Molecular Weight

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

C6H12O6

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:

C12H22O11

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:

Al2(SO4)3

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.

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