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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Frederick95 on March 14, 2008, 12:05:12 AM

Title: PV = nRT: Which universal gas constant to use?
Post by: Frederick95 on March 14, 2008, 12:05:12 AM

For the following information I have of my lab, when I use the formula PV=nRT, which value for R(universal gas constant) should I use?
8.314472 J·K-1·mol-1     or    0.0820574587 L·atm·K-1·mol-1?

Why is this?


The Lab:
I have collected oxygen gas by the downward displacement of water (results are in the last column).  This oxygen was produced in the first 60 s of the reaction.

Calculate the number of mols of oxygen gas produced per second
Remember PV=nRT and that you must account for the partial pressure of water vapour

The reactions conditions are:
Atmospheric pressue:102.6 kPa;
Temperature:290 K
Partial pressure of water vapour:1.94 kPa.

The reaction is:
Volume of 6% H2O2 (mL):10.0
Volume of distilled water (mL):0.0
Volume of 1.0 M NaI (mL):10.0
Volume of distilled water (mL):0.0
O2 production in 60 s (mL):151.0

Title: Re: PV = nRT: Which universal gas constant to use?
Post by: Lcal on March 14, 2008, 12:25:14 AM

general rule for figuring out what R constant to use is by setting up your equation for what you want to solve for and plugging in only units and cancel...pick the R constant that cancels out units so you're left with the ones you want...

(the difference between the two R values you listed is one is in J/(mol k) and the other is in L atm/(mol k)  so for example if you want your units in J per K as in a heat capacity you use the first r...if you want your answer in units of just L or K or atm you can use the second one)

Title: Re: PV = nRT: Which universal gas constant to use?
Post by: AWK on March 14, 2008, 02:04:23 AM

look at:
http://en.wikipedia.org/wiki/Gas_constant