The volume V of n moles of an ideal gas at pressure P and temperature T is given by V = (n*R*T)/P where R is a constant equal to 8.3145 J/(mole*K). At standard temperature and pressure one mole of an ideal gas occupies a volume equal to 22.4 liters.
In the problem, the gas C2H2 has a volume 6.2 L. The behavior of the gas can be approximated to that of an ideal gas. This gives the number of moles of C2H2 that occupy 6.2 L at STP as `6.2/22.4 ~~ 0.2767` .
0.2767 moles of C2H2 occupy 6.2 L at STP