Which volume, in cm3 , of 0.20 mol dm-3 naoh (aq) is needed to neutralize 0.050 mol of h2s (g)? h2s (g) + 2naoh (aq) → na2s (aq) + 2h2o (l)?

Answer is: volume of sodium hydroxide is 500 cm³.
Chemical reaction: H₂S + 2NaOH → Na₂S + 2H₂O.
From chemical reaction: n(H₂S) : n(NaOH) = 1 : 2.
n(NaOH) = 2 ·0.050 mol.
n(NaOH) = 0.1 mol.
V(NaOH) = n(NaOH) ÷ c(NaOH).
V(NaOH) = 0.1 mol ÷ 0.2 mol/dm³.
V(NaOH) = 0.5 dm³.
V(NaOH) = 0.5 dm³ · 1000 cm³/dm³.
V(NaOH) = 500 cm³.

The answer should be vaporization 

Density= mass/volume so 5.1 divided by 4.7 is  5.1 / 4.7 =1.08510638298round to the correct number of sig figs