If the o2 is collected over water at 25 c (298 k) and 724.0 mm hg total pressure, how many mols o2 will a 1.00 l sample of collected gas contain?

To answer this item, it is assumed that O2 is an ideal gas such that it follows the ideal gas law.

    PV = nRT

Solving for n,
    n = PV / RT

where n is number of moles, P is pressure, V is volume, R is the universal gas constant, T is the temperature. 

Substituting the known values,
     n = (724 mmHg/760 mmHg/1 atm)(1 L) / (0.0821 L. atm/mol.K)(298 K)

Simplifying,
   n = 0.0389

Hence, the value of n is 0.0389.