Chemistry, Third edition

122 8 · THE MOLE

Example 8.1

How many moles of hydrated copper(II) sulfate (CuSO 4 · 5H 2 O)

are contained in 5.00 g of the substance?

Answer

m(CuSO 4 · 5H 2 O)(63.5 32 4(16)5(18))249.5 u

therefore,

M(CuSO 4 · 5H 2 O)249.5 g mol^1.

The amount of copper sulfate in moles 

mass

molar mass



5.00

0.0200 mol

249.5

Example 8.2

0.059 mol of calcium carbonate (CaCO 3 ) is required in an

experiment. What mass of CaCO 3 needs to be weighed out?

Answer

M(CaCO 3 )100 g mol^1

Therefore, mass of calcium carbonate

amount of calcium carbonate in moles molar mass

0.059 100 5.9 g

Example 8.3

0.20 mol of a compound containing carbon and hydrogen has a

mass of 3.2 g. What is the molecular mass of the compound?

What is its likely formula?

Answer

molar mass

mass

amount of substance in moles



3.2

0.20

16 g mol^1

SinceM(compound)16 g mol^1 ,m(compound)16 u.

Carbon has an atomic mass of 12. The molecule is likely to be methane (CH 4 ) with

m(CH 4 ) 12  1  1  1  1 16 u

Comment

Where there could be confusion, you should always be careful to specify exactly

the substance you are referring to. For example the statement ‘1 mol of hydrogen’

could mean 1 mol of H atoms or1 mol of H 2 molecules. These have different

molar masses of 1 g mol^1 and 2 g mol^1 , respectively.