**A. Balanced equation**

The balanced equation is

#"CaH"_2("s") + "2H"_2"O"("l") rarr "Ca(OH)"_2("aq") + "2H"_2("g")#

**B. Mass of #"Ca(OH)"_2#**

#"Moles of CaH"_2 = 21color(red)(cancel(color(black)("g CaH"_2))) × ("1 mol CaH"_2)/(42.09 color(red)(cancel(color(black)("g CaH"_2)))) ="0.499 mol CaH"_2#.

#"Moles of Ca(OH)"_2 = 0.499 color(red)(cancel(color(black)("mol CaH"_2))) × ("1 mol Ca(OH)"_2)/(1 color(red)(cancel(color(black)("mol CaH"_2)))) = "0.499 mol Ca(OH)"_2#

#"Mass of Ca(OH)"_2 = 0.499 color(red)(cancel(color(black)("mol Ca(OH)"_2))) × ("74.09 g Ca(OH)"_2)/(1 color(red)(cancel(color(black)("mol Ca(OH)"_2)))) = "37 g Ca(OH)"_2# (2 significant figures)

**C. Volume of #"H"_2#**

#"Moles of H"_2 = 0.499 color(red)(cancel(color(black)("mol CaH"_2))) × ("2 mol H"_2)/(1 color(red)(cancel(color(black)("mol CaH"_2)))) = "0.998 mol H"_2#

Since 1982, the IUPAC has defined STP as a pressure of 1 bar and a temperature of 0 °C.

Under these conditions, 1 mol of an ideal gas occupies 22.71 L.

∴ #"Volume of H"_2 = 0.998 color(red)(cancel(color(black)("mol H"_2))) × ("22.71 L H"_2)/(1 color(red)(cancel(color(black)("mol H"_2)))) = "23 L H"_2# (2 significant figures)

The volume of #"H"_2# at STP is 23 L.