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Many chemical calculations partly involve calculating the formula or molecular mass of a compound. To calculate the formula mass of a compound you not only need to know its formula, but you must also be able to interpret the symbols, numbers, and brackets in the formula.

The formula mass is calculated by adding up all the atomic masses for every atom in the formula.

Note: The atomic mass of an atom is the atom's mass relative to 1/12th of the mass of a carbon-12 atom on which the standard atomic mass scale is based (e.g. H = 1, C = 12, P = 31 etc).

Task 1: Working out some formula masses

Model 1

  1. Open Yenka file Model 1.
  2. Look at the copper sulfate example that has been worked out for you. Open the Periodic Table and use it to get the atomic masses of the elements, then calculate the formula masses for the following compounds:
    (a) Lead(II) chloride, PbCl2
    (b) Aluminium oxide, Al2O3
    (c) Ammonium nitrate, NH4NO3
    (d) Sodium thiosulphate, Na2S2O3
    (a) 278; (b) 102; (c) 80; (d) 158

Task 2: Calculating formula mass when the formula needs brackets

  1. Calcium hydroxide has the formula Ca(OH)2. The subscript number after the brackets multiplies whatever is in the brackets (a bit like in maths).
    Atomic masses: Ca = 40; O = 16; H = 1
    So the formula mass = 40 + (16 + 1) x 2 = 74
    Now work out the formula mass of lead nitrate, Pb(NO3)2
    Start a new model in Yenka and verify your answer using a beaker of lead nitrate.
    Formula mass of lead nitrate is 331 (331g = 1 mole).
  2. Now work out the formula mass for:
    (a) Ammonium sulfate, (NH4)2SO4
    (b) Iron(II) nitrate, Fe(NO3)2
    (c) Aluminium sulfate, Al2(SO4)3
    (a) 132; (b) 180; (c) 342


The formula mass is calculated by adding up all the atomic masses for every atom in the formula. Calculating the formula mass is an essential part of many calculations and requires the correct reading and interpretation of a chemical formula.

You can test yourself further on calculating formula masses on Doc Brown's website.

Teacher Summary

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