AP Chemistry
posted by CJ on .
Sulfuric acid (H2SO4) is prepared commercially from elemental sulfur using the contact process. In a typical sequence of reactions, the sulfur is ﬁrst burned:
S + O2 → SO2 ,
then it is converted to SO3 using a catalyst:
2 SO2 + O2 → 2 SO3 .
The resulting SO3 is reacted with water to produce the desired product:
SO3 + H2O → H2SO4 .
How much sulfuric acid could be prepared
from 34 moles of sulfur?
Answer in units of g

This is a regular stoichiometry problem. You can work it as three problems OR you can collapse the three into one problem with a shortcut.
Method 1: Three problems.
S + O2 ==> SO2. 34 mol S; calculate mols SO2 formed. Then
2SO2 + O2 ==> 2SO3. Use mols SO2 from the first problem and calculate mols SO3 formed.
SO3 + H2O ==> H2SO4. Use mols SO3 from second problem and calculate mols H2SO4 formed.
Method 2:
Write all of the equations.
S + O2 > SO2
2SO2 + O2 ==> 2SO3
SO3 + H2SO4 = H2SO4.
Now watch this. Using the coefficients in the balanced equation we have
1 mol S = 1 mol SO2
2 mol SO2 = 2 mol SO3 or 1>1
1 mol SO3 = 1 mol H2SO4
Therefore we can see that 1 mol S forms 1 mol H2SO4 and it seems obvious then that 34 mols S will produce 34 mols H2SO4. 
So that would also equal 34 grams right?

No. The problem asked for mols and answer is 34 mols S. A mol of S has a mass of 32.066 grams; therefore, 34 mols will be 34 x 32.066 = a bunch of grams.