дано
V(C3H8) = 38.9 L
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V(возд)-?
C3H8+5O2-->3CO2+4H2O
n(C3H8) = V(C3H8) / Vm = 38.9 / 22.4 = 1.74 mol
n(C3H8) = 5n(O2)
n(O2) = 5*1.74 = 8,7 mol
V(O2) = n*Vm = 8.7 * 22.4 = 195 L
V(возд) = V(O2) *100% / 21% = 195 * 100% / 21% = 928.6 L
ответ 928.6 л