. Analysis of a compound finds it contains 25.0% of species X, by mass, and 75.0% of species Y by mass. The molar mass of species X is 25.0 g/mol, and the molar mass of species Y is 150. g/mol. If the molar mass of the compound is 600. g/mol, what is the molecular formula of the compound

Molecular formula is: X₆Y₃

Explanation:

First of all we think in centesimal composition:

25 % of X and 75 % of Y

This means, that in 100 g of compound we have 25 g of X and 75 g of Y.

Therefore, our compound is: XY

To determine the moles, we divide the % by the molar mass of each

25 g / 25 g/mol = 1 X

75 g / 150 g/mol = 0.5 Y

As we can not have decimal numbers we mutiply x2.

X₂Y

This is the empirical formula, which is not the same as molecular one. We calculated from 100 g of compound so now we finish the excersise by rules of three:

In 100 g of compound we have 1 moles of X

In 600 g we must have (600 . 1)/100 = 6 X

In 100 g of compound we have 0.5 mol of Y

In 600 g we must have (600 . 0.5)/100 = 3 Y

Molecular formula is: X₆Y₃

We can confirm it by the molar mass:

25 g/mol . 6 + 150 g/mol . 3 = 600 g/mol

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