дано

m(NaNO3) = 340 g

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V(O2)-?

2NaNO3-->2NaNO2+O2

M(NaNO3) = 85 g/mol

n(NaNO3) = m/M = 340 / 85 = 4 mol

2n(NaNO3) = n(O2)

n(O2) = 4/2 = 2 mol

V(O2) = n(O2) * Vm = 2*22.4 = 44.8 L

ответ 44.8 л