Compounds burned in Air

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Samiksha Chopra 2I
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Joined: Fri Sep 25, 2015 3:00 am

Compounds burned in Air

Postby Samiksha Chopra 2I » Tue Oct 13, 2015 11:19 pm

Between two questions like:
A compound is burned in air and produces 5.0g CO2 and 7.20g of H2o, find the empirical formula.
and
6.40 grams of a compound is burned in air and produces 5.0g CO2 and 7.20g of H2o, find the empirical formula.

Do we make the assumption in both cases that the compound consists of only C and H and the O comes in because it is burned in air? This ambiguity rises because technically the 6.40g in the second question gives enough information to find what mass percent comes from O on the assumption that the O in the product comes from the compound and is not part of the excess. In the first part though this is impossible to calculate.

If anyone can clear this up that will be very helpful.

604607619
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Re: Compounds burned in Air

Postby 604607619 » Tue Oct 13, 2015 11:41 pm

i believe the first question the compound is a hydrocarbon so it only consists of carbon and hydrogen because it doesn't give you the mass of the reactant so there is no way of finding the moles of O. However, the second question gives you the mass of the reactant so you know it is comprised of C,H, and O.

Armin Ghomeshi
Discussion 4B

Isa AbdulCader 3K
Posts: 18
Joined: Fri Sep 25, 2015 3:00 am

Re: Compounds burned in Air

Postby Isa AbdulCader 3K » Sat Oct 17, 2015 7:47 pm

To add onto the previous response, you cannot always assume that since the g of the initial compound are given it is composed of C, H, and O. In order to determine if there is any O present in the substance being burned, you have to calculate the moles of C and H present as a result of the combustion reaction, then find the grams of both and find the total number of grams of C and H. Subtract this from the grams given of the unknown compound, and if it = 0, then there is only C and H, but if there is leftover mass, that is O, and must be taken into account.


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