The ionic compound sometimes called uranium yellow is used to produce colored glazes for ceramics. It is 7.252% sodium, 75.084% uranium, and 17.664% oxygen.

Question

What is the empirical formula for this compound?
This compound has a molecular mass of 1268.06 g/mol, what is the molecular formula for this compound?

Answer 1

Take 100 g sample which gives you
7.252 g Na
75.084 g U
17.664 g O
------------
Convert to mols by mols = g/atomic mass.
mols Na = 7.252/23 = approx 0.315
mols U = 75.084/238.02 = approx 0.315
mols O = 17.664/16 = approx 1.104

Now convert these numbers into ratio to each element with the smallest number being 1.00. The easy way to do that is to divide the smallest number by itself and divide all of the other numbers by the same small number. That gives us
0.315/0.315 = 1.00 Na
0.315/0.315 = 1.00 U
1.10/0.315 = 3.50
USUALLY we round these to whole numbers; however, 0.5 is too much to round off BUT we see if we just multiplied all of the numbers by 2 we would get whole numbers so we have
2Na atoms
2U atoms
7 O atoms
and the empirical formula is Na2U2O7 for an empirical formula mass of
(2*23)+(2*238)+(7*16) = approx 634.

The molar mass is 1268; therefore, we have (empirical formula)*?number = 1268. Solve for the ? number and round it to a whole number. That's obviously 2 so the molecular formula is (Na2U2O7)2 or Na4U4O14.