First we need the molecular mass of NH₃=3H+N=3(1.0079)+14.0067=17.0304. If x is the amount in grams the number of moles is x/17.0304. 34.3dm³=0.0343 cubic m.
643.05 mmHg=85732.96 pascals.
We apply the Ideal Gas Law PV=nRT, where R=8.314 SI units.
n=x/17.0304 moles, P=85732.96 pascals, V=0.0343, T=95.3, so 85732.96(0.0343)=(x/17.0304)(8.314)(95.3).
2940.64=46.5241x, so x=2940.64/46.5241=63.2 grams approx.