дано

m(Ca) = 20 g

V практ(H2)= 8 L

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φ(H2)-?

Ca+2H2O-->Ca(OH)2+H2

M(Ca) = 40 g/mol

n(Ca) = m/M = 20 / 40 = 0.5 mol

n(Ca) = n(H2) = 0.5 mol

V теор (H2) = n*Vm = 0.5 * 22.4 = 11.2 L

φ(H2) = V практ(Н2) / V теор(Н2) *100% = 8 / 11.2 * 100% = 71.43 %

ответ 71.43%