Part 05 · Moles and calculations · Card 2 of 9
Relative formula mass
The sum of the relative atomic masses of all the atoms shown in a formula. For a molecule it is also called relative molecular mass (Mr).
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Analogy
Like adding up the weights of every brick in a model.
Example
Mr of CO2
C = 12O = 16, two of them = 32Mr = 12 + 32 = 44
In practice
You need it to work out how many moles of a compound you have.
Practice
Flashcards
- What is the formula linking moles, mass and Mr?Moles = mass / Mr.
- What is the Mr of water, H2O?18 (2 x 1 + 16).
Interview questions
- Calculate the mass of 0.5 mol of calcium carbonate, CaCO3. (Ar: Ca = 40, C = 12, O = 16)
- Mr = 40 + 12 + 3 x 16 = 100
- Mass = moles x Mr
- 0.5 x 100 = 50 g
Questions people ask
How do you calculate Mr for a compound with brackets?
Work out the mass of the group in the brackets, then multiply by the number outside. For calcium hydroxide, Ca(OH)2, the OH group is 16 + 1 = 17. Two of them make 34. Add calcium at 40 to get an Mr of 74.
Does relative formula mass have units?
No. It is a ratio compared with carbon-12, so the units cancel out. When you use it to find the mass of one mole, you add grams: Mr of 18 means 18 g per mole. In a calculation the number is used, with units added only at the end.