Chapter 3 Amount of substance

Question 1

Amount of substance (n)

Answer 1

Used to count the number of particles in a substance

Question 2

Units of amount of substance

Answer 2

Mole (mol.)

Question 3

Avogadro Constant (N)

Answer 3

6.02 x 10^23

• The number of particles in 1 mole of carbon-12 or of a substance

Question 4

Mole

Answer 4

Amount of a substance that contains 6.02 x 10^23 particles

Question 5

Relative atomic mass or relative molar mass

Answer 5

Mass (in grams) of 1 mole of atoms of an element

Question 6

Chemical formulae

Answer 6

• Molecular formulae
• Empirical formulae

Question 7

Chemical formulae: Molecular formulae

Answer 7

The number of atoms of each element in each molecule

Question 8

Chemical formulae: Empirical formulae

Answer 8

The simplest whole-number ratio of atoms of each element in a compound

Question 9

Relative molecular mass

Answer 9

Compares the mass of a molecule with the mass of an atom of carbon-12

Question 10

Relative formula mass

Answer 10

Compares the mass of a formula unit with the mass of an atom of carbon-12

Question 11

Analysis

Answer 11

Investigating the chemical composition of a substance

Question 12

Water of crystallisation in hydrated salts

Answer 12

When water molecules are part of their crystalline structure

Question 13

Assumptions made when finding the formula of a hydrated salt

Answer 13

1. All water has been lost. A good solution is to heat to constant mass
2. No further decomposition (some salts decompose further when heated (e.g. copper sulfate -> copper oxide)

Question 14

Standard solution

Answer 14

A solution of known concentration

Question 15

Molar gas volume

Answer 15

The volume per mole of gas molecules at a stated temperature and pressure (normally RTP)

Question 16

RTP

Answer 16

Room temperature and pressure

• 20 degrees celsius
• 101 kPa (1atm) pressure

Question 17

What volume does 1 mole of gas molecules at RTP take up?

Answer 17

24 dm3

Question 18

The ideal gas equation

Answer 18

pV = nRT

Question 19

Assumptions made for the molecules in an ideal gas:

Answer 19

• Random motion
• Elastic collisions
• Negligible size
• No intermolecular forces

Question 20

Stoichiometry

Answer 20

The balancing numbers which gives the ratio of the moles in each substance

Question 21

Why do chemists balance equations

Answer 21

To find:

• The quantities of reactants required to prepare a required quantity of a product
• The quantities of products that should be formed from certain quantities of reactants

Question 22

Percentage yield

Answer 22

Actual yield/theoretical yield x 100

Question 23

Theoretical yield

Answer 23

The maximum possible amount of product

Question 24

Why is theoretical yield difficult to achieve?

Answer 24

• The reaction may not have gone to completion
• Other reactions may have taken place alongside the main reaction
• Purification of the product may result in loss of some product

Question 25

Actual yield

Answer 25

The amount of product obtained from the reaction

• Usually lower than theoretical yield

Question 26

The limiting reagent

Answer 26

The reactant that is not in excess which woll be completely used up first and stop in reaction

Question 27

Atom economy

Answer 27

A chemical reaction is a measure of how well atoms have been utilised

Question 28

Characteristivs of reactions with high atom economies

Answer 28

• Produce a large proportion of desired products and few unwanted waste products
• are important for sustainability as they make the best use of natural resources

Question 29

Atom economy equation

Answer 29

sum of molar masses of desired products/ sum of molar masses of all products

x 100

Question 30

What does efficiency depend on

Answer 30

• Atom economy
• Percentage yield

e.g. a reaction could have a high atom economy but low percentage yield