[audienne]

Propane undergoes complete combustion as follows:

C3H8 (g) + 5O2(g) -------> 3CO2 (g) + 4H2O (g)

all volumes are measured at 120 C and 102 kPa

When 80 ml of propane and 500ml of oxygen are reacted
a)   one of the gases does not react completely. Which gas is it and how much of it is unreacted?

Pv = nrt

Vol of propane = 80ml = 0.08 L
No of mole (for propane)  therefore: 102 x 0.08 = n x 8.314 x 393
                                                                    n= 0.0249 mol   

Vol of oxygen = 500 ml
No of mole (for oxygen) therefore = 102 x 0.5 = n x 8.314 x 393

                                                                 N = 0.0156 mol

Propane to oxygen mole ratio = 1: 5
                                                      = 0.0249 mol of propane : 0.01245 mol of oxygen

However,  0.0156 mol of oxygen is present.  Therefore oxygen is in excess.

How much is unreacted?

500 ml – 5 x 80 ml = 100ml

b)   What volumes of carbon dioxides and water are produced in this reaction

Carbon dioxides = 3 x 80 ml  =240ml

Water = 4 x 80ml =320 ml

c)   Has the total volume of the gases increased or decreased.

The volume of the gases remain the same as the temperature does not change

d)   Find out this change in volume

Nil – see answer C

[Yggdrasil]

a,b) correct
c,d) P and T remain the same, but has n changed?

[azmanam]

Your answer to a is correct, but your numbers are wrong. 

(0.0249 mol propane)*(5 mol oxygen/1 mol propane)=(0.125 mol oxygen)

Like I said, your overall answer to a) is correct, but check your math again.

[Borek]

Note: if temperature and pressure are constant, you don't have to calculate number of moles. Their ratio will be identical as ratio of volumes.

[audienne]

Total volume of the reactant : 80 + 500 = 580 ml

Total volume of the product : 3 x 80 + 4 x 80 = 560ml

Volume has decreased by 20 ml

is it correct?

I have another question, (when p and t are constant ), mole and volume ratio will be the same. If for example the oxygen does not present in excess, the total volume of the product and reactant are still not the same. Is this correct?Why so?