Relative formula mass and the moleIB MYP Chemistry: Revision notes
Section 1
Relative atomic mass and relative formula mass
The relative atomic mass (Ar) of an element compares the mass of its atoms with carbon-12. You find it on the periodic table (for example Ar of C = 12, O = 16, Cl = 35.5).
The relative formula mass (Mr) of a compound is the sum of the Ar values of all the atoms in its formula. It has no units.
Worked example: calcium hydroxide, Ca(OH)₂. Ca = 40, O = 16, H = 1.
Mr = 40 + 2 × (16 + 1) = 40 + 34 = 74
The 2 outside the bracket multiplies everything inside it.
Forgetting that a number outside a bracket multiplies every atom inside it, for example Ca(OH)₂ has two O and two H.
Section 2
The mole
Atoms are far too small to count one at a time, so chemists count them in moles. A mole is the amount of a substance that contains 6.02 × 10²³ particles (atoms, molecules or ions). This number is the Avogadro constant.
The mass of one mole of a substance, in grams, equals its relative formula mass. For water, Mr = 18, so one mole of water has a mass of 18 g and contains 6.02 × 10²³ molecules.
To find the number of particles: particles = moles × 6.02 × 10²³.
Example: 0.25 mol of water contains 0.25 × 6.02 × 10²³ = 1.5 × 10²³ molecules.
Section 3
Moles and mass
The key equation is:
moles = mass (g) ÷ Mr
Rearranged: mass = moles × Mr and Mr = mass ÷ moles.
Example 1. How many moles are in 11 g of carbon dioxide, CO₂? Mr = 12 + 2 × 16 = 44. Moles = 11 ÷ 44 = 0.25 mol.
Example 2. What is the mass of 0.50 mol of sodium hydroxide, NaOH? Mr = 23 + 16 + 1 = 40. Mass = 0.50 × 40 = 20 g.
Write the equation, substitute with units, then calculate. Mass is in grams, and Mr has no units.
Dividing the wrong way round. Moles must come out smaller than the mass for any substance with Mr greater than 1.
Section 4
Atoms or molecules?
Always check what you are counting. One mole of oxygen atoms (O) has a mass of 16 g, but one mole of oxygen molecules (O₂) has a mass of 32 g, because each molecule has two atoms.
In the same way, 1 mol of H₂O molecules contains 2 mol of H atoms and 1 mol of O atoms.
State the particle you mean ('mol of O₂ molecules') and use the Mr of that particle.
Must Know
- Mr = sum of the Ar values in the formula (no units)
- One mole contains 6.02 × 10²³ particles (the Avogadro constant)
- moles = mass ÷ Mr; mass = moles × Mr
- particles = moles × 6.02 × 10²³
- Check whether you are counting atoms or molecules
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Relative formula mass and the mole
- A cement works in Turkey heats limestone, which is mostly calcium carbonate, CaCO₃. A chemist at the works calculates the amounts of substances being used. Relative atomic masses: Ca = 40, C = 12, O = 16.Calculate the mass of 3.0 mol of calcium carbonate.2 marks
- A sports drink company in Australia lists glucose, C₆H₁₂O₆, as an ingredient and also uses large amounts of water, H₂O. Chemists count particles in moles. The Avogadro constant is 6.02 × 10²³ per mole. Relative atomic masses: H = 1, C = 12, O = 16.Calculate the relative formula mass (Mr) of glucose.2 marks
- A teacher gives students samples of four substances and asks them to weigh out one mole of each. The students record the mass of one mole: water, H₂O, 18 g; sodium chloride, NaCl, 58.5 g; magnesium oxide, MgO, 40 g; calcium carbonate, CaCO₃, 100 g. Relative atomic masses: H = 1, C = 12, O = 16, Na = 23, Mg = 24, Cl = 35.5, Ca = 40.Describe the pattern in the data, verify it using one of the substances, and state a general rule.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).