Question

Salts A(atomic weight $7$), B(atomic weight $27$) and C(atomic weight $48$) were electrolysed under identical conditions using the same quantity of electricity. It was found that when $2.1$ g of A was deposited, the weights of B and C deposited were $2.7$ g and $7.2$ g. The valencies of A, B and C are respectively :

• A. $3,1and2$
• B. $1,3and2$
• C. $3,1and3$
• D. $2,3and2$

Answer 1

Answer: (B)

$1,3and2$

$A^{n_1^{+}}+n_1{e}^{-}\to A$

$B^{n_2^{+}}+n_2{e}^{-}\to B$

$C^{n_3^{+}}+n_3{e}^{-}\to C$

Since charge is same,

$\therefore$ Eq wt is same

$\therefore \frac{2.1}{\frac{7}{n_1}}=\frac{2.7}{\frac{27}{n_2}}=\frac{7.2}{\frac{48}{n_3}}$

$\therefore \frac{3\times n_1}{10}=\frac{1\times n_2}{10}=\frac{3\times n_3}{2\times 10}$

$\therefore n_1=\frac{n_2}{3}=\frac{n_3}{2}$

$\therefore n_1=1$
$n_2=3$
$n_3=2$

Option B is correct.