One mol is equal to the Avogadro's Number. Using this equivalence, we can obtain the number of molecules contained in 1.4 mol:
\(1.4\ mol\ PF_3\cdot \frac{6.022\cdot 10^{23}\ molec}{1\ mol} = 8.43\cdot 10^{23}\ molec\ PF_3\)
Each molecule contains 4 atoms: three atoms of fluorine and one atom of phosphorus:
\(8.43\cdot 10^{23}\ molec\ PF_3\cdot \frac{4\ atoms}{1\ molec\ PF_3} = \bf 3.37\cdot 10^{23}\ atoms\)
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