First we need to calculate the number of moles of acetylene produced:
number of moles = mass / molecular mass
moles of C₂H₂ = 6.5 / 26 = 0.25
from the reaction we deduce:
if 1 mole of C₂H₂ requires consumption of 2 moles of H₂O
then 0.25 moles of C₂H₂ requires consumption of X moles of H₂O
X = (0.25 × 2)/1 = 0.5 moles of H₂O
mass = number of moles × molecular mass
mass of H₂O = 0.5 × 18 = 9 g
