Ch 2 · Relative Masses of Atoms and Molecules · O Level Chemistry 5070
Topic 2 of 3 in Stoichiometry — O Level Chemistry 5070 — 38 lessons.
Introduction
Atoms are far too small to weigh individually, so chemists compare their masses with a standard atom instead. That comparison gives the relative atomic mass printed under every symbol in the Periodic Table, and from it you can find the mass of any molecule or formula unit. Once you can do that, you can predict how many grams of product a reaction will make, which is the foundation of every quantitative question on the paper.
Key ideas — Relative atomic mass, Ar
The relative atomic mass, A_r, of an element is the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of carbon-12.
🧑🏫 Teacher's explanation
Imagine you are weighing a bag of mixed marbles. Some marbles are light (10g) and some are heavy (20g). If you have mostly light marbles, the 'average' weight of a marble in that bag won't be exactly 10g or 20g; it will be somewhere in between, like 12g. This is exactly how atoms work. Most elements are not made of just one type of atom; they are a mixture of different 'isotopes' (versions of the same atom with different masses).
📖 Definitions
🔍 Worked example — Easy
Calculate the relative molecular mass, M_r, of water, H_2O. (Given: A_r of H = 1, A_r of O = 16)
🔍 Worked example — Standard
Calculate the relative formula mass, M_r, of Calcium Chloride, CaCl_2. (Given: A_r of Ca = 40, A_r of Cl = 35.5)
🔍 Worked example — Exam-style
An isotope of Copper has a mass of 63 and another isotope has a mass of 65. If 70% of the atoms are isotope 63 and 30% are isotope 65, calculate the relative atomic mass, A_r.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Relative molecular mass and relative formula mass, Mr
The relative molecular mass, M_r, is the sum of the relative atomic masses of all the atoms shown in the molecular formula. For water:
🧑🏫 Teacher's explanation
Think of relative mass like a 'weighting system' rather than a measurement in grams. Because atoms are incredibly tiny, we cannot weigh a single atom on a kitchen scale. Instead, we compare everything to a standard reference point: one atom of Carbon-12. We say, 'If Carbon-12 weighs exactly 12 units, how much does this other molecule weigh in comparison?'
📖 Definitions
🔍 Worked example — Easy
Calculate the relative molecular mass, M_r, of methane, CH_4. (Given: A_r(C) = 12, A_r(H) = 1)
🔍 Worked example — Standard
Calculate the relative formula mass, M_r, of Calcium hydroxide, Ca(OH)_2. (Given: A_r(Ca) = 40, A_r(O) = 16, A_r(H) = 1)
🔍 Worked example — Exam-style
Calculate the percentage by mass of Nitrogen in Ammonium nitrate, NH_4NO_3. (Given: A_r(N) = 14, A_r(H) = 1, A_r(O) = 16)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Reacting masses in simple proportions
A balanced equation tells you the ratio of particles that react. Multiplying each formula by its M_r turns that into a ratio of masses, and because mass is conserved the masses of reactants and products always balance.
🧑🏫 Teacher's explanation
Think of a chemical equation like a recipe for making sandwiches. If a recipe says '2 slices of bread + 1 slice of cheese arrow 1 sandwich', the numbers (2 and 1) tell you the ratio of ingredients. However, slices of bread and slices of cheese don't weigh the same! To know how much the final sandwich weighs, you can't just add the number of slices; you have to multiply the number of slices by how much each slice weighs.
📖 Definitions
🔍 Worked example — Easy
Calculate the mass of magnesium oxide (MgO) produced when 48 g of magnesium (Mg) reacts completely with oxygen (O_2). Use A_r values: Mg = 24, O = 16.
🔍 Worked example — Standard
In the reaction CH_4 + 2O_2 arrow CO_2 + 2H_2O, what mass of CO_2 is produced from 16 g of CH_4? (A_r: C=12, H=1, O=16)
🔍 Worked example — Exam-style
A student reacts 5.0 g of calcium carbonate (CaCO_3) with excess hydrochloric acid. The reaction is CaCO_3 + 2HCl arrow CaCl_2 + H_2O + CO_2. Calculate the mass of CO_2 produced. (A_r: Ca=40, C=12, O=16)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. Limestone is heated to make quicklime, calcium oxide.
🔍 Worked example — Easy
Calculate the relative formula mass (M_r) of magnesium chloride, MgCl_2. (Given: A_r(Mg) = 24, A_r(Cl) = 35.5)
🔍 Worked example — Standard
In the reaction Mg + O_2 arrow MgO, calculate the mass of MgO produced from 12 g of Mg. (Given: A_r(Mg) = 24, A_r(O) = 16)
🔍 Worked example — Exam-style
Limestone is heated: CaCO_3 arrow CaO + CO_2. Calculate the mass of CO_2 released when 25 g of CaCO_3 is decomposed. (Given: A_r(Ca)=40, A_r(C)=12, A_r(O)=16)
⚠️ Common errors
Common mistakes
Practice
Answers The average mass of the isotopes of an element [B1] compared to 1/12th of the mass of an atom of carbon-12 [B1]. (a) 2 + 32 + 64 = 98 [B1]; (b) 56 + 3 × 17 = 107 [B1]. Sodium chloride is ionic, so it consists of a lattice of ions and has no molecules [B1]. M_r(Fe_2O_3) = 160 [M1]; 160 g of oxide gives 2 × 56 = 112 g of iron [M1]; 320 g is twice as much, so 224 g of iron [A1]. M_r(CO_2) = 44 [M1]; 32/44 × 100 = 72.7\% [A1].