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