Дано
m(ppa HCOOH) = 100 g
W(HCOOH) = 30%
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V(H2)-?
m(HCOOH) = 100*30% / 100% = 30 g
2HCOOH+Mg-->(HCOO)2Mg+H2
M(HCOOH) = 46 g/mol
n(HCOOH) = m/M = 30 / 46 = 0.65 mol
2n(HCOOH) = n(H2)
n(H2) = 0.65 / 2 = 0.325 mol
V(H2) = n*Vm = 0.325 * 22.4 = 7.28 L
ответ 7.28 л