Question

Atomic mass of X and Z are 20 u and 40 u respectively. If X, Y and Z constitute Dobereiner’s triads, what should be the atomic mass of Y (in $u$)?

• A. 20
• B. 40
• C. 30
• D. 60

Answer 1

The correct option is C 30

According to Dobereiner's law of triads, the atomic mass of the middle element of the triad is equal to the arithmetic mean of the atomic masses of the other two elements.
Since, Y is the middle element of the triad, its mass should be the average of X and Z, that is,

$\text{Y}=\frac{\text{X+Z}}{2}=\frac{20+40}{2}=30u$