Relative atomic mass

2 marks · Hard

The element X has two stable isotopes: X-64 (mass = 64, abundance = 73%) and X-66 (mass = 66, abundance = 27%).

Calculate the relative atomic mass of X. Give your answer to one decimal place.

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

64.5 (±0.1)

Also accept: 64.5 u

Common mistakes: Transposed percentages; Arithmetic errors

Show mark scheme and worked solution

Marking points

M1 · 1 mark · Correct method: (64 × 73 + 66 × 27) / 100.

A1 · 1 mark · Correct answer: 64.5 (accept to 1 decimal place). (follow-through)

Worked solution

Relative atomic mass = (percentage of X-64 × mass of X-64) + (percentage of X-66 × mass of X-66) divided by 100:

= (4672 + 1782) / 100

= 6454 / 100

= 64.5