At STP, a 5.00L flask filled with air has a mass of 543.251g. The air in the flask is replaced with another gas and the mass of the flask is 566.107g. The density of air is 1.29g/L. What is the gas that replaced the air?

Answer 1

density air = 1.29 g/L; therefore, the 5.00 L air has a mass of 1.29*5.00 L = ?g
543.251 = mass flask + mass air
-? g = mass air from above.
---------
x = mass flask.

Then
566.107 = mass flask + unknown gas
-x = mass flask from above.
------------
y = mass unknown gas.

y*22.4/5.00 = molar mass gas.
Look on periodic chart for gas.

Answer 2

substituting all values in the above discussion would yield 131.25 g/mol. This element is Xenon. :D