Relative Masses and Moles Notes

AQA GCSE Chemistry: Revision notes

Key facts

  • Relative formula mass (Mr) = sum of the relative atomic masses (Ar) of all atoms in the formula.
  • One mole of a substance has a mass in grams equal to its Mr.
  • The Avogadro constant is 6.02×10236.02 \times 10^{23} particles per mole.
  • Moles = mass ÷ Mr, and mass = moles × Mr (Higher tier).
  • Use mole ratios from a balanced equation, with moles as the bridge between masses (Higher tier).
  • Molesmass (g)Mr\dfrac{\text{mass (g)}}{M_r}

Relative formula mass

The Mr of a compound is the sum of the Ar values of every atom in its formula.

Add the Ar of each atom, multiplying by the number of each atom shown. For HX2O\ce{H2O}: (1×2)+16=18(1 \times 2) + 16 = 18. The relative formula mass has no units.

  • MrM_rsum of all ArA_r in the formula

Worked example

Calculate the Mr of COX2\ce{CO2} (Ar: C = 12, O = 16).

What is the Mr of HX2O\ce{H2O} (Ar: H = 1, O = 16)?

The mole (HT)

A mole is a fixed number of particles, and its mass in grams equals the Mr.

Amounts of substance are measured in moles. One mole of any substance contains the Avogadro constant of particles, 6.02×10236.02 \times 10^{23}.

One mole of COX2\ce{CO2} (Mr = 44) has a mass of 44 g and contains 6.02×10236.02 \times 10^{23} molecules.

  • 1 mole6.02×10236.02 \times 10^{23} particles
  • Mass of 1 moleMr in grams

What is the mass of one mole of HX2O\ce{H2O} (Mr = 18)?

Mass and moles (HT)

Divide the mass by Mr to get moles; multiply moles by Mr to get mass.

Use the relationship between mass, moles and Mr:

moles=mass (g)Mr\text{moles} = \frac{\text{mass (g)}}{M_r}

Rearranged, mass=moles×Mr\text{mass} = \text{moles} \times M_r. You need the substance's Mr first.

  • MolesmassMr\dfrac{\text{mass}}{M_r}
  • Massmoles×Mr\text{moles} \times M_r

Worked example

How many moles are in 22 g of COX2\ce{CO2} (Mr = 44)?

What is the mass of 2 mol of COX2\ce{CO2} (Mr = 44)?

Masses in equations (HT)

The balancing numbers give a mole ratio, so convert mass to moles, use the ratio, then convert back.

A balanced symbol equation shows the mole ratio of reactants and products. Convert the known mass to moles, use the ratio to find moles of the other substance, then convert to mass.

In reverse, convert masses to moles, find the simple whole number ratio, and use it to balance the equation.

  1. 1

    Mass to moles

    moles = mass ÷ Mr for the substance you know

  2. 2

    Use the equation ratio

    balancing numbers give moles of the other substance

  3. 3

    Moles to mass

    mass = moles × Mr for the substance you want

Mass to mass using an equation

Why must you convert to moles before using the balancing numbers?

Try an exam question

Calculate the mass of 0.25 mol of calcium carbonate, CaCOX3\ce{CaCO3}. (Ar: Ca = 40, C = 12, O = 16)

[4 marks]

That's the notes covered.

Carry on to the next subtopic.