C4. Chemical Calculations

Question 1

What is the law of conservation of mass?

Answer 1

The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants

Question 2

Write a balanced equation of magnesium reacting with hydrochloric acid.

Answer 2

Mg(s) + 2HCl (aq) –> MgCl₂ (aq) + H₂ (g)

Question 3

Define relative atomic mass and relative formula mass

Answer 3

RAM - average mass of atoms in an element taking into account masses and abundance of its isotopes, relative to ¹²C

RFM- sum of RAM’s of all atoms in the formula

Question 4

What is the relative formula mass of:

A) CaF₂?
B) C₂H₁₂O₆?

Answer 4

CaF₂ - (Ar values: Ca= 40, F = 19)

40 + 19 + 19 = 78

C₂H₁₂O₆- (Ar values: C=12, H=1, O=16)

(12×6)+(1×12)+(16×6) = 180

Question 5

The following reaction occurs in a test tube under a Bunsen Burner:

4 MgO (s) + CH₄ (g) –> 4 Mg (s) + 2H₂O (g) + CO₂ (g)

The carbon dioxide and water escape from the test tube.

Use the equation to explain why.

Answer 5

They are both gases

Question 6

What is Avogadro’s constant?

Answer 6

The number of atoms, molecules or ions in a mole of a given substance. The value of the constant is 6.02×10²³

Question 7

What is the formula that links mass, molecular mass and moles together?

Answer 7

Mass = Mr × Moles

Question 8

What is the mass of 20 moles of calcium carbonate, CaCO₃?

Answer 8

Mass = Mr x Moles

Mr=100

100 × 20 = 2000g

Question 9

Calculate the amount of carbon dioxide in moles in 0.32 g of carbon dioxide?

Relative atomic masses (Ar): carbon = 12, oxygen = 16

Answer 9

Moles= Mass / Mr

0.32 / 44 = 0.007

Question 10

Nitrogen and hydrogen form ammonia shown by the following equation:

N₂ + 3 H₂ ⇌ 2 NH₃

Calculate the mass of nitrogen needed to form 6.8 tonnes of ammonia

Relative atomic mass (Ar): H=1; N=14

Answer 10

Step 1- Work out the number of moles of ammonia (Mr of ammonia= 17)

6800000/17 = 400000 moles of ammonia

Step 2- Use the balanced equation and number of moles of ammonia to work out the number of moles of nitrogen

The ratio of nitrogen to ammonia is 1:2

Therefore the number of moles of nitrogen is 400000/2= 200000

Step 3 - Work out the mass of nitrogen (Mr of N₂ is 28)

200000 × 28= 5600000 = 5.6tonnes

Question 11

State what we mean by a limiting reactant in a chemical reaction?

Answer 11

In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used. The reactant that is completely used up is called the limiting reactant because it limits the amount of products

Question 12

How much oxygen will be given off from 40.8g of hydrogen peroxide?

Answer 12

Step 1: Write the balanced equation 2H₂O₂ –> 2 H₂O + O₂ Mr of H₂O₂ = 34

Step 2: Number of moles in 40.8g : 40.8/34 = 1.2 moles

Ratio in the balanced equation of H₂O₂ : O₂ = 2:1

Step 3: Therefore number of moles of O₂ = 0.6 moles

Step 4: Mass of oxygen = 0.6 × 32 (Mr of O₂) = 19.2

Question 13

What is the two formulae that link concentration, mole/mass and volume together?

Answer 13

Concentration (g per dm³) = Mass (g)/ Volume (dm³)

Concentration (mole per dm³) = nr of moles/volume (dm³)

Question 14

31.0 cm³ of potassium hydroxide solution neutralised 25.0cm³ of 2.0 moldm-³ nitric acid

HNO₃ + KOH –> KNO₃ + H₂O

Calculate the concentration of the potassium hydroxide solution in moldm-³

Answer 14

Step 1: Calculate the moles of HNO₃ used = Concentration × volume

2 × 0.025dm³ (25/1000 to convert the units) = 0.05 moles

Step 2: Calculate the moles of KOH

Ratio is 1:1 therefore numbers of moles of KOH = 0.05

Step 3: Calculate the concentration of KKH

Volume = Moles/concentration; 0.05/ 0.031 = 1.61

Question 15

What is the molar volume of a gas at room temperature and pressure?

Answer 15

1 mole of a gas at room temperature and pressure occupies 24dm³

Question 16

What is titration?

Answer 16

A technique for finding the concentration of a solution by reacting a known volume of this solution of a known concentration

Question 17

How do you conduct a titration?

Answer 17

a) Rinse the pipette with a solution of unknown concentration. Use the pipette to measure out the known volume of this solution
b) Add an indicator (a substance that changes colour at the end of titration)
c) Rinse the burette with a solution of known concentration. Discard the liquid. Use a burette to gradually add the solution of a known concentration
d) When indicator changes colour (at the end point), the volume added is recorded
e) It is important to get concordant volume results - they have to lie close to each other
f) Suitable calculations are performed to find the concentration

Question 18

Why is not always possible to obtain the theoretical amount of product in a chemical reaction?

Answer 18

The reaction may not go to completion because it reversible
Some of the product may be lost when it is separated from the reaction mixture
Some of the reactants may react in ways different to the expected reaction (side reactions may occur)

Question 19

How is the percentage yield of a product in a chemical reaction?

Answer 19

% Yield = Actual mass of a product × 100% ÷ Maximum theoretical mass of product

Question 20

What is the % yield of NH₃ is 40.5g NH₃ is produced from 20.0 mol H₂ and excess N₂?

Answer 20

Step 1 - Write the balanced equation

N₂ + 3H₂ –> 2 NH₃

Step 2- Calculate the theoretical amount of NH₃, Moles MH₃ (ratio of H₂ to NH₃ is 3:2); of 20/1.5 = 13.3 moles

13.3 × 17 (Mr of NH₃) = 227

Step 3- Calculate percentage yield of NH₃

40.5/227= 17.8%

Question 21

What is atom economy?

Answer 21

A measure of the amount of starting materials that end up as useful products.

It is a ratio of the relative formula mass of desired product to the sum of relative formula masses of reactants

Question 22

Look at the equations for the two reactions that produce CuCl₂

Reaction I: CuCO₃ + 2 HCl –> CuCl₂ + H₂O + CO₂

Reaction II: CuO + 2 HCl –> CuCl + H₂O

Reactive formula masses: CuO =79.5; HCl = 36.5; CuCl₂ = 134.5; H₂O = 18

Which reaction has a better atom economy?

Answer 22

Reaction II (look at the reactants):

Total formula mass of the reactants = 152.5

Formula mass of CuCl₂ = 134.5

(134.5/152.5) × 100% = 88.2%