Question

Pentane, C5H12, will react as shown below:

Answer 1

1) That looks right

2) Step 1: Moles=Mass/Mr, so it's 144/72= 2mol
    Step 2: Stoichiometry means the ratio of oxygen to pentane is 1:8 so moles of       oxygen is moles of pentane times 8. 2*8=16mol of oxygen
    Step 3: 16= 9.6*10^24

3) The ratio of oxygen to carbon in CO2 is 1:2
    so it's 192/3 = 64
    64*2=128g
Moles = Mass/Mr = 128/32 = 4mol = 2.41*10^24 atoms of oxygen
Basically you needed to work out the mass of oxygen using ratios before using a moles equation instead of just working with moles

4) True

5) 1C5H12+ 8O2----- 5CO2+ 6 H2O
      32g        32g

Moles of oxygen = Mass/Mr = 32/32= 1mol so you'd need 0.125mol of pentane to react fully
Moles of pentane= Mass/Mr = 32/72=0.4mol so you'd need 3.2mol of oxygen to react fully

This makes oxygen the limiting factor, because you need more of it to react fully with the pentane, so we use oxygen 

Stoichiometry means for every 8mol of oxygen there's 5mol of CO2
So, 1/8*5=0.635mol of CO2