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Calculate the number of moles of $O_2$,$CO_2$ and $Cl_2$ present in our flask of $1\; L$ by aid of the following table :
$n_{O_2}=\frac{1,27}{32}= 0,04 \; mol$ $n_{CO_2}=\frac{1,76}{44}= 0,04 \; mol$ $n_{Cl_2}=\frac{2,83}{71}= 0,04 \; mol$
Formulate Avogadro's law !
Equal volumes of all gases, at the same temperature and pressure, have the same number of moles (and therefore molecules too)