So I know the answers are correct but I got -33g of butane and -99 g of co2 why are they not negative? 

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The given reaction is:

`C_4H_10 + 13/2 O_2 -> 4CO_2 + 5 H_2O`

In the given reaction, butane undergoes combustion and generates carbon dioxide and water. The energy produced in the reaction is 2658 KJ.

To generate, 1.5 x 10^3 KJ or 1500 KJ of energy, 1500/2658 th mole of butane is needed. The molecular mass of butane is 58 gm/mole (= 4 x 12 + 1 x 10).

The mass of butane required is 58 x 1500/2658 = 32.73 gm

And the amount of CO2 generated would be

= 44 x 4 x 1500/2658 = 99.32 gm.

The answers would not be in negative, since the question asks for the amount of butane consumed and the amount of carbon dioxide generated. A negative answer, in reply to these questions, would mean butane is generated and carbon dioxide is consumed; which is clearly not the case. 

The negative answers would have been relevant if the question asked for the amount of butane generated and amount of carbon dioxide consumed. 

Hope this helps. 

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