Amount of substance

Question 1

Relative atomic mass (Ar)

Answer 1

The average mass of an atom of an element, relative to 1/12th the relative atomic mass of an atom of a carbon-12.

Question 2

Relative molecular mass (Mr)

Answer 2

Average mass of a molecule in relation to 1/12 the relative atomic mass of a carbon 12 atom

Question 3

Relative formula mass

Answer 3

Used for ionic compounds because they done exists as molecules

Question 4

Avogadro constant

Answer 4

Number of particles = number of moles x avogadros constant

Question 5

Empirical formula

Answer 5

• the simplest whole number ratio of atoms of each element in a compound

1) find masses/ % from experiment, this will be in the question

2) work out moles of each element by doing mass % / Mr

3) divide both moles by the smallest number of moles

4) convert to a whole number ratio

Question 6

Ionic equations

Answer 6

-ignore anything other than aqueous reactants + products. Leave s/l/g and ion out
- simplify equations by considering ions present. Some ions don’t take part in overall reaction - spectator ions

1) write out equation with state symbols
2) write out ions of aqueous reactants/products
3) cancel out ions that are the same on each side
4) left with ionic equation

Question 7

Limiting reagents

Answer 7

1) work out moles of both reactants doing mass/mr

2) look at the mole ratio

3) if 2:1, you need 2x the amount of moles of the second reactant.

4) if you have 2x or more, the first reactant is in excess, other is limiting

5) in calculations after finding limiting reagent, use the limiting one for calc

Question 8

Percentage yield

Answer 8

(Actual yield/theoretical yield) x100

• tells is how efficient the process was, how much wa slots by practical process of obtaining a product, or as a result of reactions that didn’t go to completion

Question 9

Why is the actual yield always less than maximum theoretical yield?

Answer 9

• incomplete reactions, in which some of reactants do not react to form the product
• practical losses during experiment, such as pouring or filtering
• side reactions
• gas products may be lost into the environment

Question 10

Atom economy

Answer 10

(Molecular mass of desired product/sum of molecular masses of all reactants) x100

• tells us in theory how many atoms are wasted in a reaction

00% atom economy when only one product

Question 11

Why is high atom economy good ?

Answer 11

• less waste product = more environmentally friendly
• more efficient use of raw materials = more sustainable = more profit

Question 12

% apparatus uncertainty

Answer 12

(Uncertainty of apparatus/reading) x100

Then multiply the answer by h