Question

a flask of 0.30 L was weighted after it had been evacuated.It was then filled with a gas of unknown molecular mass at 760 mm of Hg and temperature of 300 K. The increase in mass of flask was found to be 0.997 g. Determine the molecular mass​

Answer 1

The molecular mass​ : 81.72 g/mol

Further explanation

In general, the gas equation can be written  

$\large {\boxed {\bold {PV = nRT}}}$

where  

P = pressure, atm , N/m²

V = volume, liter  

n = number of moles  

R = gas constant = 0.082 l.atm / mol K (P= atm, v= liter),or 8,314 J/mol K (P=Pa or N/m2, v= m³)

T = temperature, Kelvin  

P = 760 mmHg=1 atm

T = 300 K

V = 0.3 L

Number of moles :

$\tt n=\dfrac{PV}{RT}\\\\n=\dfrac{1\times 0.3}{0.082\times 300}\\\\n=0.0122$

The molecular mass (MW) :

$\tt MW=\dfrac{mass}{n}\\\\MW=\dfrac{0.997~g}{0.0122}\\\\MW=81.72~g/mol$