Question

The three imaginary elements X, Y and Z represent a Dobernener’s triad. If the atomic mass of element X is 14 and that of element Y is 46, then the atomic mass of element Z will be :

(a) 28 (b) 60 (c) 78 (d) 72

Answer 1

(c) 78
The atomic mass of the element Z is 78.

According to Dobereiner's law of triads, the atomic mass of $Y=\frac{(X+Z)}{2}$
Substituting the atomic masses we get:

$46=\frac{(14+Z)}{2}$
$92=14+Z$

$\therefore Z=78$.