You need to use the Ideal Gas Law equation for this problem. The Ideal Gas Law equation is:

PV = nRT

P = the pressure (for this problem, it's in atm)

V = the volume in L

n = the number of moles of the gas

R = the Gas Constant (it's 0.082057 L* atm / (mol*K)

T = the temperature in Kelvin (K)

The problem says that the moles of gas "exerts a pressure of 2.00 atm". So P = 2.00 atm. The problem also says that the gas is placed in a 3.00 L container, so V = 3.00 L. n = the number of moles of gas and is what we want to find. The temperature must be in Kelvin, but we are given a temperature of 25 degrees C. We must convert the temperature using this conversion: K = 273.15 + Celsius. So we do: K = 273.15 + 25.

Thus, K = 298.15. So T = 298.15 K. We already found R = 0.082057 L* atm/(mol*K)

Now we can put all our information in the Ideal Gas Law Equation and solve for n:

PV = nRT

2 * 3 = n * 0.082057 * 298.15

6 = n * 0.082057 * 298.15

6 = n * 24.4652946

6 / 24.4652946 = n

0.2452 moles = n

So you will need 0.2452 moles (or if you want to round it to 2 significant figures, 0.25 moles).