00:01
All right, so here we have a hydrocarbon that has burned.
00:04
It's made 7 .587 grams of co2 and 1 .553 grams of h2o.
00:14
Now, when we have this, we don't really know the balanced equation.
00:17
We don't know the formula of the hydrocarbon.
00:21
And we're trying to come up with the empirical formula.
00:24
So what we do know is that the carbohydrate is made only of carbons and hydrogens.
00:29
And so any carbon, when we have a combustion reaction, the hydrocarbon reacts with oxygen to make water vapor and co2.
00:45
And so we know all of the carbon that's produced comes from our hydrocarbon.
00:50
So all the carbon that came from the hydrocarbon must be in co2 now.
00:54
And all the hydrogen must be in h2o.
00:57
So what we can do is we can find the mass of carbon and hydrogen.
01:02
In overall, and then use that to find the molecular formula, molecular and empirical formula.
01:10
So to find how many grams of carbon are in this 2 .25 gram sample of the hydrocarbon, we'll start with how much co2 is made.
01:21
And so we want to go from grams of co2 into just grams of carbon in general.
01:26
So the way that we do that is we go grams to moles to moles to grams, just like we do with stoichiometry with balanced equations.
01:33
So first to go from grams to more, moles of co2, we'll need to use the molar mass.
01:37
So the molar mass of co2 is 44 .01 grams per one mole.
01:48
And then because we are not interested in co2, but we're interested in just carbon atoms, or just carbon in general, we'll go from moles of carbon co2 into just moles of carbon.
01:58
And so we can use the molecular formula here.
02:02
We see that there's one carbon, one mole of carbon per one mole of co2 molecule.
02:09
So now we're our moles of carbon, but we want grams of carbon.
02:14
So again, we can use the molar mass.
02:16
We know that one mole of carbon is 12 .01 grams.
02:20
So this tells us the mass of carbon in our hydrocarbon sample.
02:25
So 7 .587 divided by 44 .01 times 12 .01 is 2 .07 grams of carbon.
02:38
Now you can do the same thing for finding the grams of hydrogen, but instead of co2, we'll use the mass of h2o.
02:47
But because this hydrocarbon only has carbon and hydrogens, we can just use subtraction to get the amount of hydrogen that there is.
02:53
So the overall sample is 2 .25 grams, and the amount of carbon that we have is 2 .0, let's get a 4 -sig on there, is 2 .070 .7.
03:20
So this will give us the mass of just hydrogen...