Basic question: need help

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Joined: Sun Dec 29, 2019 5:47 pm

Basic question: need help

Post by lotus0618 » Thu Jul 02, 2020 2:13 am

Every time I come across this kind of question, I would find myself spending a lot of time on it or get it wrong, Can you please let me know the GENERAL approach and basic logics I should use when it comes to this type of problem? I know this problem is very basic but how

“If a patient’s original lung volume (V1) is decreased by 33% (V2), what’s the relationship between the total pressure in the patient’s lungs before (p1) and after (p2) this change?

The correct answer: P2= 3/2 P1

How I approached this on the test, and this took forever:
V1 - 33% = V2= 1 - 33% = 67%
And then I’m stuck there

Thank you!
Posts: 48
Joined: Wed May 27, 2020 7:25 pm

Re: Basic question: need help

Post by NS_Tutor_Nancy » Thu Jul 02, 2020 9:32 am


For this one, remember Boyle's Law which tells us the pressure-volume relationship for gases: P1 * V1 = P2 * V2 (this is basically your combined gas law - (P1*V1)/T1 = (P2*V2)/T2 - but ignoring temperature since temperature is not changing here). Then the question tells you V2 is V1 decreased by 33%, or 1/3, meaning that V2 = 2/3 * V1.

Now let's plug in and simplify. P1 * V1 = P2 * 2/3 V1
The V1s cancel out on each side so P1 = P2 * 2/3
Then just solve for P2: P2 = 3/2 P1

I hope that helps!
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