AQA Combined Chemistry 5.3

Question 1

Count the number of atoms in the following compounds:

a) CaO
b) Al2O3
c) H2SO4

Answer 1

a) 2 atoms (1 x Ca and 1 x O)
b) 5 atoms (2 x Al and 3 xO)
c) 7 atoms (2 xH, 1 x S and 4 x O)

Question 2

Count the number of elements in the following compounds:

a) CaO
b) Al2O3
c) H2SO4

Answer 2

a) 2 elements (Ca and O)
b) 2 elements (Al and O)
c) 3 elements (H, S and O)

Question 3

Calculate the mass of magnesium needed to produce 4.6 tonnes of magnesium oxide
2Mg + O2 - 2MgO

Answer 3

Step 1; make sure the equation is balanced
Step 2; calculate the Mr of 2Mg and the Mr of 2MgO
Mr of 2Mg = 48, Mr of 2MgO = 80
Step 3; you now know 48 tonnes of Mg would produce 80 tonnes of MgO
Step 4; find how much Mg you would need to produce 1 tonne of MgO (48/80) Mg and (80/80) MgO. Therefore 0.6 tonnes of Mg would produce 1 tonne of MgO
Step 5; X both sides by 4.6 to work out how much Mg would be needed to make 4.6 tonnes of MgO - (0.6 x 4.6) = 2.76 tonnes

Question 4

Calculate the mass of CO2 produced when 50g of CaCO3 is thermally decomposed
CaCO3 - Ca0 + CO2

Answer 4

Step 1; equation needs to be balanced
Step 2; calculate Mr of CaCO3 and CO2 = 100 and 44
Step 3; you now know 100g of CaCO3 would produce 44g of CO2
Step 4; Find what 1g of CaCO3 would produce
(100/100) CaCO3 and (44/100) CO2
1g of CaCO3 would produce 0.44g of CO2
Step 5; X both sides by 50 to work out what 50g of CaCO3 would make
50g CaCO3 would make (o.44 x 50) of CO2 = 22g

Question 5

Calculate the relative formula mass of eacgh of the compounds below; (Mg = 24; Cl = 35.5; Ca = 40; O = 16; Al = 27)

a) CaO
b) MgCl2
c) Al2O3

Answer 5

a) CaO - 40 + 16 = 56
b) MgCl2 - 24 + (35.5 x 2) = 95
c) Al2O3 - (27 x 2) + (16 x 3) = 102

Question 6

Write a balanced symbol equation for the following reaction:

48g of Mg reacts with 32g of O2 to produce 80g of MgO

Answer 6

Number of moles = mass in g/Mr
Mg - 48/24 = 2; O2 - 32/32 = 1; MgO - 80/40 = 2
Ratio = 2Mg: 102: 2MgO
Balanced equation = 2Mg + O2 - 2MgO

Question 7

A piece of magnesium is added to acid. What would happen to the mass reading on the balance and why?

Answer 7

It would decrease because there is no lid on the conical flask so gas can escape and therefore mass is ‘lost’ from the flask

Question 8

What is a ‘limiting reactant’?

Answer 8

The first reactant that gets used up in a reaction and causes the reaction to stop

Question 9

Calculate the uncertainty in the following sets of results;

a) 1, 3, 6, 7, 10
b) 22, 25, 26, 25, 24

Answer 9

Uncertainty = range/2
a) 1, 3, 6, 7, 10
Range; 10 - 1 = 9. Uncertainty = (9/2) = +4.5
b) 22, 25, 26, 25, 24
Range; 26 - 22 = 4. Uncertainty = (4/2) = +2

Question 10

Why is one reactant often added in excess to a reaction?

Answer 10

To ensure that all of the other reactants are used up

Question 11

Calculate the percenatge mass of hydrogen in:
(N = 14; H = 1; S = 32; O = 16)
a) NH3
b) H2SO4

Answer 11

% mass = Ar x no. of atoms of the element/Mr of compound

a) NH3: (3x1)/17 = 0.176
0. 176 x 100 = 18%
b) H2SO4: (2x1)/98 = 0.02
0. 02 x 100 = 2%

Question 12

How many molecules, atoms or ions are in one mole of a substance?

Answer 12

6.02 x 10^23^

Question 13

What does the law of the conservation of mass state?

Answer 13

No atoms are lost or made during a chemical reaction

Question 14

How many moles are in the following?

a) 46g of CaO?
b) 50g of O2?

Answer 14

Number of moles = mass in g/ Mr
a) Mr of CaO = 40 + 16 = 56
Number of moles = 46g / 56 = 0.82 moles
b) Mr of O2 = 16
Number of moles = 50g / 32 = 1.6 moles

Question 15

A piece of magnesium is added to acid. What would happen to the mass reading on the balance and why?

Answer 15

It would stay the same because there is a lid on the conical flask so no gas can escape

Question 16

Describe the equations below in words in terms of the number of moles of each substance reacting

1) 2Mg + O2 - 2MgO
2) H2SO4 + 2NaOH - Na2SO4 + 2H2O

Answer 16

1) 2 moles of magnesium react with 1 mole of oxygen to produce 2 moles of magnesium oxide
2) 1 mole of sulfuric acid reacts with 2 moles of sodium hydroxide to produce 1 mole of sodium sulfate and 2 moles of water

Question 17

Calculate the concentration of sodium hydroxide solution when 44g of sodium hydroxide is dissolved in 0.5dm^3^ of water

Answer 17

44g / 0.5dm^3^

= 88g/dm^3^

Question 18

What is the mass of 1 mole of a substance, in grams, equal to?

Answer 18

The relative formulas mass of the substance

Question 19

What equation links concentration, number of moles of solute and volume of solvent?

Answer 19

Concentration = number of moles in solute/ volume of solvent

Question 20

What equation links concentration, mass of solute and volume pf solvent?

Answer 20

Concentration = mass of solute/volume of solvent

Question 21

What are the two possible units for concentration?

Answer 21

g/dm^3^ or mol/dm^3^

Question 22

What is the value of the Avogadro Constant?

Answer 22

6.02 x 10^23^ per mole

Question 23

Calculate the concentration of sodium hydroxide solution when 3.5 moles of sodium hydroxide is dissolved in 100 cm^3^ of water. Give your answer in mol/dm^3^

Answer 23

Convert volume of water to dm^3^
100cm^3^/1000 = 0.1dm^3^
3.5 moles / 0.1 dm^3^ = 35mol/dm^3^