equations and calc qs

Gallium can exist as two stable isotopes: Ga-69 and Ga-71.

60.1% of gallium atoms are Ga-69 atoms, and the rest are Ga-71 atoms.

Calculate the relative atomic mass of gallium. (3)

% isotopic abundance of Ga-71 = 100 - 60.1 = 39.9

relative atomic mass = (69 × 60.1) + (71 × 399) ÷ 100

= (4146.9 + 2832.9) ÷ 100 = 69.8

• 3 marks for correct answer

• otherwise 1 mark for working out % abundance of Ga-71

• 1 mark for corredly substituting in values for abundances and atomic masses)

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