Question

$25$ mL samples of distilled water, tap water and boiled water required, respectively, $1$ mL, $13$ mL and $5$ mL of soap solution to form a permanent lather. The ratio of temporary to permanent hardness in the tap water is :

• A. $3:2$
• B. $2:3$
• C. $1:2$
• D. $2:1$

Answer 1

Answer: (D)

$2:1$

Volume of soap solution used effectively in tap water $=13-1=12$ mL
$\therefore$ Temporary + Permanent hardness $=12$ mL

Volume of soap solution used effectively in boiled water $=5-1=4$ mL
Permanent hardness $=4$ mL
$\therefore$ Temporary + Permanent hardness $=12$ mL

Permanent hardness $=4$ mL
Temporary hardness $=8$ mL

$\therefore \frac{Temporaryhardness}{Permanenthardness}=\frac{8}{4}=2:1$