Question

Selenium, an element used in the manufacture of solar energy devices, forms an oxide that contains only one atom of selenium (SeOx) and is 37.8% oxygen by mass. What is the molecular formula of the oxide? (Hint: find “x”)

Answer 1

Answer: The molecular formula of the compound will be $SeO_3$

Explanation:

The number of moles is defined as the ratio of the mass of a substance to its molar mass. The equation used is:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$ ......(1)

Let the mass of the compound be 100 g

Given values:

% of O = 37.8%

% of Se = [100 - 37.8] = 62.2%

Mass of O = 37.8 g

Mass of Se = 62.2 g

We know:

Molar mass of Se = 79 g/mol

Molar mass of O = 16 g/mol

Putting values in equation 1, we get:

$\text{Moles of Se}=\frac{62.2g}{79g/mol}=0.787 mol$

$\text{Moles of O}=\frac{37.8g}{16g/mol}=2.36 mol$

Calculating the mole fraction of each element by dividing the calculated moles by  the least calculated number of moles that is 0.787 moles

$\text{Mole fraction of Se}=\frac{0.787 }{0.787 }=1$

$\text{Mole fraction of O}=\frac{2.36}{0.787 }=2.99\approx 3$

Taking the mole ratio as their subscripts.

The ratio of Se : O = 1 : 3

Hence, the molecular formula of the compound will be $SeO_3$