions
7.1

• n_KOH = mass  =         10               = 0,179 mol
                  m          (39 + 16 + 1)
  According to the equation, 2 mol KOH → 1 mol K₂SO₄ 
  Therefore, 0,179 mol KOH → ( 0,179 × 1) mol K₂SO₄ 
                                                       2
  = 0,089 mol K₂SO₄ 
  Mass = n × M = 0,089 × (2 × 39 + 32 + 64)  = 15,5 g K₂SO₄ 

7.2

• nMg  = mass  = 0,4 = 0,167 mol
                 M        24
  According to the equation, 1 mol magnesium → 1 mol hydrogen
  Therefore, 0,167 mol hydrogen would be produced
  Volume at STP = n × 22,4 = 0,167 × 22,4  = 3,74 dm³ 

Question 8: Long questions
8.1

• M of Al = 27               M of O = 16
  M of Al₂O₃ = (27 + 27 + 16 + 16 + 16) = 102 
  
  % Al = 54 × 100  = 52,9% 
            102

8.2

• n = c × v  = 0,1 × 0,25             = 0,025 mol 
  Mass = n × M = 0,025 × [54 +(32 × 3) + (16 × 12)] = 0,025 × 342 
  Mass = 8,55 g 

Question 9: Long questions
9.1 A precipitate is a solid  that forms when two solutions are mixed  and form an insoluble salt.
9.2.1 KCl(aq) + NaNO₃(aq) → KNO₃(aq) + NaCl(aq) 
9.2.2 CaSO₄(aq) + (NH₄)₂CO₃ → CaCO₃(s)  + (NH₄)₂SO₄(aq) 
9.2.3 BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)  + 2 NaCl(aq) 
9.2.4 K₂SO₄(aq) + Pb(NO₃)₂(aq) → 2 KNO₃(aq) + PbSO₄(s) 
9.3.1 No precipitate 
9.3.2 CaCO₃ 
9.3.3 BaSO₄ 
9.3.4 PbSO₄ 

Question 10: Long questions
10.1 Add some HCl to a small amount of each of the three samples.  The one that bubbles and dissolves is the magnesium carbonate. Take the remaining two samples and make them into solutions. Add some AgNO₃ solution to each one.A creamy yellow precipitate forms in both. Add some NH₄OH solution to each.  The test tube containing the precipitate that dissolves when treated with the ammonium solution is the bromide. Therefore, the other tube must be the potassium iodide.
10.2 MgCO₃ + 2 HCl → MgCl₂ + CO₂ + H₂O

• correct reactants
• correct products 
• balanced correctly

Question 11: Long questions
11.1

• M of Ca = 40           M of O = 16
  Mass ratio of Ca:O in CaO must be 40:16 or 5:2 
  But we have 4,5 g Ca, so mass ratio Ca:O = 4,5:x 5 x = 2 × 4,5 (cross multiply)
  x = 2 × 4,5 / 5 
  = 1,8 g O 
  Therefore, total mass of CaO formed = 4,5 + 1,8 = 6,3 g CaO 
  OR
  2 Ca + O₂  → 2 CaO 
  2 mol Ca   → 2 mol CaO
  So 1 mol Ca   → 1 Mol CaO
  40 g Ca   → 56 g CaO 
  4,5 g Ca   → x g CaO 
  x = 4,5 × 56 = 6,3 g CaO 
                 40

11.2

• CH₄ + 2 O₂ → CO₂ + 2 H₂O 
  2 Al + 3 Cl₂ → 2 AlCl₃ 
  6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂ 

Question 12: Long questions
12.1 Mass of oxygen = 1,6270 g – (0,3853 + 0,0518 + 0,2247 + 0,2981) = 0,6671 g 
n = mass
         M
carbon n = 0,3853 = 0,0321 mol 
                      12
hydrogen n = 0,0518 = 0,0518 mol 
                          1
nitrogen n = 0,2247 = 0,0161 mol
                       14
phosphorus n = 0,2981 = 0,0096 mol 
                             31
oxygen n = 0,6671  = 0,0417 mol 
                      16
12.2 M of ATP = (10 × 12 + 16 × 1 + 13 × 16 + 5 × 14 + 3 × 31) = 507 
Relative formula mass of empirical formula = 507
Relative molecular mass = 507 
The molecular formula is the same as the empirical formula. 

Question 13: Long questions
13.1 KCl(aq) + AgNO₃(aq) → AgCl(s) + KNO₃(aq) 
13.2

• number of moles of AgNO₃ = c × v 
  = 0,05 × 0,024 
  = 0,0012 mol 
  According to the equation, 1 mol of KCl reacts with 1 mol AgNO₃.
  Therefore, 0,0012 mol KCl is required. 

13.3 c = n  = 0,0012  = 0,08 mol.dm^-3 
              v      0,015

Question 14: Long questions
14.1

• C – Yellow precipitate formed when AgNO₃ is added does not dissolve in NH₄OH
          Must be an iodide (sodium iodide) 
• A – A white precipitate formed when barium chloride is added indicates a sulphate. 
  Must be sodium sulphate.
• B – A substance that dissolves in acid and forms a gas indicates the production of CO₂ 
  Must be sodium carbonate. 

14.2 BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2 NaCl(aq) 
14.3 Na₂CO₃(aq) + 2 HCl(aq) → 2 NaCl(aq) + CO₂(g) + H₂O(l)