Solving a Vertical Cylinder Pressure Problem

[lawgurl]

Here is my problem.

A vertical cylinder of cross-sectional area .04m^2 if fitted with a frictionless piston with a mass of 13kg. Assume acceleration of gravity is 9.8 m/s^2. If there is 1 mol of an ideal gas in the cylinder at 315 K, find the pressure in the cylinder. Assume the system is in equilibrium.

Here is what I've been doing.

P=Force/Area Force= Pressue * Area

Since it is in equilibrium, the downward force of the piston should equal the upward force of the gas.

PA = (Piston mass)(gravity)

P(.04) = (13)(9.8)
P = 3185 Pa

3185 is not the right answer according to my online answer checker. What am I doing wrong?

 

[lawgurl]

Questionable Solution

Okay, I found why my answer is wrong. Apparently I need to at 101300 Pa (1 atm) to my answer. If anyone can explain why I add one atmosphere, that would be greatly appreciated.

 

[Astronuc]

Assume the piston seals the vertical cylinder. The piston has a mass (13 kg) but also, there is atomspheric pressure outside the cylinder, i.e. the air in which we live, and that pressure is 1 atm = 14.7 psia = 101325 Pa or 101.325 kPa. At equilibrium, the pressure inside the cylinder must equal the pressure applied from the outside which is the sum of the (weight of the piston)/(area of piston), or (13 kg)(9.8 m/s²)/(0.04 m²) + the atmospheric pressure 101325 Pa.

mg = force = N if m (kg) and g (m/s²).

Pressure = force/area = N/m² = Pa.