Reacting Quantities

[mandy]

Octane is one of the main constituents of petrol. It burns according to the equation:
2C₈H₁₈(g) + 25O₂(g) ------> 16CO₂(g) + 18H₂O(g)
What mass of octane must have been used if 50.0L of carbon dioxide, measured at 120^oC and 1.10atm, was produced?

Could you please help me with this?
What are the steps?

[chem-nerd]

use PV = nRT to get n(CO₂)  (remember to convert units to K and kPa first)

n(octane) = 2/16 n(CO₂)

m(octane) = n x Mr

[mandy]

wow, thank you so much !

[mandy]

Hydrogen gas reacts with chlorine gas according to the equation:
H₂(g) + Cl₂(g) -------> 2HCl(g)
3.0L of hydrogen and 7.0L of chlorine are allowed to react as much as possible. What will be the volume of the gaseous mixture at the end of the reaction, assuming all volumes are measured at the same temperature and pressure?

What would I do now? Do I use the hydrogen or chlorine, because when I tried, neither worked.

[chem-nerd]

in this sort of equation where all the reactants and products are gases, you can assume the volumes to be the same as the number of moles when T and P are kept constant. (PV = nRT, thus V = n)

Thus, 3 mol (L) of H₂ would react with 3 mol (L) of Cl₂ to produce 6 mol (L) if HCl. (H₂ is the limiting reactant, Cl₂ is in excess by 4 mol (L))

The volume at the end would be the 4L excess Cl₂ plus the 6L HCl produced, thus 10L

[mandy]

okay, got it. thank you again :]