### PV=nRT

Posted:

**Mon Jan 13, 2020 10:27 am**When do we use the equation PV=nRT? And how would we use it?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=50&t=56183

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Posted: **Mon Jan 13, 2020 10:27 am**

When do we use the equation PV=nRT? And how would we use it?

Posted: **Mon Jan 13, 2020 10:32 am**

Sam McNeill 1E wrote:When do we use the equation PV=nRT? And how would we use it?

You would use this equation when you are trying to convert between partial pressure and concentration. Let's say, for example, you were given the Kc of a reaction and the partial pressure of some of the reactants. Since you were given the partial pressure, you must use the ideal gas equation to convert pressure into concentration. This would allow for you to find the concentration of the unknown product or reactant (whatever is not given in the problem).

Posted: **Mon Jan 13, 2020 10:34 am**

For our purposes, we use the Ideal Gas Equation to convert between partial pressures and concentrations so that we can calculate equilibrium constants with consistent units. You would use it by rearranging the equation, resulting in P=(n/V)RT where we know that (n/V)=concentration. So, you would now use P=(concentration)RT to plug in your values and convert between partial pressures and concentrations.

Posted: **Mon Jan 13, 2020 12:21 pm**

have we gone over this in class in an example equation?

Posted: **Mon Jan 13, 2020 12:24 pm**

Concentration of gases would be one of the only reasons we would use the ideal gas law at this point of the class.

Posted: **Mon Jan 13, 2020 12:25 pm**

805097738 wrote:have we gone over this in class in an example equation?

not really; lavelle primarily showed us pv=nrt and how we can get the calculation for concentration (concentration=p/rt) from the equation. there are a lot of practice problems in the textbook that are about going from partial pressure (p) and using that to find concentration with pv=nrt, then finding the equilibrium.

Posted: **Mon Jan 13, 2020 12:26 pm**

You can use it to convert between partial pressure and concentration.

Posted: **Mon Jan 13, 2020 4:52 pm**

PV=nRT

This equation is used to convert between partial pressure of a gvien substance and the concentration of this same substance. This would be useful if we were given Kc and the partial pressures of certain substances and asked to solve for the unknown concentration of some substance. This is now it works:

PV=nRT can be rewritten as P=(nRT)/V. Then, n/V is the ratio of units that refers to concentration (moles per Liter). Thus, P=(nRT)/V can be rewritten as P=(concentration)(RT). Furthermore, we can solve for (concentration) and get (concentration)=P/RT. If given a temperature and a pressure, you can calculate the concentration of a substance.

This equation is used to convert between partial pressure of a gvien substance and the concentration of this same substance. This would be useful if we were given Kc and the partial pressures of certain substances and asked to solve for the unknown concentration of some substance. This is now it works:

PV=nRT can be rewritten as P=(nRT)/V. Then, n/V is the ratio of units that refers to concentration (moles per Liter). Thus, P=(nRT)/V can be rewritten as P=(concentration)(RT). Furthermore, we can solve for (concentration) and get (concentration)=P/RT. If given a temperature and a pressure, you can calculate the concentration of a substance.

Posted: **Mon Jan 13, 2020 4:56 pm**

this is the ideal gas law and you can use it to convert between partial pressure and concentration. concentration=P/RT

Posted: **Mon Jan 13, 2020 5:44 pm**

Sam McNeill 1E wrote:When do we use the equation PV=nRT? And how would we use it?

In the context of what we learned, we use the PV=nRT equation to convert between pressure and molarity (and vice versa). In this class, we use this equation to convert between Kp and Kc. For example, if the question gives you concentrations and asks for Kp, you can use PV = nRT to convert the concentrations to pressures, and then use the pressures to find Kp.