A 2900-mL flask is initially open to air at 23 ∘C and 1 atm pressure. It's then closed and immersed in boiling water. When it has reached equilibrium, the flask is opened and air is allowed to escape. Then it's closed and cooled back to 23 ∘C.
a. What's the maximum pressure reached in the flask?
b. How many moles escape when the air is released?
c. What's the final pressure?
- NCSLv 73 years agoFavorite Answer
a. Assuming the flask has only air in it and that we can treat air as an ideal gas,
pV = nRT gives us, when the stoppered flask is heating up, that
(p/T)_a = (p/T)_b since V and n are constant.
1atm / 296K = p / 373K
p = 1.26 atm ◄ maximum
b. At STP, our flask holds
2.9L * 1mol/22.4L = 0.1295 moles
But we're not at STP. Holding V and p constant,
0.1295mol * 273K = n * 296K
n = 0.1194 mol
When we unstopper, we can hold V and T constant:
1.26atm / 0.1194mol = 1atm / n'
n' = 0.0948 mol
which means that n - n' = 0.0246 mol has escaped
c. When we stopper it up and let it cool, we can hold V and n constant:
1atm / 373 = p / 296
p = 0.79 atm
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