April 21, 2019, 04:47:26 AM
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Topic: Gas Laws - MCAT format questions - how to answer???  (Read 725 times)

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Offline GPCR pathway

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Gas Laws - MCAT format questions - how to answer???
« on: January 17, 2019, 02:30:14 AM »
I am not sure how these type of question formats be answered:
1. A chemistry student prepares an aqueous solution of a gas, with 0.3 g of the gas at ordinary condition. If the pressure is doubled what will be the mass of the gas will be dissolved?

2. If the absolute temperature of a gas is increased six times and its pressure is tripled, then the volume will?

3. If the absolute temperature of a gas is quadrupled and its pressure is halved, then the volume will?

these questions are popular and often used in standardized tests with limited time and I know that there is a technique/trick on how to solve this kind of questions fast, I just forgot how. Please send help  :'(

A photo of the solution will be also very much appreciated

Offline Borek

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Re: Gas Laws - MCAT format questions - how to answer???
« Reply #1 on: January 17, 2019, 03:54:48 AM »
No idea about any "tricks", it is mostly a matter of knowing what is proportional to what. That can be easily read from the Henry's law (1) and PV=nRT (2 and 3).
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Offline Enthalpy

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Re: Gas Laws - MCAT format questions - how to answer???
« Reply #2 on: January 17, 2019, 11:53:06 AM »
Is the text of 1) complete? It doesn't even say "saturated" nor "equilibrium".

Offline GPCR pathway

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Re: Gas Laws - MCAT format questions - how to answer???
« Reply #3 on: January 18, 2019, 02:59:34 AM »
Figured out easier to follow steps:
1. know which law
2. input given
3. change remaining variables to 1
4. find X

A chemistry student prepares an aqueous solution of a gas, with 0.3 g of the gas at ordinary condition. If the pressure is doubled what will be the mass of the gas will be dissolved?

I used P1V1=P2V2:

1P(0.3g)=2P(X)

[(1)(0.3)] ÷ 2 = X

X = 0.6 g

Offline mjc123

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Re: Gas Laws - MCAT format questions - how to answer???
« Reply #4 on: January 18, 2019, 04:46:47 AM »
Your answer is coincidentally correct, but your reasoning is not.
You use P1V1 = P2V2, but 0.3g is not a volume.
1*0.3/2 is not 0.6
You went wrong on step 1 - know which law! (You have already been told which law applies to solutions of gas.)

Offline Vidya

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Re: Gas Laws - MCAT format questions - how to answer???
« Reply #5 on: February 04, 2019, 01:25:19 AM »
Figured out easier to follow steps:
1. know which law
2. input given
3. change remaining variables to 1
4. find X

A chemistry student prepares an aqueous solution of a gas, with 0.3 g of the gas at ordinary condition. If the pressure is doubled what will be the mass of the gas will be dissolved?

I used P1V1=P2V2:

1P(0.3g)=2P(X)

[(1)(0.3)] ÷ 2 = X

X = 0.6 g
PV =k(boyles law) is the law applied on gases and not on the solution of gases.
You are actually using Henry's law
Pressure of the gas is directly proportional to the mole fraction of the gas in the solution.
p= k X(mole fraction)
this is modified
this can be mathematically
P1m2 =P2m1 ...where m1 and m2 are masses of the gas at p1 and p2 for the given amount of the solution .
 now p2=2p1
so m2 = 2*0.3

Offline Vidya

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Re: Gas Laws - MCAT format questions - how to answer???
« Reply #6 on: February 04, 2019, 01:28:17 AM »


2. If the absolute temperature of a gas is increased six times and its pressure is tripled, then the volume will?

3. If the absolute temperature of a gas is quadrupled and its pressure is halved, then the volume will?

these questions are popular and often used in standardized tests with limited time and I know that there is a technique/trick on how to solve this kind of questions fast, I just forgot how. Please send help  :'(

A photo of the solution will be also very much appreciated
Here you can use gas laws
do you know which one?
combined gas law
P1V1/T1= P2V2/T2

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