A balloon under 1 atm of pressure contains 10.0 g of neon gas. With the temperature kept constant, 15.0 g of xenon gas is added. What happens?

The volume of the ballon expands 1.2 times

Explanation:

The pressure in the ballon will be always the same. And as temperature is also the same the only parameters that change are moles of gas and volume.

The volume is directely proportional to  moles of the gas under pressure and temperature constant (Avogadro's law). That means:

V₁ / n₁ = V₂ / n₂

Where V is volume and n moles of 1, initial statet and 2, final state of the gas

Initial moles of Neon gas (20g/mol) are:

10.0* (1mol / 20g) = 0.5 moles are equal to initial moles that occupies X liters of volume.

As you add 15.0g of Xe (Molar mass: 131g/mol):

15.0g * (1mol / 131g) = 0.11 moles + 0.55 moles = 0.66 moles that are final moles.

Replacing in Avogadro's law:

V₁ / n₁ = V₂ / n₂

X L / 0.55moles = V₂ / 0.66 moles

1.2X L = V₂

That means the volume of the ballon expands 1.2 times