Дано:  

m (K) = 20 г.

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V (Cl2) - ?

M(K) = 39 g/mol

n(K) = m/M = 20 / 39 = 0.51 mol

2n(K) = n(CL2)

n(CL2) = 0.51 / 2 = 0.255 mol

2K + Cl2 = 2KCl

V(CL2)  = n*Vm = 0.255*22.4 = 5.712  L

ответ 5.712 л