Answer
$i=2.484$
Work Step by Step
First of all, we find the number of moles
$n_{MgCl_2}=(4.5701g\space MgCl_2)(\frac{1mol\space MgCl_2}{95.211g\space MgCl_2})=0.048000mol$
and $n_{H_2O}=(43.238g\space H_2O)(\frac{1molH_2O}{18.0152g\space H_2O})=2.4001mol\space H_2O$
Now $i=\frac{\frac{P^{\circ}_{H_2O}n_{H_2O}}{P_{soln}}-n_{H_2O}}{n_{MgCl_2}}$
We plug in the known values to obtain:
$i=\frac{\frac{(0.3804atm)(2.4001mol)}{0.3624atm}-2.4001mol}{0.048000mol}=2.484$
