CaH2+H2O->Ca(OH)2+H2 unbalanced If 21 g of CaH2 is consumed in the reaction B.How many grams of Ca(OH)2 are formed? C.How many liters of H2 gas are produced at STP?

B. 37 g of ##"Ca(OH)"_2## are produced; C. 23 L of ##"H"_2## gas are formed at STP.

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.