Question

N2(g) + 3H2(g)  2NH3(g) at STP

How many litres of ammonia gas is produced using 5 litres of H2?

N2 + 3H2 ---> 2NH3
- the ratio between H2 and NH3 is 3 : 2
- it's given 5 litres of H2, so the volume of NH3 formed is 5/3*2 = 10/3 = 3.33 (L)

Sig FIG ANSWER 3 L

How many moles of N2 are needed?
- 5 litres of H2 converts to 0.22 moles of H2
- the ratio between N2 and H2 is 1 : 3, so the moles of N2 needed is 0.074 mol

SIG FIG ANSWER 0.1 MOL

Answer 1

The answer for 3.33 looks ok but you're allowed 1 s.f. (from 5L) so wouldn't you round 0.074 to 0.07 and not 0.1 although both 0.07 and 0.1 have 1 s.f.

Answer 2

Your math and conclusions are correct, but suggest keeping in mind the numbers actually refer to 'molar volumes' of gas. Such could pose some confusion when working with gas volumes in practical lab settings. That is,
one should specify temperature and pressure conditions to account for departures from STP conditions. Yes, ratios will be consistent with what your calculation shows, but actual volumes in a lab setting may not be what's expected from theoretical calculations. :-)

Answer 3

so for 0.074 mol is 0.07 mol

Answer 4

yes,You are allowed 1 s.f.because of the 5 L and there is 1 s.f. in 0.07. The 0 is a decimal placer and not a significant digit.