Quantitative chemistry

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₍₉₎

Question 3

Define relative atomic mass and relative formula mass

Answer 3

RAM - average mass of atoms in an element talking into account masses and abundance of its isotopes, relative to ¹²C.
RFM - sum of RAM’s of all atoms in the formula.

Question 4

The following reaction occurs in a test tube under a bunsen burner:
4MgO₍ₛ₎ + CH₄₍₉₎ → 4Mg₍ₛ₎ + 2H₂O₍₉₎ + CO₂₍₉₎
The carbon dioxide and water escape from the test tube.
Use the equation to explain why:

Answer 4

They are both gases.

Question 5

The experiment was repeated three times. Calculate the mean mass of magnesium produced and suggest how you could increase the precision of the results

Mass of magnesium oxide used in g:
Exp. 1 - 4.0g
Exp. 2 - 4.0g
Exp. 3 - 4.0g

Mass of magnesium produced in g:
Exp. 1 - 3.3g
Exp. 2 - 3.5g
Exp 3. - 3.2g

Answer 5

(3.3 + 3.5 +3.2) / 3 = 3.3

To improve precision, measure to more decimal places or use a more sensitive balance / apparatus.

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 x 10²³.

Question 7

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

Answer 7

Mass = Mr x moles

Question 8

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

Answer 8

Mass = Mr x moles
Mr = 100
100 x 20 = 2,000g

Question 9

Calculate the amount of carbon dioxide in moles in 0.32g of carbon dioxide.
Relative atomic masses (Aᵣ):
Carbon = 12
Oxygen = 16

Answer 9

Moles = mass / Mr

0.32 / 44 = 0.007mols

Question 10

Nitrogen and hydrogen form ammonia shown by the following equation:
N₂₍₉₎ + 3H₂₍₉₎ ⇌ 2NH₃₍₉₎
Calculate the mass of nitrogen needed to form 6.8 tonnes of ammonia.
Relative atomic masses (Aᵣ):
H = 1
N = 14

Answer 10

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

6.8 / 17 = 0.4mols of ammonia (in tonnes).

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

The ratio is nitrogen to ammonia is 1:2.

Therefore the number of moles of nitrogen is 0.4 / 2 = 0.2mols.

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

0.2 x 28 = 5.6 tonnes of nitrogen.

Question 11

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

Answer 11

In a chemical reaction involving 2 reactant, it is common to use an excess of one of the reactants to ensure 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

Hydrogen peroxide decomposes in water to form water and oxygen. How many grams of oxygen gas will be give off from 40.8g of hydrogen peroxide?

Answer 12

Step 1 - write the balanced equation:

2H₂O₂₍ₗ₎ → 2H₂O + O₂₍₉₎

Step 2 - find the number of moles in 40.8g: 40.8 / 34 = 1.2mols.

Ratio in balanced equation is 2:1.

Step 3 - Therefore the number of O₂ mols is 0.6mols.

Step 4 - mass of oxygen = 0.6 x 32 = 19.2g

Question 13

Write down the 2 formulae that link concentration, mole/mass and volume together

Answer 13

Concentration (g/dm³) = mass (g) / volume (dm³)

Concentration (mol/dm³) = moles / volume (dm³)

Question 14

31.0cm³ of potassium hydroxide solution neutralised 25.0cm³ of 2.0mol/dm³ acid.

HNO₃ + KOH → KNO₃ + H₂O

Calculate the concentration of the potassium hydroxide solution in mol/dm³

Answer 14

Step 1 - calculate the moles of HNO₃ used = concentration x volume.

2 x 0.025dm³ (25/1000 in order to convert to dm³) = 0.05mols.

Step 2 - calculate the moles of KOH.

Ratio is 1:1 therefore number of moles of KOH is 0.05mols

Step 3 - calculate the concentration of KOH.

Volume = mols / concentration; 0.05 /0.031 = 1.61mols/dm³.

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 (20°C) and room pressure (1 atm) occupies 24dm³.

moles = volume of gas (dm³) / 24dm³

Question 16

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

Answer 16

• The reaction may not go to completion because it is 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 17

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

Answer 17

% Yield = (actual mass of a product / maximum theoretical mass of product) x 100

Question 18

Calculate the percentage yield from the following data

Answer 18

AY - (actual yield), PY (predicted yield), PY% (percentage yield)
AY - 45g, PY - 100g, PY% - 45%
AY - 12g, PY - 50g, PY% - 24%
AY - 8g, PY - 40g, PY% - 20%

Question 19

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

Answer 19

Step 1 - write the balanced equation

N₂ + 3H₂ → 2NH₃

Step 2 - calculate the theoretical amount of NH₃ (ratio of H₂ to NH₃ is 3:2). 20mols H₂ therefore (20 x 2) / 3 = 13.3mols.

13.3 x 17 (Mr of NH₃) = 227g

Step 3 - calculate percentage yield of NH₃

40.5 / 227 = 17.8%

Question 20

What is atom economy?

Answer 20

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 21

Look at the equations for the 2 reactions that produce CuCl₂

Reaction 1: CuCO₃₍ₛ₎ + 2HCl₍ₐq₎ → CuCl₂₍ₐq₎ + H₂O₍ₗ₎ + CO₂₍₉₎
Reaction 2: CuO₍ₛ₎ +2HCl₍ₐq₎ → CuCl₂₍ₐq₎ + H₂O₍ₗ₎

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

Which reaction has a better atom economy?

Answer 21

Reaction 2 (look at reactants).
Total formula mass of reactants = 152.5
Formula mass of CuCl₂ = 134.5

(134.5 / 152.5) x100 = 88.2%