Question

The Dobereiner's Triad given to you is Li, Na and K. If the atomic masses of Na and K in the Dobereiner's Triad is 23 and 39 respectively. Find the atomic mass of Lithium.

• A. 7.5
• B. 7
• C. 6.5
• D. 6

Answer 1

The correct option is B

7

We are given that the Li, Na and K are a Dobereiner's Triad.

Let the atomic mass of Lithium be $x$

We know that the average of the atomic masses of Li and K is equal to Na as they are in the same Dobereiner's Triad.

So we can calculate the atomic mass of Lithium like this

$\frac{x+39}{2}=23$

$x+39=23\times 2$

$x+39=46$

$x=7$

So the atomic mass of Lithium is 7