STP = standard temperature and pressure
Use the formula:
d = MP/RT
where:
d = density = 1.25 g/L
M = molar mass (unknown)
P = pressure = 1 atm (standard pressure)
R = universal gas constant = 0.8206 L*atm/mol*K
T = temperature = 0C or 293.15 K (standard temperature)
1.25 g/L = M*(1 atm)/(0.08206 L*atm/mol*K)*(273.15 K)
M = 28.0 grams/mol
