How many moles of oxygen will occupy 728 al 760 mm Hg and 25C? please keep 3 significant figures ?

Suppose it is "728 ml" instead of "728 al".

Pressure, P = 760/760 atm = 1 atm
Volume, V = 728 ml = 0.728 L
No. of moles, n = ? mol
Gas constant, R = 0.08206 L atm / (mol K)
Temperature, T = (273 + 25) K = 298 K

Gas law: PV = nRT
Then, n = PV/(RT)

No. of moles, n = 1 × 0.728 / (0.08206 × 298) mol = 0.0300 mol

It depends on what units belong to "728".