дано
mтехн(Al) = 20 g
W(прим) = 15%
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V(H2)-?
mчист(AL) = 20 - (20*15% / 100%) = 17 g
2Al+6HCL-->2AlCL3+3H2
M(Al) = 27 g/mol
n(Al) = m/M = 17 / 27 = 0.63 mol
2n(Al) = 3n(H2)
n(H2) = 3*0.63 / 2 = 0.945 mol
V(H2)= n*Vm = 0.945*22.4 = 21.168 L
ответ 21.168 л