дано
m(O2) = 28 g
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n(H2O)-?
m(H2)-?
2H2+O2-->2H2O
M(O2)= 32 g/mol
n(O2) = m/M = 28 /32 = 0.875 mol
n(O2) = 2n(H2O)
n(H2O) = 2*0.875 = 1.75 mol
2n(H2) = 2n(H2O) = 1.75 mol
M(H2)= 2 g/mol
m(H2) = n*M = 1.75 * 2 = 3.5 g
ответ 3.5 г , 1.75 моль