June 07, 2020, 03:26:52 AM
Forum Rules: Read This Before Posting

### Topic: Combined gas law  (Read 223 times)

0 Members and 1 Guest are viewing this topic.

#### dimka20

• Very New Member
• Posts: 2
• Mole Snacks: +0/-0
##### Combined gas law
« on: May 21, 2020, 12:49:36 AM »
Can someone explain why p1= 10 atm and t1= 546k? Shouldn't these be STP? Because the way they set up the equation it means that the increased pressure increases the volume but shouldn't increased pressure always lower the volume? and shouldn't increased temperature increase the volume? Flipping the numbers changes the answer.

https://imgur.com/a/3wlyXQ0

This problem is looking for density but aren't the ideas supposed to be the same? As pressure inc, volume decrease.

https://imgur.com/a/50Zg5Q9

#### AWK

• Retired Staff
• Sr. Member
• Posts: 7331
• Mole Snacks: +515/-86
• Gender:
##### Re: Combined gas law
« Reply #1 on: May 21, 2020, 02:46:25 AM »
The combined gas law contains 3 gas parameters: p, V, and T, but this does not mean that you have to use all of them in the calculations. After all, from Washington to New York you don't necessarily have to go through Tokyo. If any of the gas parameters are identical on both sides of the equation, it can be eliminated by reducing the combined gas law to the previous one (Amontons, Gay-Lussac, or Charles). These are historical laws that you should know about, but all tasks of this type should be calculated using the ideal gas equation that allows you to take into account different amounts of different gases (masses, moles). And at all, for these calculations, it is not necessary to know the value of the gas constant R in any units (we compare two different states of the same or different gases by dividing by sides both equations written for these states).
AWK

#### Borek

• Mr. pH
• Deity Member
• Posts: 25784
• Mole Snacks: +1686/-400
• Gender:
• I am known to be occasionally wrong.
##### Re: Combined gas law
« Reply #2 on: May 21, 2020, 04:01:12 AM »
Pressure of 10 atm was just given in the problem.

No idea where they got 546 K from, IMO should be 273 K in both questions.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### pm133

• Regular Member
• Posts: 47
• Mole Snacks: +5/-0
##### Re: Combined gas law
« Reply #3 on: May 21, 2020, 09:11:57 AM »
546K comes from the question which has a starting temp of 273 degrees Celsius. A nice wee "gotcha" there from the questioner.

#### Borek

• Mr. pH
• Deity Member
• Posts: 25784
• Mole Snacks: +1686/-400
• Gender:
• I am known to be occasionally wrong.
##### Re: Combined gas law
« Reply #4 on: May 21, 2020, 11:49:26 AM »
546K comes from the question which has a starting temp of 273 degrees Celsius. A nice wee "gotcha" there from the questioner.

Nice, good catch
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### AWK

• Retired Staff
• Sr. Member
• Posts: 7331
• Mole Snacks: +515/-86
• Gender:
##### Re: Combined gas law
« Reply #5 on: May 21, 2020, 12:16:35 PM »
Both examples come from MCAT General Chemistry Review 2018-2019: Online + Book, Volume 4