Question

The manufacture of aluminum involves the production of cryolite . Cryolite has the percent composition:
%Na = 32.851 %, %Al = 12.851 %, and % F = 54.298 .
(15pts) What is the empirical formula of this compound?
(5 pts)   What is the empirical formula mass of this compound?

Answer 1

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

Na ⇒ 32.581 g
Al ⇒ 12.851 g
F ⇒ 54.298 g

2) Convert the masses to moles:

Na ⇒ 32.581 g / 23 = 1.42
Al ⇒ 12.851 g / 27 = 0.48
F ⇒ 54.298 g / 19 = 2.86

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

Na ⇒ 1.42 g / 0.48 = 2.96 ~ 3
Al ⇒ 0.48 g / 0.48 = 1
F ⇒ 2.86g / 0.48 = 5.96 ~ 6

4) Write the empirical formula:

Na₃AlF₆

5) Determine the molecular formula:

"EFW" ⇒ (3*23) + 27 + (19*6) = 210

empirical formula of this compound = Na₃AlF₆

empirical formula mass of this compound = 210 g/mol