Relative atomic mass

3 marks · Easy

Below is information about two isotopes of lithium: Isotope 1: ^{6}Li, abundance 8%. Isotope 2: ^{7}Li, abundance 92%.

(a)

Write the equation for calculating the relative atomic mass of lithium using these data.

[1 mark]

(b)

Calculate the relative atomic mass of lithium to one decimal place.

[2 marks]

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.

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(a) (8 × 6 + 92 × 7)/100

(b) 6.9

Common mistakes: 6.9 (wrong calculation by ignoring percentage)

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Marking points

B1 (a) · 1 mark · Correct equation for the calculation (substitution into relative atomic mass formula).

M1 (b) · 1 mark · Correct substitution and working using abundances and mass numbers.

A1 (b) · 1 mark · Final answer correct to one decimal place: 6.9. (follow-through)

Worked solution

a) (8 × 6 + 92 × 7)/100

b) = (48 + 644) / 100 = 692 / 100 = 6.92; to one decimal place: 6.9