Question

$RH _{2}$ (ion exchange resin) can replace $Ca ^{2+}$ in hard water
$RH _{2}+ Ca ^{2+} \longrightarrow RCa +2 H ^{+}$
$1 \,L$ of hard water after passing through $RH _{2}$ has $pH \,2$. Hence, hardness in ppm of $Ca ^{2+}$ is:

• A. 200
• B. 100
• C. 50
• D. 125

Answer 1

Answer: (A)

$RH _{2}+ Ca ^{2+} \longrightarrow RCa +2 H ^{+}$

Energy $Ca ^{2+}$ ion is replaced by two $H ^{+}$ ions $pH =2$

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

$\therefore \left[ Ca ^{2+}\right]=\frac{10^{-2}}{2} M=\frac{10^{-2} \times 40}{2} \,g\, L ^{-1}$

$\therefore \,\, ppm$ of $Ca ^{2+}=\frac{10^{-2} \times 40 \times 10^{3}}{2 \times 10^{6}} Ca ^{2+}=200$