GCSE · Chemistry · AQA · Spec 8462

Relative formula mass (Mr)

You can find any formula's relative mass by adding up its atoms. Do it, and the element with the most atoms can carry surprisingly little of the mass.

Guess first

Four of methane's five atoms are hydrogen. So is most of its mass hydrogen?

What percentage of each compound's MASS comes from the element named? Drag each marker to your best guess. No calculating yet: go with your gut.

Build each Mr, atom by atom

Here is methane burning in oxygen. Tap each substance. Think of its formula as a shopping list: each atom's relative atomic mass is the price, the small number is how many you bought, and the Mr is the bill.

Tap a molecule to see what it does in the reaction.

Predict, then check

Now use the big numbers. They tell you how many of each formula to count.

CH₄ + 2O₂ → CO₂ + 2H₂O. Counting everything the equation shows, the reactants add up to 16 + (2 × 32) = 80. What do the products add up to?

Back to the guessing game: work one out properly

Problem

What percentage of the mass of carbon dioxide, CO₂, is carbon? (Relative atomic masses: C = 12, O = 16)

Your turn

Magnesium in magnesium chloride

What percentage of the mass of magnesium chloride, MgCl₂, is magnesium? (Relative atomic masses: Mg = 24, Cl = 35.5)

  1. Magnesium's mass in the formula: one Mg atom, so 1 × 24 = 24.
  2. missing step
Which line is step 2?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Add up every atom a formula shows, then find out which element really carries the mass.

What you need to know

  • Relative formula mass (Mr): the relative atomic masses of all the atoms in a formula, added together.
  • Count each atom as many times as the formula shows it: in CO₂, oxygen counts twice, so Mr = 12 + (2 × 16) = 44.
  • Percentage by mass of an element = (element's mass in the formula ÷ Mr) × 100.
  • In a balanced equation, the total Mr of the reactants in the quantities shown equals the total Mr of the products in the quantities shown.

The big picture

Relative formula mass, Mr, is found by adding the relative atomic masses of all the atoms in a formula, counting each atom as many times as the formula shows it. To find an element's percentage by mass, divide its mass in the formula by the Mr and multiply by 100. In a balanced equation, the total Mr of the reactants in the quantities shown equals the total Mr of the products.

Key points

1Mr has no units: it is a relative mass, so it is just a number.
2A small number after a symbol multiplies that atom only; a big number in front of a formula counts whole formulas.
3An element's share of the atoms is not its share of the mass: hydrogen is four of methane's five atoms but only 25% of its mass.
4The element's mass in the formula is its relative atomic mass × the number of its atoms shown.
5Divide the part by the whole: element's mass ÷ Mr. A percentage by mass can never be more than 100%.
6Totals match across a balanced equation because the same atoms are on both sides, just rearranged.

Worked example

Problem

Aluminium oxide has the formula Al₂O₃. Calculate the percentage by mass of aluminium in aluminium oxide. (Relative atomic masses: Al = 27, O = 16)

⚠ Watch out

Counting each element once. The small 2 in MgCl₂ means two chlorine atoms, so Mr = 24 + (2 × 35.5) = 95, not 59.5. Its cousin: judging mass share by atom count. Hydrogen is most of methane's atoms but only a quarter of its mass.

🧠

Memory hook

Read a formula like a shopping list: price of each atom × how many you bought, then total the bill. The bill is the Mr. One item's share of the bill is its percentage by mass.

✓

Check yourself

Water, H₂O, is two-thirds hydrogen atoms. Is hydrogen more or less than a third of its mass? Work it out (H = 1, O = 16). Answer: 2 ÷ 18 × 100 = 11.1%.

Flashcards

(12)
What is relative formula mass (Mr)?
The sum of the relative atomic masses of all the atoms in a formula, each counted as many times as the formula shows it.
In CO₂, what does the small 2 tell you?
There are two oxygen atoms in the formula, so oxygen contributes 2 × 16 = 32 to the Mr.
In 2H₂O, what does the big 2 in front tell you?
There are two whole water molecules. The Mr of H₂O is still 18; the big 2 matters when you total an equation (2 × 18 = 36).
How do you find an element's mass in a formula?
Multiply its relative atomic mass by the number of its atoms shown in the formula. Four H in CH₄ give 4 × 1 = 4.
Percentage by mass of an element: which two values do you need, and what do you do with them?
The element's mass in the formula and the compound's Mr. Then (element's mass ÷ Mr) × 100.
Hydrogen is four of methane's five atoms. What percentage of methane's mass is hydrogen?
25%: 4 ÷ 16 × 100. Each hydrogen atom (relative atomic mass 1) is far lighter than the carbon atom (12).
What percentage of carbon dioxide's mass is carbon?
27.3%: 12 ÷ 44 × 100 = 27.27…
Mr of MgCl₂? (Mg = 24, Cl = 35.5)
24 + (2 × 35.5) = 95. Getting 59.5 means you counted chlorine only once.
In a balanced equation, how does the total Mr of the reactants compare with the total Mr of the products?
They are equal, as long as each substance is counted in the quantity the equation shows.
Why do the Mr totals match across a balanced equation?
The same atoms are on both sides. A reaction rearranges atoms into new substances; it does not create or destroy them.
Does Mr have a unit?
No. It is a relative mass, so it is just a number.
Your percentage by mass comes out above 100%. What has gone wrong?
You divided upside down. Put the element's mass on top and the Mr underneath.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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