Question

H2(g) + Cl2(g) HCl(g)
Consider the unbalanced equation above. 3.95 L of H2 and 1.00 L of Cl2 are reacted under the same conditions of temperature and pressure.
(a) How many liters of HCl will be produced?
(b) How many liters of reactant are in excess?

i found part a to be 2.00 L what do i need to do to find part b

Answer 1

2.000L for part a is correct.
H2 + Cl2 ==> 2HCl

When working with gases remember that you can take a short cut and use L directly as mols IF everything stays at the same T and P. So I do this.
...........H2 + Cl2 ==> 2HCl
I.........3.95..1.00....0
C........-1.00.-1.00...1.00
E.........2.95...0.....1.00

So you have used 1.00 mol of the 3.95 mols to leave 2.95 mols H2 remaining un-reacted. Of course that's the same as
3.95 L H2 initially less 1.00 L reacted = 2.95 L H2 remaining un-reacted.