In this problem I want to determine the emperical
formula for a compound, when what I'm given is from
combustion analysis, how much carbon dioxide
and how much water forms
And so the idea's that I'm going to react something that
CxHyOz, reacted with oxygen with some stoichiometric
coefficient that I don't know, and I'l make CO2
and water as the two products
3.095g CO2, 1.90g of water
and we started with 1.621g of the unknown compound
Essentially I want to determine x, y and z
Where these are integer values. The smallest integer
values doesn't give us the actual chemical formula
But the ermpircal formula has the right ratio of carbon to hydrogen to oxygen
What I need to do is determine first
this is the mass, I need to determine the number of moles
of water and of CO2 in order to
determine the number of moles of carbon and
hydrogen in my starting compound
What I'm gonna do is 3.095g of CO2
We use the molecular weight for CO2
So grams of CO2 per mole. That's gonna give me the
moles of CO2. So I'l look up the atomic weights
And I'm gonna substitute this value in here and I'm
That will allow me to determine the moles of CO2
and likewise, 1.902g of water, and I realize I just put
down the molecular weight for carbon
I need to put in here the molecular weight for CO2
So I'm gonna write CO2 molecular mass
It's gonna be one carbon plus two oxygens
So the molecular weight for CO2 and then we do the
same calculation for grams of water per mole
So I can determine the moles of water
So I can put the molecular mass of water
and then I can calculate the number of moles of each of these
So here's the numbers using a calculator. The moles
of CO2 and moles of water formed
And so this means that in my starting compound and
0.0703mol of carbon, and that means in the starting compound
I have 0.1056 times two moles of hydrogen
becuase of two moles of hydrogen atoms for every
mole of water. And so the number of moles of carbon
number of moles of hydrogen I can use to
then determine the mass. I want the mass so
I can determine the mass of oxygen
So 0.0703mol of carbon times 12.01g of carbon
per mole of carbon, and units cancel. This is the
grams of carbon in that unknown compound
And I'm going to do this same thing
Though I did this multiplication
Now I'm going to multiply this by 1.008g of hydrogen
per mole of hydrogen. This is going to be the grams of
hydrogen in my starting unknown compound
So I have the grams of carbon, the grams of hydrogen
And I know that the total grams of the starting compound
1.621g. I can then calculate the grams of oxygen
the grams of oxygen atoms would be our total
minus the number of grams of carbon, minus the
number of grams of hydrogen
So I calculate the grams of oxygen
Now I can take the grams of oxygen
I can convert this to moles of oxygen
15.99g of oxygen per for one mole of oxygen
So now I have the number of moles of oxygen
We make a list of the element, so carbon, hydrogen
oxygen, and the number of moles so carbon
0.0703, hydrogen two times 0.156 or 0.2112
moles of hydrogen and we just calculated
the number of moles of of oxygen
So if I went to get the minimum interger ratios
I would divide each of these numbers by
0.0352. This would be one, this is pretty close to two
and this number. And so then our empirical formula
would be C2H6O