Answer
a) $126.1\ g/mol$
b) $246.4\ g/mol$
Work Step by Step
Atomic weights: $Mg: 24.305u,\ O: 15.999u,\ S: 32.06u,\ C: 12.011u,\ H:1.008u$
$H_2O$: 2 H + 1 O: $2\times1.008+15.999=18.01\ g/mol$
a) $H_2C_2O_4\dot{}2\ H_2O$: 2 H+ 2 C + 4 O + 6 $H_2O$:
$2\times1.008+2\times12.01+4\times15.999+2\times18.01=126.1\ g/mol$
b) $MgSO_4\dot{}7\ H_2O$: 1 Mg+ 1 S + 4 O + 7 $H_2O$:
$24.305+32.06+4\times15.999+7\times18.01=246.4\ g/mol$
