Question

Hardness of water is 400ppm. The molarity of CaCO3 in this water is?

Answer 1

400 ppm mean 400gm in 10^6 ml solution

and molarity = (wt/Mw) x (1000 / V in ml)
so 
molarity = (400/100) x (1000 /10^6)
molarity = 4 x 10^-3 M

i hope it will help you
regards

Answer 2

Answer: Molarity of $CaCO_3$ in this water is 0.004M

Explanation: Hardness of water is measured in terms of $CaCO_3$ equivalents.

Given: Hardness of water = 400 ppm = 400mg/l

0.4 g of $CaCO_3$ is present in 1 liter of solution

$Moles=\frac{\text{Given mass}}{\text{Molar mass}}=\frac{0.4g}{100g/mol}=0.004moles$  

Molarity of a solution is defined as the number of moles of solute dissolved per Liter of the solution.

$Molarity=\frac{moles}{\text {Volume in L}}=\frac{0.004moles}{1L}=0.004M$