Question

The atomic masses of X and Z are 20 u and 40 u, respectively. If X, Y and Z form a Dobereiner’s triad, what should be the atomic mass of Y?

• A. 20 u
• B. 40 u
• C. 30 u
• D. 50 u

Answer 1

The correct option is C 30 u

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,

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