What is the density of ammonia gas, nh3, at 31°c and 751 mmhg?

First, we will use the general gas formula to get the number of moles.
PV = nRT where:
P is the pressure of gas = 751 mmHg = 100125.096375 Pascal
V is the volume = 1 liter = 0.001 m^3
n is the number of moles we want to calculate
R is the gas constant = 8.314 J/(K. mol)
T is the temperature = 31 degrees celcius = 304.15 degree kelvin

Substitute in the above equation to get the number of moles as follows:
100125.096375 * 0.001 = n * 8.314 * 304.15
n = 0.039595 moles

Now, we will use the number of moles to get the mass as follows:
number of moles = mass / molar mass
mass = number of moles * molar mass
number of moles = 0.039595 moles
molar mass of ammonia (NH3) = 14 + 3(1) = 17 grams
Substitute to get the mass as follows:
mass = 0.039595 * 17 = 0.673122 grams

Last step is to get the density as follows:
density = mass / volume
mass = 0.673122 grams
volume = 1 liter
density = 0.673122 / 1 = 0.673122 grams/liter = 0.000675 kg/L