3 Quantitative Chemistry

Question 1

Conservation of mass (2)

Answer 1

• The total mass of the products is equal to the total mass of the reactants.
• No atoms are lost or made during a chemical reaction

Question 2

What is the relative formula mass of Mg(OH)₂

Answer 2

Mg(OH)₂ = Mg + 2(O+H)
= 24 + 2(16+1)
= 24 + 34
= 58

Question 3

Why can there be a change in mass in a reaction?

Answer 3

Because a reactant or a product is a gas and its mass has not been taken into account.

Question 4

Percentage uncertainty equation

Answer 4

Percentage uncertainty = (range / mean) *100

Question 5

What can happen if there is a gas in the product?

Answer 5

The mass of products appear to decrease because the gas escapes:
HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
142.3g → 142.1g

Question 6

What can happen if there is a gas in the reactants?

Answer 6

The mass of products appear to go up. We cannot measure gases in the air
2Mg + O₂⁽ᵍ⁾ → 2MgO
55.0g → 55.1g

Question 7

Avogadro’s Number

Answer 7

6.02 * 10²³ = 1 mole = mol

Question 8

Calculate 3 moles of Cl₂ molecules.

Answer 8

3 * (6.02 * 10²³) molecules

Question 9

The mass of 1mol of a substance is equal to…

Answer 9

…its relative mass in 𝗴𝗿𝗮𝗺𝘀.

Question 10

mol equation

Answer 10

mol = mass(g) / Mᵣ

Question 11

Percentage yield equation

Answer 11

% yield = (actual yield / theoretical yield) *100

Question 12

Q: If 48g of product were made and the theoretical yield was 80g, what was the precentage yield?

Answer 12

%yield = (actual / theoretical)100
%yield = (48/80)100
= 0.6*100 = 60
A: 60%

Question 13

Why is there never a 100% yield in a reaction? (3)

Answer 13

• Side reactions
• Loss of product during purification
• Incomplete reaction

Question 14

Limiting reactant (3)

Answer 14

• The reactant that is completely used up in a reaction.
• It stops the reaction from going any further and any further products being produced.
• The other reactant is said to be in excess (there’s more of it that can react)

Question 15

Percentage atom economy equation

Answer 15

Percentage atom economy

= (total mass of atoms in desired product / total mass of atoms in the reaction) *100

Question 16

Q: What is the percentage atom economy of the following:

2Mg + O₂ → 2MgO

Answer 16

2Mg + O₂ } in the reaction
2MgO } desired product
% atom economy = 80 / (48+32) = 80/80 = 100
A: 100%

Question 17

Concentration equation

Answer 17

concentration = moles / volume
mol/dm³ = mol / dm³

Question 18

How do you convert cm³ to dm³?

Answer 18

÷ 1000

Question 19

Q: Convert the concentration into g/dm³:

2.5g solute dissolved in a 500cm³ solution.

Answer 19

g/dm³
2.5g / dm³ } 2.5 / 500cm³
500cm³ = 0.5dm³
2.5g / 0.5dm³ = 5
A: 5g/dm³

Question 20

1 mol of ___ occupies ___ at room temperature and room temperature pressure.

Answer 20

1) gas

2) 24dm³