Assume the molar mass of X to be 100 g/mol and the mass of the sample is 100 g.

Hence the moles of X and Y are:

$\mathbf{mol}\mathbf{}\mathbf{X}\mathbf{=}\mathbf{40}\mathbf{}\mathbf{g}\mathbf{}\mathbf{X}\mathbf{\left(}\frac{\mathbf{1}\mathbf{}\mathbf{mol}\mathbf{}\mathbf{X}}{\mathbf{100}\mathbf{}\mathbf{g}\mathbf{}\mathbf{X}}\mathbf{\right)}\mathbf{=}$**0.40 mol X**

$\mathbf{mol}\mathbf{}\mathbf{Y}\mathbf{=}\mathbf{60}\mathbf{}\mathbf{g}\mathbf{}\mathbf{Y}\mathbf{\left(}\frac{\mathbf{1}\mathbf{}\mathbf{mol}\mathbf{}\mathbf{Y}}{\mathbf{50}\mathbf{}\mathbf{g}\mathbf{}\mathbf{Y}}\mathbf{\right)}\mathbf{=}$**1.20 mol Y**

The elements X and Y form a compound that is 40% X and 60% Y by mass. The atomic mass of X is twice that of Y.

What is the empirical formula of the compound?

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