1.2 Amounts of Substance

Question 1

Relative atomic mass (Ar)

Answer 1

The weighted average mass of all the isotopes of an element relative to 1/12th the mass of an atom of carbon-12

Question 2

Relative molecular mass (Mr)

Answer 2

The mass of a molecule of the compound relative to 1/12th the mass of an atom of carbon-12

Question 3

What is relative formula mass used for?

Answer 3

Ionic compounds

Question 4

What is relative molecular mass used for?

Answer 4

Covalent substances

Question 5

Relative formula mass (Mr)

Answer 5

The mass of one formula unit of an ionic compound relative to 1/12th the mass of an atom of carbon-12

Question 6

A mole

Answer 6

The amount of substance that contains as many particles as there are in exactly 12g of carbon-12

Question 7

What is the symbol for the Avagadro Constant?

Answer 7

L

Question 8

What are the units of the Avagadro constant?

Answer 8

Mol⁻¹

Question 9

What is the mass of one mole of a substance?

Answer 9

It’s Mr in grams.
E.g. 1 mole of carbon = 12.0g

Question 10

Conversion of mg to g

Answer 10

×10⁻³

Question 11

Conversion of kg to g

Answer 11

×10³

Question 12

Conversion of tonnes to g

Answer 12

×10⁶

Question 13

Conversion of cm³ to dm³

Answer 13

×10⁻³

Question 14

Equation for number of moles

Answer 14

Mass (in g) / Mr

Question 15

Equation for number of atoms

Answer 15

n (moles) x L (Avogadro’s constant)

Question 16

Equation for concentration

Answer 16

n (moles) / volume (dm³)

Question 17

Ideal gas equation

Answer 17

PV = nRT

Question 18

Units used for the ideal gas equation

Answer 18

P = Pa
V = m³
n = mol
R = JK⁻¹mol⁻¹
T = Kelvin

Question 19

Empirical formula

Answer 19

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

Question 20

Molecular formula

Answer 20

The actual number of atoms of each element present in a molecule

Question 21

Conversion from degrees C to Kelvin

Answer 21

+273

Question 22

What are the units of molar mass?

Answer 22

g mol⁻¹

Question 23

What 2 assumptions are made when using the ideal gas equation?

Answer 23

1. No intermolecular forces between molecules
2. Volume occupied by molecules in negligible relative to the volume of the container

Question 24

What are two acid-base indicators?

Answer 24

Methyl orange
Phenolphthalein

Question 25

Percentage error

Answer 25

(Uncertainty / value) ×100

Question 26

Methyl orange colour in acid

Answer 26

Red

Question 27

Methyl orange colour in base

Answer 27

Yellow

Question 28

Phenolphthalein colour in acid

Answer 28

Colourless

Question 29

Phenolphthalein colour in base

Answer 29

Pink

Question 30

Atom econemy

Answer 30

(Molecular mass (Mr) of desired product/sum of molecular masses of all products) ×100

Question 31

Reasons for low percentage yeild

Answer 31

• Incompleate/reversible reactions
• Side reactions
• Mechanical losses
• Impurities in the reactants

Question 32

What is a back titration?

Answer 32

A titration used for substances that are insoluble in water. They are instead reacted with acid to form a solution.

Question 33

Water of crystallisation

Answer 33

The water molecules that form an essential part of the crystalline structure of a hydrated salt