## F.23 Question [ENDORSED]

Posts: 62
Joined: Thu Jul 27, 2017 3:01 am

### F.23 Question

The CO2 produced by the combustion of hydrocarbons contributes to global warming. Rank the following fuels
according to increasing mass percentage of carbon: (a) ethene, C2H2; (b) propanol, C2H2OH; (c) heptane, C2H16.

Do you simply take the amount of carbon from each hydrocarbon and divide that by the hydrocarbon's molar mass? And for the answer, do you write it as x% of (hyrdrocarbon)?

K Stefanescu 2I
Posts: 68
Joined: Fri Sep 29, 2017 7:04 am

### Re: F.23 Question  [ENDORSED]

Yes. For example, for ethene, the mass percentage of carbon would be calculated as follows: ((2* 12.011)/(2*12.011 + 2*1.008)) * 100= 92.3%
I think that most questions requesting mass percentage require that the answer be given as a percentage. I would do so in any case, just to be safe.

Don't forget to repeat this process for each fuel and then rank them from least to greatest C%.

VahagnAldzhyan
Posts: 25
Joined: Thu Jul 27, 2017 3:01 am

### Re: F.23 Question

Yes. You have to take the molar mass of each compound and then determine how much percent Carbon makes up in each compound.

Yizhou Liu 3L
Posts: 19
Joined: Fri Sep 29, 2017 7:07 am

### Re: F.23 Question

Yes. Also, there's an easy way to do it. Each hydrocarbon in this problem has two carbons, which means the numerators are the same. So you can just compare the denominator by calculating the mass of H2，H2OH and H16.

Posts: 16
Joined: Sat Jul 09, 2016 3:00 am

### Re: F.23 Question

say a problem asked you to find the mass percentage composition and another problem asked you to find the moles of a certain mass. would the process ultimately be the same? You just multiply by 100 when finding the percent composition

Chem_Mod
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### Re: F.23 Question

No, the process is not practically the same. When finding the mass percentage of a particular compound, you would need to have the total mass of that substance on which elemental analysis was performed, then, divide the mass of the element in the sample by the mass of the total mass of substance analyzed. So, its basically 1 part of the sample (which happens to be 1 particular element) divided by the whole sample.

Finding the number of moles from a particular mass, in the context of an empirical or molecular formula, is different for whether you are talking about molecules or atoms in that (for the former) if you have an unknown sample, you need to find the molecular formula and then divide the entire sample's mass by its molecular weight (molecular weight can only be found by the molecular formula. When determining an empirical or molecular formula, you work with mass, not moles, because if you compared moles of one element in the sample to moles of another element in a sample, you are not taking into account that the different elements contribute to the total mass of the sample to different degrees depending on the element's molecular weight. Now, if you are determining an empirical formula, you do compare the number of moles of each element present (from the given masses), but this is to find out the subscripts in the formula (the Ratio of moles), not the "mass percentage" of each. Mass percentage and mole percent are very different.

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