дано
m(PbS) = 25 g
η(SO2) = 77%
--------------------
V практ (SO2)-?
2PbS+3O2-->2PbO+2SO2
M(PbS) = 207+32 = 239 g/mol
n(PbS) = m/M = 25 / 239 = 0.1 mol
2n(PbS) = 2n(SO2) = 0.1 mol
V теор (SO2) = n*Vm = 0.1 * 22.4 = 2.24 l
V практ(SO2) = 2.24 * 77% / 100% = 1.72 L
ответ 1.72 л