This problem can be solved using the ideal gas law, PV=nRT. We can rearrange it as follows to solve for n, the number of moles of gas:
n = PV/RT
R = .0821 L-atom/mol-K (ideal gas constant)
Temperature needs to be in Kelvins: 25.5ºC + 273 = 298.5 K
Volume must be in liters: V = 29.65 ml X (1 L/1000 ml) = 0.02965 L
n = (708.1 atm)(0.02965 ml)/(0.0821 L-atm/mol-K)(298.5 K)
n = 0.857 moles, so the correct answer is b.
There are several values for R with different units. Be sure to use the value that has the same pressure units as given in the problem, in this case atmospheres. Make sure that other units match up, like using liters for volume. If you include all of the units in the calculation and cancel them you can verify that you set the problem up correctly.