Molecular Weight

To calculate the molecular mass of Glucose we need to know the molecular formula for Glucose. This is:


First we must multiply the atomic mass of each element (Carbon, Hydrogen and Oxygen) by the number of atoms of that element (six, twelve and six).

Six atoms of Carbon                 ~      6 (No of atoms) times 12 (atomic mass of Carbon)    = 72

Twelve atoms of Hydrogen     ~    12 (No of atoms) times 1  (atomic mass of Hydrogen) = 12

Six atoms of Oxygen                 ~     6 (No of atoms) times 16 (atomic mass of Oxygen)    = 96

Now we add these three numbers together to get the molecular mass of Glucose.

72 + 12 + 96 = 180

So the molecular mass of Glucose is 180

If you have understood this, you should be able to calculate the molecular mass of sucrose:


You should be able to calculate any molecular mass from a molecular formula: you can use my periodic table to lookup atomic masses.

Now you could try calculating the molecular mass of an inorganic substance. The molcular formula for Aluminium Sulphate is:


This shows TWO atoms of Aluminium, THREE atoms of Sulphur and TWELVE atoms of Oxygen (the “3” multiplies everything inside the brackets).

Aluminium        ~     2 times    27 = 54

Sulphur              ~     3 times    32 = 96

Oxygen               ~    12 times   16 = 192

54 + 96 + 192 = 342

So the molecular mass of Aluminium Sulphate is 342

If you understand this, you can do the calculation for any inorganic substance.