[heavenly_angel]

I need help with this question. I've asked several people and haven't really been able to get something I understand.

Gasoline is a blend of different hydrocarbons (compounds that contain only hydrogen and oxygen). Calculate the volume of carbon dioxide, at 300C and 110kPa, that will pass through the muffler of a car during the combustion of a full tank (50.0g) of gasoline is C7H16. The chemical equation for this reaction is:

C7H16(l) + 11 O2(g) --> 7 CO2(g) + 8 H2O(g)

So far I know for sure that P = 110 kPa, T = 573 K

I think R = 22.4 L but am not sure if I have to modify this number based on the number of moles I have in the equation.

N = 7.00 moles because there's 7 moles of CO2?

V is what we're going to solve the equation to find out

I know I want to use the equation of V = nRT/P

That's about as far as I was able to get. I'm not sure if those last two vaules are correct or not and how I cancel out units in my equation to be left with the approperiate ones

[Grumples]

Firstly, you should clean up your instructions, since they are somewhat confusing and straight out wrong in some ways ("[hydrocarbons] contain only hydrogen and oxygen" for instance).


Anyway, yes, you should use the equation PV=nRT.  They've given you P and T; and R is always a constant.  

First step: since you are asked for volume of CO2, you'll need to find the moles of CO2 produced.  to do that, you'll need to find the moles of gasoline burned.  find that by dividing the weight of gas by the molecular weight.  since 7 moles of CO2 are produced per mole of gasoline, multiply that number by 7 to find moles of CO2.  Using that mole number, plug it into the PV=nRT.  However, remember two things: firstly, pressure must always be in atmospheres, so convert kPa to atm's.  secondly, your value for R is wrong.  R is always .0821 (L*atm)/(n*K).  

Plug in values and you get:

(110/101.3)atms * Volume = .5n * (300+273)K * .0821

so V = 21.7L if my calculations are correct.

[heavenly_angel]

I posted what the question asked to the letter. IF there's a mistake it's on the fault of the company issuing the books, not me.

Thanks for the help

[AWK]

You have 50 g of C7H16. Calculate moles of C in this sample (50/100)x7=3.5
Then use Ideal gas law, but find the proper value of R (for pressure in kPa)

[mike]

Here are some values of R (gas constant) for use with PV=nRT with a variety of units.

[Borek]

Different R values are in the 'miscellaneous' section of chemicalforums (links on the left top).