C4: Chemical Calculations

Question 1

How to work out the relative formula mass?

Answer 1

The relative atomic masses of all the atoms in the molecular formula added together.

Question 2

How to calculate the % mass of an element in a compound?

Answer 2

Ar x number of atoms of that element / Mr of the compound x 100

Question 3

How to work out the number of moles?

Answer 3

Mass in g / Mr

Question 4

How to work out reacting masses?

Answer 4

1. Write tick and ? on balanced equation
2. Find moles (= 𝒎𝒂𝒔𝒔/𝑴𝒓)
3. Find moles ? (use balanced equation)
4. Find mass ? (= 𝒎𝒐𝒍𝒆𝒔 x 𝑴𝒓)

Question 5

How to calculate reacting ratios?

Answer 5

Find the Mr of what is given.
Put it in a ratio.
Divide by the smallest mole.
Write it in an equation.

Question 6

How to work out the limiting reagent?

Answer 6

Convert reactants into moles
Divide by coefficient
Reactant with smaller number is limiting

Question 7

How to calculate percentage yield?

Answer 7

Actual mass of product produced / maximum theoretical mass of product possible x 100

Question 8

How to calculate atom economy?

Answer 8

Relative formula mass of desired products / relative formula mass of all reactants x 100

Question 9

What are 5 things that you must consider when choosing which reaction is best?

Answer 9

Percentage yield
Reaction rate
Reversibility (equilibrium position)
Energy cost
Cost of reactants

Question 10

How to calculate concentration?

Answer 10

Amount of solute (g) / volume of solution (dm3)

Amount of solute (g) / volume of solution (cm3) x 1000

Question 11

How to calculate the number of moles in titration?

Answer 11

Concentration x volume / 1000

Question 12

How to get g/dm3 from mol/dm3?

Answer 12

Concentration x Mr