Relative atomic mass

1 mark · Easy

Naturally occurring boron is made up of two isotopes: 20% of its atoms have a mass number of 10, and the rest have a mass number of 11. What is the relative atomic mass of boron to the nearest whole number?

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11 (±0.5)

Also accept: 11

Common mistakes: 10.8 (to 1 decimal place, but the question requests whole number)

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Marking points

A1 · 1 mark · Correct relative atomic mass (11) to the nearest whole number.

Worked solution

Relative atomic mass = (20 × 10 + 80 × 11) / 100 = (200 + 880) / 100 = 1080 / 100 = 10.8.

To the nearest whole number, this is 11.