How do you find molar mass?

1 Answer
Oct 27, 2015


Use the mass numbers from the periodic table.


The molar mass is the mass of one mole.

Use the mass numbers from the periodic table. Mass numbers are usually the number below the chemical symbol; the numbers that are not integers (whole numbers). The other numbers are the atomic numbers (aka proton numbers).

The molar mass of an element (i.e. single atoms of the same type) is simply the mass number in grams. To calculate the molar mass of molecules you need to multiply the mass number by the number of that type of atom in the molecule, i.e. there are two hydrogen atoms, one sulphur atom and four oxygen atoms in sulphuric acid (#H_2SO_4#).

Some examples (2 s.f.):
1. Helium (He) has a mass number of 4.0. So its molar mass is 4.0 g.
2. Sulphur's molar mass is 32 g.
3. The molar mass of oxygen gas (O₂ ) is 2 × 16 = 32 g.
4. Sodium hydrogen sulphate (#NaHSO_4#) has a molar mass of… #m= 23+1+32 + (4 × 16)=120g#.