Chapter 8: Enthalpy and Thermochemistry

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[30binanfd]

Hello. I'm working on the question and answers for chapter 8 and I'm thoroughly confused. The sixth question (bottom of the first page) asks to calculate the amount of O2 consumed when 5 grams of sucrose is combusted. This should be simple but I just don't understand where I went wrong. Here is what I did;

Using 5 grams of sucrose and dividing that by the mw of sucrose, 342 g/mol, I get the number of moles of sucrose, 5/340 or 1/68. now multiplying this by the stoichiometric ratio of sucrose to oxygen, 12 mol O2 to 1 mol of sucrose, I get 12/68 mols of O2. Multiplying this by the mw of O2, 32 g/mol, I get 384/68 or 5.6 g of O2 consumed.

After reading the answer, I realized that the calculations that I did were exactly the same and I came up with the wrong answer even when I did it on a calculator. Also, the answer uses a mw of 382 g/mol for sucrose which is incorrect as this is the mw of 12 moles of oxygen. It makes sense that around 5 grams of O2 would be used up since 12 moles of oxygen would have around the same mw as one mole of sucrose but the answers are all less than 1.

[fbbb331745]

I got the same answer you did. There must be an error somewhere in the problem

[reese.1491564]

I got the exact same answer as the 2 of you did. The math shown is correct but the final answer is wrong.

[DLBMCAT]

I ended up with the same result too. If you take the inverse of 342/1920 you will get 0.178 g, but I believe that is wrong because there is no reason to take the inverse.