Question

$R{H}_2$ (ion exchange resin) can replace $C{a}^{2+}$ in hard water:

$R{H}_2+C{a}^{2+}\to RCa+2H^{+}$

$1L$ of hard water after passing through $R{H}_2$ has $pH=2$. Hence, hardness in ppm of $C{a}^{2+}$ is :

• A. $200$
• B. $100$
• C. $50$
• D. $125$

Answer 1

Answer: (A)

$200$

The pH of the solution is $2$.

Hence, $\left[H^{+}\right]={10}^{-2}M$

The concentration of calcium ion is $\left[C{a}^{+}\right]=\frac{{10}^{-2}}{2}M=5\times {10}^{-3}M$

In g/L, the concentration of calcium ions $=\frac{{10}^{-2}}{2}\times 40g/L$

In ppm, the concentration of calcium ions $=\frac{{10}^{-2}}{2}\times \frac{40\times {10}^6}{{10}^3}=200ppm$