# Question #df090

Aug 23, 2017

The average mass of a gallium atom is 69.7 u.

#### Explanation:

The average mass is a weighted average of the isotopic masses:

You multiply each atomic mass by its relative importance (percentage).

$\text{Average mass" = "0.60 × 68.93 u + 0.40 × 70.92 u" = "41.4 u + 28.4 u" = "69.7 u}$

Note: The answer can have only two significant figures, because that is all you gave for the abundance of gallium-69.

However, I calculated the result to three significant figures for you.