Question

Tin (Sn) exists in earths crust as SnO2. Calculate the percent composition by mass of Sn and O in SnO2.

Please help me with this question I'm so confused

Answer 1

Sn atomic mass = 118.71
Oxygen atomic mass = 16.00
Number of tin atoms = 1
Number of oxygen atoms = 2
Total mass = (118.71*1)+(16.00*2)
= 150.71

(118.71/150.71)*100 = percentage of tin composition in SnO2
((16.00*2))/150.71)*100 = percentage of oxygen composition in SnO2

Answer 2

So do they have to be separate percentages or do you add them together

Answer 3

You don't get it??? The calculation gave you the % Sn and the % Oxygen. You don't do anything with them You might notice they add to 100% and they should.
Think about it. If you had 10 lbs oranges and 10 lbs apples in a basket what percentage apples and what percentage oranges do you have?
(10 lbs apples/total of 20 lbs) *100 = 50%
Likewise % Sn = (grams Sn/total grams)*100 = ?
I'll bet you could do the apples/oranges problem in your head. So don't let changing apples to tin and oranges to oxygen make any difference in your thinking skills.