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The empirical formula of a metal oxide which has 54 % metal (M) and 46 % oxygen (O) is (Atomic mass of M = 27 amu and O= 16 amu)

  • A.\(\mathrm{M_3O_2}\)
  • B.\(\mathrm{MO_2}\)
  • C.\(\mathrm{M_2O_3}\)
  • D.\(\mathrm{M_2O_5}\)
  • E.\(\mathrm{M_2O}\)

Answer: C

The empirical formula is M2O3.

Divide each percentage by the atomic mass to get the mole ratio:

\[ \begin{aligned} \mathrm{M} &: \frac{54}{27} = 2 \\ \mathrm{O} &: \frac{46}{16} = 2.875 \end{aligned} \]

The ratio M : O is 2 : 2.875, or 1 : 1.44, which is closest to 1 : 1.5, that is 2 : 3. (A metal of atomic mass 27 is aluminium, and Al2O3 contains about 53 % metal, matching the data.)

M2O and MO2 give ratios of 1 : 0.5 and 1 : 2, and M3O2 and M2O5 are further from 1 : 1.44.

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