How to do gas law problems without calculator (ex. Dr.Collin's packet)

[believee]

I was just going through Dr.Collin's chemistry packet and realized that it's almost impossible for me to do the gas law problem (PV=nRT) without using calculator.

Please refer to 5-2 and 5-6 for the problems
(I am not sure if I am allowed to post the problems here, we basically have to solve for the moles, using n=PV/RT). The numbers don't work out nicely due to R value.

1) Do they give you value of R with unit on PCAT? Or do we have to memorize this (0.082L*atm/mol*K)?
2) How do you do these without using calculator? They usually go on to decimals, thanks to the R value..

---

Members don't see this ad.

 

[willgotoapharmschool]

I am pretty sure we need to memorize that constant! 🙂 I don't have the packet with me right now, so I have no clue what question you are referring to! but I sometimes use 82/1000 or 1/12, which is about 0.0833333...

 

[believee]

I will post a similar question (not worded similarly, but conceptual wise) to the one on the Collin's:

Q. Given V=1000mL, P=3atm, T=298K, find the number of moles.

 

[believee]

I am pretty sure we need to memorize that constant! 🙂 I don't have the packet with me right now, so I have no clue what question you are referring to! but I sometimes use 82/1000 or 1/12, which is about 0.0833333...

Wow thank you!! 🙂 1/12 would make it a lot better, although it will still take me a bit to do the division in my head. ahh I am so used to using calculators esp. for divisions

 

[WSUWarrior012]

I have a little trick. I use .08 instead of .08222. What I do is if I see that I have .08*300, I say that its 24 since 8 % of each hundred adds up to it. It makes your life so much easier and saves plenty of time!

 

[believee]

I have a little trick. I use .08 instead of .08222. What I do is if I see that I have .08*300, I say that its 24 since 8 % of each hundred adds up to it. It makes your life so much easier and saves plenty of time!

Wow so I just round 0.082 to 0.082 and 298K to 300K?? That's really clever, as I just need to move the decimal places around and multiply the two numbers as if I am multiplying single digit #s. Thank you sooo much!!!

 

[facio]

thanks for the tips. that seems to be really smart thing to do.

 

[nxq875]

typically I just round the 0.08333 to 0.1
from what I've seen, the answer choices are not so close to each other than the rough rounding wouldn't give you a definitive answer

 

[facio]

I think the 8% trick or 0.08 rounding up by WSUWarrior012 would be simple enough for quick calculation and still close to the ansewer which gives you that confidence. Thanks again, WSUWarrior012.

 

[blueheron]

typically I just round the 0.08333 to 0.1
from what I've seen, the answer choices are not so close to each other than the rough rounding wouldn't give you a definitive answer

I did this. Got me a 97. When you are running out of time in the math section you because you did it with .08 don't complain. Like stated the answer choices aren't that close.