To find the relative formula mass (Mr) of a compound when given the number of moles and the mass, you can use the formula:
\[ \text{Relative formula mass (Mr)} = \frac{\text{mass (g)}}{\text{number of moles}} \]
In this case, you're given:
- Mass = 84 g
- Number of moles = 1.4 moles
Now, plug the values into the formula:
\[ \text{Mr} = \frac{84 , \text{g}}{1.4 , \text{moles}} = 60 , \text{g/mol} \]
Thus, the relative formula mass (Mr) of the compound is 60 g/mol.