First, we can use the ideal gas law equation to find the initial number of moles of gas in the balloon:
PV = nRT
Where:
P = pressure (1.05 atm)
V = volume (5.0 L)
n = number of moles of gas
R = ideal gas constant (0.0821 L.atm/mol.K)
T = temperature (20 ºC + 273 = 293 K)
Plugging in the values:
(1.05 atm)(5.0 L) = n(0.0821 L.atm/mol.K)(293 K)
n = (1.05*5) / (0.0821*293) ≈ 0.211 moles
Now, we can use the ideal gas law equation again to find the final volume at the new altitude and temperature:
PV = nRT
Where:
P = pressure (0.65 atm)
n = number of moles of gas (0.211 moles)
R = ideal gas constant (0.0821 L.atm/mol.K)
T = temperature (-15 ºC + 273 = 258 K)
V = final volume
Plugging in the values:
(0.65 atm)(V) = (0.211 moles)(0.0821 L.atm/mol.K)(258 K)
V = (0.211*0.0821*258) / 0.65 ≈ 6.55 L
Therefore, the new volume of the balloon when it rises to an altitude where the pressure is 0.65 atm and the temperature is -15 ºC will be approximately 6.55 L.