Question

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

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

Answer 1

The correct option is A 30 u

According to Döbereiner's law of triads, the atomic mass of the middle element of a 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,

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