Question

The mineral cassiterite is a compound of tin and oxygen. Chemical analysis of cassiterite shows that the mass percentages of tin, Sn, and oxygen, O, are 78.8% and 21.2%, respectively. Determine the formula of this compound.

I found the molar mass and correct equation SnO4.
Sn-118.71
O-64

Do I do 78.8/118.71? =.66
21.2/64=.33

.33/.33=1 O
.66/.33=2 Sn

1 to 2 molar ratio.

So is it Sn20?
Did I do this correctly?

Answer 1

The first part of what you did is confusing. Here is what you do.
Take 100 g sample which gives you
78.8 g Sn
21.2 g O

Convert to mols
78.8/118.71 = about 0.66
21.2/16 = about 1.32

Now find the ratio of the two to each other with the smaller number being no less than 1.00. The easy way to do that is to divide the smaller number by itself which assures that one of being 1.00.
0.66/0.66 = 1.000 Sn
1.32/0.66 = 2.00 O
Empirical formula is SnO2