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Question

If Li, Na, and an unknown element X, form a Döbereiner's triad, then the atomic mass of X is ______ amu.
  1. 39

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Solution

The correct option is A 39
In a Döbereiner's triad, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements.

Calculation:

Atomic mass of Li = 7 amu
Atomic mass of Na = 23 amu
Let the atomic mass of the element X be Z.
Since Li, Na, and X form a Dobereiner's triad,

(7+Z)2 = 23
7+Z=46
Z=39

So, the atomic mass of X is 39 amu.

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