How do i do this with out a calculator?

[hoylematt]

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What is the final pressure of a gas that expands from 2.0L at 10 degrees celcius to 20L at 100 degrees celcius, if its pressure was initially 6atm?

I understand how to set it up and I did so in about 10 seconds, then I look at the answer choices and see that holy crap they actually want me to calculate it and it has to be close. (as in rounding will own your face)

Please teach me how to do the arithmetic like this in my head. 😡

BTW the setup is:
[(2.0)(6.0)]/283 = [P2(20)]/373

 

[Chemboy23]

Same prob here man, I hate doing these things w/o a calculator. The acid/base part get even better (and the Ksp)...oh the joy these things bring to my life 🙂

Btw, someone told me they would actually have it set up for you on the real test (give you the setup and not necessarily the exact answer). We can only hope so!

Good luck!

 

[dwallin]

In stead of trying to "do the whole thing in your head" get it into the form where you would punch it into your calculator and just look for things to simplify, like 2/20 can reduce to 1/10 then you still have 6/10 on the top, that reduces to 3/5, as well you will have 373/273, that reduces to roughly 37/27 and you can get a rough idea that that's 12/9 which reduces to 4/3, so you're left with 3/5 * 4/3 = 12/15 = 4/5 = .8 roughly, does that make sense?

 

[hoylematt]

That was a sick fraction attack. I guess I see how that can work out. I get so frustrated knowing how to set up a problem and then looking around frantically for my calculator because i have always relied on it.

Sad day.

 

[ToothTickler]

On my DAT, the answer choices were all in exact answers (meaning it was in fraction set up format). But yeah, trying to approximate these things is a pain. I say practice practice practice doing fractions like dwallin posted. If anything it will help on the QR.

 

[DQLEUCD]

What is the final pressure of a gas that expands from 2.0L at 10 degrees celcius to 20L at 100 degrees celcius, if its pressure was initially 6atm?

I understand how to set it up and I did so in about 10 seconds, then I look at the answer choices and see that holy crap they actually want me to calculate it and it has to be close. (as in rounding will own your face)

Please teach me how to do the arithmetic like this in my head. 😡

BTW the setup is:
[(2.0)(6.0)]/283 = [P2(20)]/373

First you do this (6 atm/10 (factor of how much volume went up) x (373/283) (373/283 is roughly 4/3 so u can do it easier). You then just find the closest number to it so answer should be about 4/5 or .8 ATM. Actually answer is .79 ATM which is really close. On the DAT's they will not give you big numbers to calculate. Just simply each into fractions and round to the nearest whole number.