A compound was found to contain 33.0% Si and 67.0% F by mass if the molar mass of the compound is 340 g/mol calculate its molecular formula

Answer 1

First, you can use the percent composition to determine the mass of each element within the compound.

The compound is 33% Silicon so:

( x / 340 g) x 100 = 33%

( x / 340 g) = 33/100

(x / 340 g) = 0.33

x = 0.33 * 340 g

x = 112.2 g Si

It is also 67% Fluorine so:

( y / 340 g) x 100 = 67%

( y / 340 g) = 67/100

(y / 340g ) = 0.67

y = 0.67 * 340 g

y = 227.8 g F

Then you can use the calculated mass to determine the moles of each element within the compound using the molar mass of each element.

112.2 g Si ( 1 mol Si / 28.1 g ) = 3.99 = 4 mol Si

227.8 g F (1 mol F / 19 g ) = 11.99 = 12 mol F

molecular formula: Si4F12

Answer 2

Another way to do this is, especially if they ask for the empirical formula also, is to assume you have 100 g of the substance. Then the % is equal to grams.

33 g Si x 1 mol Si / 28.09 g = 1.175 moles Si

67 g F x 1 mol F / 18.99 g = 3.528 moles F

Divide both by 1.18 in an attempt to get whole numbers of moles and we have...

1.175/1.175 = 1.00 mol Si

3.528/1.175 = 3.00 mol F

Empirical formula = SiF₃ molar mass = 28 + 3*19 = 85

340 g/85 g = 4 empirical formulae in the molecular formula

Molecular formula = Si₄F₁₂