3.1.2 Amount Of Substance

Question 1

What is relative atomic mass?

Answer 1

Average mass of one atom / 1/12 mass of one carbon-12 atom

Question 2

What is relative molecular mass?

Answer 2

Average mass of one entity / 1/12 mass of one carbon-12 atom

Question 3

What is Ar?

Answer 3

Relative atomic mass

Question 4

What is Mr?

Answer 4

Relative molecular mass

Question 5

When is relative formula mass used?

Answer 5

When a compound is ionic

Question 6

What is Avogadro’s constant? (2)

Answer 6

The number of particles in one mole

6.023 x10^23

Question 7

What is the molecular formula?

Answer 7

The actual whole number of atoms of each element in a molecule of a compound

Question 8

What is the empirical formula?

Answer 8

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

Question 9

How is the empirical formula of a compound calculated? (3)

Answer 9

Divide proportions of elements by their respective Ar
Divide these values by the smallest
Multiply the calculated values by a number that makes all of them as near to a whole number as possible

Question 10

How is the molecular formula calculated from an empirical formula? (3)

Answer 10

Calculate the ‘empirical mass’ from the given formula
Divide the molecular mass by the empirical mass
Multiply the ratios of the empirical formula by this value to make the molecular formula

Question 11

How is the number of moles calculated?

Answer 11

Mass given (g) / Mass of one mole (Mr)

Question 12

How is the number of particles calculated?

Answer 12

Number of moles x Avogadro’s constant

Question 13

How is the mass of one particle calculated?

Answer 13

Mass of one mole (Mr) / Avogadro’s constant

Question 14

How is concentration/molarity calculated?

Answer 14

Concentration (moldm^-3 / gdm^-3) = mass (mol/g) / volume (dm^3)

Question 15

How is the limiting factor of an equation calculated? (2)

Answer 15

Calculate the number of moles of the desired product that would form with each given mass of reactant
The reactant that makes the least product is the limiting factor

Question 16

How is the volume of gas calculated?

Answer 16

pV = nRT
p = pressure (Pa)
V = volume (m^3)
n = number of moles
R = Rydberg's constant (8.31 J K^-1 mol^-1)
T = temperature (K)

Question 17

What assumptions does the pV = nRT equation rely on? (2)

Answer 17

Particles are treated as hard spheres of negligible size

Particles experience no attractive forces

Question 18

How is atom economy calculated?

Answer 18

(molecular mass of desired product) / (sum of molecular masses of all reactants) x 100

Question 19

What are the advantages of a high atom economy? (5)

Answer 19

Efficient use of raw materials
Less waste to be disposed of
Less harmful to environment
More sustainable
Easier and cheaper process so products are cheaper and more accessible

Question 20

What is atom economy?

Answer 20

A measure of the proportion of reactant atoms that form the desired product

Question 21

What is theoretical yield?

Answer 21

The mass of product that should form, assuming no chemicals are ‘lost’ in the process and the reaction is fully completed

Question 22

How is percentage yield calculated?

Answer 22

(actual yield) / (theoretical yield) x 100

Question 23

Why is the actual yield always lower than the theoretical yield? (3)

Answer 23

Not all starting chemicals react fully
Some chemicals are ‘lost’ (e.g. left on filter paper, side reactions)
Reaction may be reversible

Question 24

How is density calculated?

Answer 24

Density (g cm^-3 / g mL^-1) = mass (g) / volume (cm^3 / mL)

Question 25

What is water of crystallisation?

Answer 25

Water that occurs inside crystals

Question 26

What is a hydrated compound?

Answer 26

A compound that contains water

Question 27

What is an anhydrous compound?

Answer 27

A compound that does not contain water

Question 28

What is the notation for water of crystallisation? (2)

Answer 28

When compound only contains lattice water or has an undetermined structure:
Hydrated compound•nH₂O or Hydrated compoundxnH₂O

A hydrate with coordinated water:
Hydrated compound(H₂O)n

Question 29

How is water of crystallisation calculated?

Answer 29

Anhydrous mass : water mass = Mr of compound : Mr of water