Question

If a `21*g` mass of calcium hydride is reacted with excess water, (i) what mass of calcium hydroxide will result; and (ii) what volume of dihydrogen will be evolved at `"STP"`?

Answer 1

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

Explanation:

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.