Calculating molecular masses
Previous Page: Relative Molecular Mass
We can use the relative atomic mass scale from the periodic table to calculate the mass of one mole of any molecular substance or ionic substance by addition of the relevant atomic masses taking into account the number of atoms of each element present in the formula. This is illustrated in the example below.
Example 1:
Calculate the molecular mass of Hydrogen Chloride HCl.
Solution:
The molecular mass of HCl = atomic mass of H + atomic mass of Cl
= 1.0 + 35.5
= 36.5g
Example 2:
Calculate the molecular mass of Copper (II) Nitrate Cu(NO3)2.
Solution:
The molecular mass of Cu(NO3)2 = 63.5 + [(14 + {3 x 16}) x 2]
= 63.5 + [62 x 2]
= 187.5g
Example 3: (an alternate notation)
Calcium phosphate has the formula Ca3(PO4)2. Calculate the mass of one mole of calcium phosphate.
3 mole of Ca
2 mole of P
8 mole of O.
| 1 mole of calcium is | 40.08g, so | 3 mole is | 120.24 g |
| 1 mole of phosphorus is | 30.9738g, so | 2 mole is | 61.9476g |
| 1 mole of oxygen is | 15.9994g, so | 8 mole is | 127.9952g |
| 1 mole of Ca3(PO4)2 is | 310.18 g |
You shouldn’t record past the least precise measurement; 2dp for calcium, so 2dp for the answer.
Example 4:
100g of Ca3(PO4)2 is how many mole?
Solution:
Now try a range of these problems for practice.