# An evacuated flask is found to have a mass of 110.020g. When filled with O2, the flask has a mass of 110.660g. When filled .......with an unknown gas(x), it has a mass of 110.380g. Calculate the...

An evacuated flask is found to have a mass of 110.020g. When filled with O2, the flask has a mass of 110.660g. When filled ....

...with an unknown gas(x), it has a mass of 110.380g. Calculate the molar mass of gas X?

jeew-m | Certified Educator

This quesion is related to the Avagardro's law. In his law Avagadro states that in same pressure and temperature equal volume of gasses have same number of molecules.

We can use this principal here.

Molar mass of oxygen `(O_2)` = 32g/mol

Weight of `O_2` in the flask `= (110.66-110.02) = 0.64g`

Moles of `O_2` in the flask `= 0.64/32mol = 0.02mol`

So occording to the Avagadro's law the gas X also should have 0.02moles since we have same pressure,temperature and volume.

Weight of the gas X in the flask `= (110.38-110.02) = 0.36g`

Molar mass of `X = 0.36/0.02 = 18` g/mol

Molar mass of X gas is 18g/mol.

In actual speaking water has this molar mass. So we can assume that water vapour is the other gas.

Assumptions

• All the gasses act as ideal gasses
• All the gasses were in same temperature and pressure during the time