Question

The elemental analysis of a compound is determined in your lab. It is: 32.79% sodium, 13.202 % aluminum, and 54.19 % fluorine. If the molar mass of this compound is 420 g/mole what is it molecular formula?

Answer 1

1) Assume 100 g of the compound is present. This changes the percents to grams:

Na ⇒ 32.79 g
Al ⇒ 13.202 g

F ⇒ 54.19 g

2) Convert the masses to moles:

Na ⇒ 32.79 g / 22.98 g/mol = 1.426 mol
Al ⇒ 13.202 g / 26.98 g/mol = 0.489 mol

F ⇒ 54.19 g / 18.99 g/mol = 2.853 mol

3) Divide by the lowest, seeking the smallest whole-number ratio:

Na ⇒ 1.426 /0.489mol
Al ⇒ 0.489/0.489 mol

F ⇒ 2.853/0.489 mol

4) Write the empirical formula:

Na₃AlF₆

_{therefor the molecular formula is}

Na₆Al₂F₁₂ = molecular weight is equal to 420