How many molecules of n2 gas can be present in a 2.5 l flask at 50c and 650 mmhg?

First step is to change temperature and pressure into SI units:
50 degree celcius = 50 + 273 = 323 degree kelvin
650 mmHg = 650/760 = 0.855 atm

Second step is to calculate the number of moles of N2 that can be found under the mentioned conditions using the rule:
PV = nRT where:
P is the pressure, V is the volume, n is the number of moles, R is the gas constant (= 0.08206) and T is the temperature.
Substituting in the equation we get:
0.855 x 2.5 = n x 0.08206 x 323
n = 0.0806 moles

Third step is to get the number of molecules in 0.0806 moles by multiplying the number of moles by Avogadro's number:
number of molecules = 0.0806 x 6.02 x 10^23 = 0.485212 x 10^23 molecule