The hardness of a water sample containing $10^{-3}$M $MgS{O}_4$ expressed as $CaC{O}_3$ equivalents (in ppm) is ________. 

[molar mass of $MgS{O}_4$ is $120.37$ g/mol]

$10^{-3}$ molar $MgS{O}_4=10^{-3}$ moles of $MgS{O}_4$ present in $1$ Litre.

$n_{CaC{O}_3}=n_{MgS{O}_4}$

$PPM$ (in term of $CaC{O}_3)=\frac{10^{-3}\times 100}{1000}\times 10^6$

                                               $=100ppm$