Question

Measurements show that unknown compound X has the following composition: element mass % chromium 68.4% oxygen 31.5% Write the empirical chemical formula of X.

Answer 1

Answer: The empirical formula of X  is $Cr_2O_3$

Explanation:

If percentage are given then we are taking total mass is 100 grams.

So, the mass of each element is equal to the percentage given.

Mass of Cr = 68.4 g

Mass of O = 31.5 g

Step 1 : convert given masses into moles.

Moles of Cr =$\frac{\text{ given mass of Cr}}{\text{ molar mass of Cr}}= \frac{68.4g}{52g/mole}=1.31moles$

Moles of O =$\frac{\text{ given mass of O}}{\text{ molar mass of O}}= \frac{31.5g}{16g/mole}=1.97moles$

Step 2 : For the mole ratio, divide each value of moles by the smallest number of moles calculated.

For Cr = $\frac{1.31}{1.31}=1$

For O = $\frac{1.97}{1.31}=1.5$

The simple whole number ratio of Cr : O= 2: 3

Hence the empirical formula is $Cr_2O_3$