The atomic mass of lithium(Li) and sodium(Na) are 7 u and 23 u respectively. Lithium, sodium, and potassium form Dobereiner's triad. What is the atomic mass of potassium(K)?

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Solution

The correct option is C 39
In Dobereiner’s triad the arithmetic mean of the atomic masses of the first and third element will be approximately equal to the atomic mass of the second element in that triad.
Therefore, Atomic mass of sodium $= \frac{(A t o m i c m a s s o f l i t h i u m + A t o m i c m a s s o f p o t a s s i u m)}{2}$
i.e., $23 = \frac{(7 + A t o m i c m a s s o f p o t a s s i u m)}{2}$
Atomic mass of potassium $= (23 \times 2) - 7 = 39$

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