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Relative formula mass (Mr) is the sum of the relative atomic masses of all the atoms in a formula, so water's is 18. Free notes and 11 questions answered.
Calculating Relative Formula Mass is part of Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations in GCSE AQA Higher Chemistry.
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Relative formula mass (Mr) is the sum of the relative atomic masses of all the atoms in a formula, so water's is 18. Free notes and 11 questions answered.
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Use this lesson to stay inside Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations while connecting the detail back to the wider Topic 3: Quantitative chemistry topic.
Read the core explanations from Calculating Relative Formula Mass before testing yourself from memory.
When chemists mix substances, they need to know exactly how much of each chemical to use. You can't just guess - too little and the reaction won't work, too much and you waste expensive chemicals. This is where relative formula mass becomes your secret weapon.
Think of it like baking a cake. The recipe tells you the proportions - 200g flour, 100g sugar, 2 eggs. In chemistry, the 'recipe' is the balanced equation, and relative formula mass tells you the 'weight' of each ingredient.
Before we can calculate formula masses, we need the atomic masses. These are the numbers on your periodic table - hydrogen is 1, carbon is 12, oxygen is 16. These aren't random numbers; they tell us how heavy each type of atom is compared to carbon.
Here's the golden rule: Mr = sum of (number of atoms × relative atomic mass) for each element. Let's break this down with water, $H_2O$.
Worked example: Calculating Mr for Water (H₂O)
Calculate the relative formula mass of water, $H_2O$. (Ar: H = 1, O = 16)
Answer: Mr of H₂O = 18
Notice we don't write units for relative formula mass - it's a ratio, not an actual mass. This catches many students out in exams.
Some formulas look scary with brackets and big subscripts. The trick is to work systematically and not panic. Calcium hydroxide, $Ca(OH)_2$, is a perfect example.
Worked example: Calculating Mr for Calcium Hydroxide Ca(OH)₂
Calculate the relative formula mass of calcium hydroxide, $Ca(OH)_2$. (Ar: Ca = 40, O = 16, H = 1)
Answer: Mr of Ca(OH)₂ = 74
Here's where relative formula mass becomes really powerful. In any balanced chemical equation, the total mass of reactants equals the total mass of products. This isn't just theory - it's the law of conservation of mass.
Let's prove this with the formation of water: $2H_2 + O_2 \rightarrow 2H_2O$
Worked example: Proving Conservation of Mass
Show that mass is conserved in: $2H_2 + O_2 \rightarrow 2H_2O$ (Ar: H = 1, O = 16)
Answer: Mass is conserved: 36 units on both sides
This works for every balanced equation. If the masses don't match, either your calculation is wrong or the equation isn't balanced properly.
Relative formula mass questions are mark-friendly if you know the technique. Examiners award method marks even if your final answer is wrong, so showing your working is crucial.
Common Exam Mistakes: Writing 'Mr = 44g' instead of 'Mr = 44'. Forgetting to multiply atoms in brackets. Not showing working for method marks. Mixing up Mr with actual mass in grams.
Master these calculations and you'll find quantitative chemistry much easier. Every mole calculation, every yield calculation, every concentration problem starts with getting the relative formula mass right.
Exam precision note: calculate the Mr of each substance first, then apply any coefficient in the balanced equation. If the equation is balanced, the sum of coefficient x Mr for the reactants equals the sum of coefficient x Mr for the products.
Balanced equations and relative formula mass fit together neatly. Once you multiply each substance by its coefficient, the total relative mass on the reactant side must equal the total relative mass on the product side because the same numbers of each type of atom are present before and after the reaction.
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Relative formula mass (Mr) is calculated by adding up the relative atomic masses of all the atoms in a formula: multiply each element's relative atomic mass by its number of atoms, then add them together. For example, Mr of H₂O = (2 × 1) + 16 = 18.
The relative atomic mass of carbon is 12. Relative atomic masses compare the average mass of an atom with 1/12 of the mass of a carbon-12 atom, and they are found on the periodic table: hydrogen is 1, carbon is 12 and oxygen is 16.
The relative formula mass (Mr) of water, H₂O, is 18. Water has 2 hydrogen atoms and 1 oxygen atom, so with H = 1 and O = 16 the calculation is (2 × 1) + (1 × 16) = 2 + 16 = 18.
Mr stands for relative formula mass: the sum of the relative atomic masses (Ar) of all the atoms in a chemical formula. It is found by multiplying each element's atomic mass by how many atoms of that element are present, then adding them all up.
The relative formula mass of calcium hydroxide, Ca(OH)₂, is 74. The bracket means 2 lots of OH, so there is 1 Ca, 2 O and 2 H: (1 × 40) + (2 × 16) + (2 × 1) = 40 + 32 + 2 = 74.
To calculate Mr for a formula with brackets, deal with the brackets first: (OH)₂ means 2 lots of OH. For calcium hydroxide, Ca(OH)₂, there are 1 Ca, 2 O and 2 H atoms, so Mr = 40 + (2 × 16) + (2 × 1) = 74.
Relative formula mass has no units because it is a ratio, not an actual mass in grams. Writing 'Mr = 44g' instead of 'Mr = 44' is a common mistake, as is mixing up Mr with actual mass in grams.
To show mass is conserved, multiply each substance's Mr by its coefficient in the balanced equation and compare the totals. For 2H₂ + O₂ → 2H₂O, the reactants give 2 × 2 + 32 = 36 and the products give 2 × 18 = 36, so mass is conserved.
The relative formula mass of oxygen gas, O₂, is 32, because it has two oxygen atoms with a relative atomic mass of 16: 2 × 16 = 32. In the same way, hydrogen gas, H₂, has an Mr of 2 × 1 = 2.
When using Mr in a balanced equation, calculate the Mr of each substance first, then multiply it by the number in front of its formula. In 2H₂ + O₂ → 2H₂O, the reactants total (2 × 2) + 32 = 36 and the products 2 × 18 = 36.
If the total relative masses of reactants and products don't match, either the calculation is wrong or the equation isn't balanced properly. In every balanced equation the sum of coefficient × Mr for the reactants equals the sum for the products, because the same atoms are present before and after.