An oxide of a metal M (atomic mass $56$ ) contains $70%$ M by mass (O = 16). The formula of the oxide is:

Answer: D

Moles in 100 g: M

= 70/56 = 1.25

, O

= 30/16 = 1.875

. Ratio M : O

= 1 : 1.5 = 2 : 3

, so the oxide is

M X_{2} O X_{3}

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