$RH_2$(ion exchange resin)can replace $Ca^{2+}$ in hand water as,$RH_2+Ca^{2+}\rightarrow RCa+ 2H^+$.1 litre of hard water passing through $RH_2$ has PH is 2.Hence hardness in PPM of $Ca^{2+}$ is:

Question

$(a)\;200\qquad(b)\;100\qquad(c)\;50\qquad(d)\;125$