K and PV=nRT

$PV=nRT$

Kayla Booker 1F
Posts: 100
Joined: Wed Sep 30, 2020 9:43 pm

K and PV=nRT

How do you know whether to use K=[P]/[R] or PV=nRT in problems involving partial pressure, because there were problems on the modules that used partial pressure, but K=[P]/[R] was used.

Andrew Wang 1C
Posts: 101
Joined: Wed Sep 30, 2020 10:11 pm
Been upvoted: 5 times

Re: K and PV=nRT

You would use PV=nRT when you are converting from partial pressure to concentration, or vice versa. So if the problem asks for Kc, but gives you a measurement in atm, you would use PV=nRT to convert that value to the concentration and then plug it into Kc=[P]/[R].

For problems that ask for Kp, you'd plug in the pressure of each reactant/product instead of their concentrations; the overall formula of K=P/R is the same.

Posts: 173
Joined: Wed Sep 30, 2020 9:32 pm
Been upvoted: 5 times

Re: K and PV=nRT

The ideal gas law, PV=nRT is only used to convert from pressure to concentration, or vice versa. You always use K=[P]/[R] to find the equilibrium constant, but if you were given some values of the products or reactants as concentrations and other values as pressures, you can use the ideal gas law to convert all the values into the same units, and then put them into the equation for the equilibrium constant. For a question involving partial pressures, you would rearrange PV=nRT to P=(nRT)/V, and use that equation to convert any values given as concentrations into pressures, which you would plug into K=[P]/[R].

Hope this helps!

Annika Tamaki 1E
Posts: 103
Joined: Wed Sep 30, 2020 10:03 pm
Been upvoted: 1 time

Re: K and PV=nRT

You can use PV=nRT to convert the atm to concentration. PV=nRT can be written as P=nRT/V. n/V=concentration, so P=conc(RT). Then, you can solve for concentration and then plug that value into K=[P]/[R].

Emmeline Phu 1G
Posts: 122
Joined: Wed Sep 30, 2020 9:52 pm
Been upvoted: 2 times

Re: K and PV=nRT

Hi! As mentioned in the lecture, the ideal gas law, PV=nRT is used to calculate either partial pressure given the concentration or calculate concentration given the partial pressure. We use K= [P]/[R] in cases where we need to calculate the equilibrium constant given one or all of the equilibrium concentrations of the reaction or to calculate the equilibrium concentrations of one of the reactants or products given the equilibrium constant and other information regarding the initial concentrations in the reaction. Hope this helps! :)

Karl Yost 1L
Posts: 179
Joined: Thu Dec 17, 2020 12:19 am

Re: K and PV=nRT

It depends on how the question is phrased. If the question gives a value of Kc or asks for Kc and you are given partial pressures (in atm.), you will have to convert the partial pressures to concentrations using the ideal gas law. If, on the other hand, the questions gives a value of Kp or asks for Kp and you have partial pressure values, there will be no conversion necessary and you can just use Kp=P(products)/P(reactants).

Phoebe Joseph
Posts: 58
Joined: Tue Nov 17, 2020 12:19 am

Re: K and PV=nRT

Also, he said you generally use Partial Pressure for gases, so I think it will help to know that in a lot of problems.

Kiyoka Kim 3C
Posts: 108
Joined: Wed Sep 30, 2020 9:59 pm

Re: K and PV=nRT

I think it would depend on the problem and you would use the necessary conversions to get the answer you need. For instance, if you are asked to find the partial pressure of the product given its equilibrium concentration, you would use PV=nRT to convert concentration to pressure.

Sabrina Galvan 3J
Posts: 100
Joined: Wed Sep 30, 2020 9:35 pm
Been upvoted: 1 time

Re: K and PV=nRT

Partial pressures are mainly used when the products and reactants are both gases. Therefore, if a problem gives you their concentrations, you would use PV=nRT, and manipulate the equation to give you P=(n/V)RT, where n/V is equal to the concentration. After calculating their pressures, you would then plug them in with the K=[P]/[R], including the stoichiometric exponents.

Lilly Catarozoli 1B
Posts: 119
Joined: Wed Sep 30, 2020 9:53 pm

Re: K and PV=nRT

It depends on what the problem asks for! Use PV=nRT when you are converting from partial pressure to concentration or the opposite. For problems that ask for Kp, you'd plug in the pressure of each reactant/product instead of their concentrations. Hope this helped!