Quantitative Chemistry

Question 1

What is the relative atomic mass (Ar)?

Answer 1

The average mass of an atom of an element compared to 1/12th of a carbon-12 atom

Question 2

What is the relative formula mass (Mr)?

Answer 2

The sum of the relative atomic masses (Ar) of all the atoms in a formula

Question 3

How is Mr calculated?

Answer 3

Add the Ar values of each atom in the compound

Question 4

What is a mole?

Answer 4

A measure of the amount of substance; 1 mole contains 6.02 x 10^23 particles

Question 5

What is Avogadro’s constant?

Answer 5

6.02 x 10^23 particles per mole

Question 6

How do you calculate the number of moles?

Answer 6

Moles = Mass / Mr

Question 7

How do you calculate mass using moles?

Answer 7

Mass = Moles x Mr

Question 8

How do you calculate Mr of H2O?

Answer 8

H = 1, O = 16 → Mr = 2(1) + 16 = 18

Question 9

Calculate the number of moles in 10g of water (Mr = 18)

Answer 9

10 ÷ 18 = 0.56 mol

Question 10

Calculate the mass of 2 moles of CO2 (Mr = 44)

Answer 10

2 x 44 = 88g

Question 11

What do the numbers in a balanced equation represent?

Answer 11

The ratio of moles of each substance involved

Question 12

What does a big number in front of a formula mean in a chemical equation?

Answer 12

The number of moles of that substance

Question 13

What is the formula for concentration?

Answer 13

Concentration = Mass / Volume

Question 14

What units are used for concentration?

Answer 14

g/dm³

Question 15

Convert 250 cm³ to dm³

Answer 15

250 ÷ 1000 = 0.25 dm³

Question 16

Calculate concentration of 5g of NaCl in 250cm³ solution

Answer 16

5 ÷ 0.25 = 20 g/dm³

Question 17

What is a limiting reactant?

Answer 17

The reactant that is used up first and limits the amount of product formed

Question 18

Why is the limiting reactant important?

Answer 18

It determines the maximum yield of a product

Question 19

How can you tell which reactant is limiting?

Answer 19

Compare moles from balanced equation with amount available

Question 20

What is percentage yield?

Answer 20

(Actual yield ÷ Theoretical yield) x 100

Question 21

Why is yield never 100%?

Answer 21

Losses due to evaporation, side reactions, or incomplete reactions

Question 22

Calculate % yield: expected 50g, got 40g

Answer 22

40 ÷ 50 x 100 = 80%

Question 23

What is atom economy?

Answer 23

(Mr of desired products ÷ Mr of all products) x 100

Question 24

Why is atom economy important?

Answer 24

Higher atom economy = less waste = more sustainable

Question 25

What is the ideal atom economy?

Answer 25

100% — all atoms used in desired product

Question 26

Why are reactions with low atom economy less sustainable?

Answer 26

They produce more waste and use more resources

Question 27

How can reactions be improved for atom economy?

Answer 27

Choose different reactions or use waste products

Question 28

What steps are used to calculate masses in reactions?

Answer 28

1. Balance equation 2. Calculate moles 3. Use ratio 4. Convert to mass

Question 29

If 2H2 + O2 → 2H2O, how many moles of H2O from 4 mol H2?

Answer 29

4 mol H2 → 4 mol H2O (1:1 ratio)

Question 30

If 2H2 + O2 → 2H2O, how many moles of O2 needed for 4 mol H2?

Answer 30

2 mol O2

Question 31

Define theoretical yield

Answer 31

The maximum amount of product possible from a reaction

Question 32

Define actual yield

Answer 32

The amount of product actually obtained from a reaction

Question 33

Define reactant

Answer 33

A substance used up in a chemical reaction

Question 34

Define product

Answer 34

A substance formed in a chemical reaction

Question 35

Why do you divide by 1000 when converting cm³ to dm³?

Answer 35

To match the units of volume used in concentration (dm³)

Question 36

If mass increases in a reaction, what likely happened?

Answer 36

A gas was taken in (e.g. oxygen)

Question 37

If mass decreases in a reaction, what likely happened?

Answer 37

A gas was released (e.g. CO2 left the system)

Question 38

Calculate the Mr of CaCO3

Answer 38

Ca = 40, C = 12, O = 16 → Mr = 40 + 12 + (3×16) = 100

Question 39

How many moles are in 25g of NaOH? (Mr = 40)

Answer 39

25 ÷ 40 = 0.625 mol

Question 40

What mass of NaCl is in 0.5 mol? (Mr = 58.5)

Answer 40

0.5 × 58.5 = 29.25g

Question 41

A solution has 10g of solute in 500cm³. What is its concentration?

Answer 41

10 ÷ 0.5 = 20 g/dm³

Question 42

Convert 750 cm³ to dm³

Answer 42

750 ÷ 1000 = 0.75 dm³

Question 43

How many particles are in 2 moles of a substance?

Answer 43

2 × 6.02 × 10²³ = 1.204 × 10²⁴ particles

Question 44

If 3 mol of hydrogen reacts with 1.5 mol of oxygen, what is the limiting reactant? (2H2 + O2 → 2H2O)

Answer 44

H2 is limiting (needs 3 mol O2)

Question 45

Why is it important to use excess of one reactant?

Answer 45

To ensure the other reactant is completely used up

Question 46

What happens to the concentration of a solution if water is added?

Answer 46

It decreases (dilution)

Question 47

Why is it better to have a high atom economy?

Answer 47

It reduces waste and increases sustainability

Question 48

How can you increase the actual yield in a reaction?

Answer 48

Improve purification, reduce losses, or ensure complete reaction

Question 49

A reaction produces 72g of product from a theoretical 90g. What is the % yield?

Answer 49

(72 ÷ 90) × 100 = 80%

Question 50

Calculate atom economy: desired product Mr = 60, total Mr = 150

Answer 50

(60 ÷ 150) × 100 = 40%

Question 51

What does a low percentage yield mean?

Answer 51

Much less product was obtained than expected

Question 52

Why might actual yield be less than expected?

Answer 52

Incomplete reaction, side reactions, or loss during separation

Question 53

What is the mole ratio in the equation: 2Mg + O2 → 2MgO?

Answer 53

2:1:2 (Mg:O2:MgO)

Question 54

How do you calculate the mass of a product in a reaction?

Answer 54

1. Moles of reactant → 2. Use ratio → 3. Calculate mass of product

Question 55

Why do we use balanced equations in calculations?

Answer 55

To know the correct mole ratio of reactants and products

Question 56

What is meant by 'in excess' in a reaction?

Answer 56

A reactant that is not fully used up

Question 57

Why might mass appear to change during a reaction?

Answer 57

Gas may enter or leave the system

Question 58

What does a percentage yield above 100% suggest?

Answer 58

Error in measurement or contamination

Question 59

Can atom economy ever be more than 100%?

Answer 59

No, it's impossible by definition

Question 60

Why are reactions with high atom economy preferred in industry?

Answer 60

They are more cost-effective and reduce waste

Question 61

Define concentration in terms of particles

Answer 61

Amount of solute particles per unit volume of solution

Question 62

What are the units of volume in quantitative chemistry?

Answer 62

Usually dm³ for solutions

Question 63

Is percentage yield affected by atom economy?

Answer 63

No, yield is practical; atom economy is theoretical