Question

An oxide of a metal ($M$) contains $40\% $ by mass of oxygen. Metal ($M$) has atomic mass of $24$.The empirical formula of the oxide is:
A.${M_2}O$
B.$MO$
C. ${M_2}{O_3}$
D.${M_3}{O_4}$

Answer 1

Hint: The empirical formula is the simplest formula for a compound, described as the ratio of subscripts of the smallest whole number of elements in the formula. It's also known as the most fundamental formula. An empirical formula is a substance's formula given with the smallest integer subscript. The empirical formula gives the number of atoms in a given ratio in the compound. The $\% $ composition of a chemical is directly proportional to its empirical formula.
Formula used:
Number of moles = (Given mass/ molar mass)

Complete answer:
An oxide of a metal ($M$) contains $40\% $ by mass of oxygen.
$100g$ of the oxide contains $40g$ of oxygen.
Therefore, mass of metal will be $100g - 40g = 60g$
Metal ($M$) has atomic mass $24$.
Using the above given formula to calculate the moles, we get
$40g$ of metal corresponds to $\dfrac{{60}}{{24}} = 2.5mol$
Oxygen has an atomic mass of $16$ .
$60g$ of oxygen corresponds to $\dfrac{{40}}{{16}} = 2.5mol$
Ratio of $M$ and $O = 2.5:2.5$
Hence, ratio of moles of $M$ and $O$ $ = 1:1$
Therefore, the empirical formula of the compound is $MO$ .
Hence, the correct option is B.$MO$

Note:
Analytical, molecular, and structural chemical equations are the three main types. Molecular formulas provide the number of each atom type in a molecule, and structural formulas show how the atoms in a molecule are bound together. Empirical formulas show the simplest total-number ratio of atoms in a compound.