Question

A gas phase reaction takes place in a syringe at a constant temperature and pressure. If the initial volume before reaction is 60 mL and the final volume after the reaction is complete is 40 mL, which of the following reactions took place. (Note: You can assume that you start with stoichiometric amounts of the reactants, the reaction goes to completion and that the gases behave ideally.)

2NO(g) + O2(g)->2NO2(g)

2NO2(g)->N2O4(g)

2NOBr(g)->2NO(g) + Br2(g)

2N2O5(g)->2N2O4(g) + O2(g)

2N2O5(g)->4NO2(g) + O2(g)

Answer 1

Recall that always

Ptotal = Pgases present

then,

if initailly

Vtotal = 60 mL

Vfinal = 40 mL

there is clearly a decrease in moles so...

ignore:

2NOBr(g)->2NO(g) + Br2(g)

2N2O5(g)->2N2O4(g) + O2(g)

2N2O5(g)->4NO2(g) + O2(g)

since those will increase in volume

focus on:

2NO(g) + O2(g)->2NO2(g)

2NO2(g)->N2O4(g)

ratios:

3:2 for 2NO(g) + O2(g)->2NO2(g)

2:1 for 2NO2(g)->N2O4(g)

note that 2:1 ratio can be, since this implies half, i.e. 100% --< 50%; in this case 60 mL should go to 30 mL which is NOT the case

3:2 for 2NO(g) + O2(g)->2NO2(g)

this is the best answer, since raito is kept

40 mL / 60 mL = 2/3

which is what we needed

2NO(g) + O2(g)->2NO2(g)