Answer
8.80 x 10^19 molecules
Work Step by Step
We are given the pressure, the temperature, and the volume of 1L. Therefore, we can use the ideal gas law:
PV = nRT to find the number of moles, which is n = PV/RT = (3.00 x 10^-3atm) * (1L) / (0.0821 * 250K) = 1.46 x 10^-4 moles
Then since we have moles, we can find the number of molecules by multiplying by Avogadro's number, 6.022 x 10^23, this gives:
8.80 x 10^19 molecules
