Stoich help

[Annax_i]

I've done this question a billion times and get weird answers (it's probably just me, the question is "simple" )

2C8H18 (g) + 25O2 (g) --> 16CO2 (g) + 18H20 (g)
Calculate mass of octane if 50L of carbon dioxide measured at 120 degree celcius and 1.1 atm was produced/

This is what I did:
n(CO2) = 113.64 mol
n(C8H18) = 1/8 X 113.64 = 14.20mol

V=nRT/P
V=14.2 X 8.31 X 393
=416 ?

Thankyou for the help

[Lear]

How did you find the moles of CO2 in the first step? You will need to use pv=nrt to find the moles of CO2 first.

[huity]

Hey Annax_i,

Don't worry at all about the question being "simple", the fact that you're asking for help shows initiative and a willingness to learn (super important in all subjects)! Plus, there are a few common mistakes which students make in these kinds of questions

Just as Lear said:
Step 1) Use pv=nrt (make sure you convert p=1.1atm into kPa and 120 degree celsius into Kelvins). Rearrange to find n. This represents n(CO2) NOT n(c8H18) since you have used V(CO2)=50L in the calculation.
Step 2) As you have done correctly, use mole ratios to find n(C8H18).
Step 3) m(C8H18)= n(C8H18)/M(C8H18). Make sure you use n(C8H18), which you just found, not n(CO2) from Step 1).

If you don't understand anything I wrote/ you're still getting different answers, be sure to pop your calculations on here