Calcium carbide (cac2) reacts with water to produce acetylene (c2h2): cac2 (s) 2h2o (g) → ca(oh)2 (s) c2h2 (g) production of 6.5 g of c2h2 requires consumption of g of h2o.

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

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)