Determining pH

Moderators: Chem_Mod, Chem_Admin

Bronson Mathos 1H
Posts: 94
Joined: Wed Sep 30, 2020 9:36 pm

Determining pH

Postby Bronson Mathos 1H » Sun Jan 24, 2021 6:24 am

Hello, I am a bit lost on determining the pH of acids and bases and was wondering when one knows that taking the -log will wield the pH or the pOH?

Shana Patel 1C
Posts: 96
Joined: Wed Sep 30, 2020 9:36 pm

Re: Determining pH

Postby Shana Patel 1C » Sun Jan 24, 2021 6:28 am

Taking the -log[H+] will give you the pH. After finding the pH, subtract the number from 14 to give you pOH.
pH + pOH = 14

Keshav Patel 14B 2B
Posts: 94
Joined: Wed Sep 30, 2020 9:40 pm

Re: Determining pH

Postby Keshav Patel 14B 2B » Sun Jan 24, 2021 8:11 am

You have to do the -log of the concentrations of OH- and H30+ to find the pH or pOH.

Alisa Nagashima 1B
Posts: 40
Joined: Tue Nov 17, 2020 12:19 am

Re: Determining pH

Postby Alisa Nagashima 1B » Sun Jan 24, 2021 9:22 am

pH = -log[H+]
pOH = -log[OH-]

It really depends on what concentrations you are taking the log of. hope this helps!

Javier Perez M 1H
Posts: 80
Joined: Wed Sep 30, 2020 9:52 pm

Re: Determining pH

Postby Javier Perez M 1H » Sun Jan 24, 2021 9:43 am

-log(x) function is used to find both the Ph and Poh given the concentrations of OH- or H+. These concentrations are plugged in as x. However, if you are solving and have a Ka value then you cannot directly determine the Poh, you must calculate Ph then -14 and you'll get your Poh.

Kamille Kibria 2A
Posts: 90
Joined: Wed Sep 30, 2020 9:52 pm

Re: Determining pH

Postby Kamille Kibria 2A » Sun Jan 24, 2021 12:59 pm

pH = -loh[H+] and pOH = -log[OH-]

however in the problems that we have been given in the hw and stuff, you usually have to make an ICE table to get to that answer. you are given the Ka or Kb or you would have to figure it our using a chart or the Kw value (1E-14). then you find the initial, change, and equilibrium concentrations of everything to solve for x. then you plug x into -log(x) to get either the pH or pOH

Ryan_Kien_1L
Posts: 92
Joined: Wed Sep 30, 2020 9:42 pm
Been upvoted: 1 time

Re: Determining pH

Postby Ryan_Kien_1L » Sun Jan 24, 2021 1:12 pm

-log of H+ concentrations yields pH. -log of OH- concentration yields OH-.

t_rasul2I
Posts: 98
Joined: Wed Sep 30, 2020 9:38 pm

Re: Determining pH

Postby t_rasul2I » Sun Jan 24, 2021 1:24 pm

like people have answered, you take the -log of the H+. However, to make it easier and know when to use pOH and pH, convert everything to the ka, pka, or pH and convert the answer after you have solved for the problem (assuming the problem doesn't have any intermediate steps requiring pOH)

Shalyn Kelly 3H
Posts: 84
Joined: Wed Sep 30, 2020 9:55 pm

Re: Determining pH

Postby Shalyn Kelly 3H » Sun Jan 24, 2021 1:49 pm

Hi! For -log just match pH or pOH to what you are taking the -log of.
pH = -log [H+]
pOH = -log [OH-]
Also keep in mind as mentioned above that pH + pOH = 14

Jenaye Brelland 2I
Posts: 95
Joined: Wed Sep 30, 2020 9:35 pm

Re: Determining pH

Postby Jenaye Brelland 2I » Sun Jan 24, 2021 3:53 pm

The -log[H+] will result in the pH. The -log[OH-] will result in the pOH. Remember that p in pH means -log. So pH is -logH+ and pOH is -logOH-.

Julianna_flores3E
Posts: 91
Joined: Wed Sep 30, 2020 9:52 pm

Re: Determining pH

Postby Julianna_flores3E » Sun Jan 24, 2021 3:58 pm

Yes taking the -log of [OH-] and [H3O+] will result in the pH and pOH. Likewise, if you have the pH or pOH and need to find the concentrations of either you would put the pH or pOH as the negative exponent of 10. In some of the Hw for week 2 this could have been useful. Hope this helps.

Ethan Goode 2H
Posts: 129
Joined: Wed Sep 30, 2020 9:51 pm

Re: Determining pH

Postby Ethan Goode 2H » Sun Jan 24, 2021 3:59 pm

It depends what you are taking the log of. If it is the concentration of hydronium ions, then it is pH, and if it of hydroxide ions it is pOH.

haley f
Posts: 92
Joined: Wed Sep 30, 2020 9:49 pm

Re: Determining pH

Postby haley f » Sun Jan 24, 2021 6:48 pm

If you take the -log of the concentration of the acid [H+], then it will yield pH. If you take the -log of the concentration of the base [OH-] then it will yield the pOH of the solution.

Tatyana Bonnet 2H
Posts: 92
Joined: Wed Sep 30, 2020 9:48 pm

Re: Determining pH

Postby Tatyana Bonnet 2H » Sun Jan 24, 2021 6:52 pm

The -log of the H+ concentration will give you the pH and the -log of OH- gives you the pOH. If you wanted to get the concentration from the pH or pOH you would do 10 to the power of negative pH or the pOH. Just remember the pH is from the concentration of H+ and and pOH from OH-, the same letters correspond. p just means to take the -log of something, like pKa is -log of Ka.

Chinmayi Mutyala 3H
Posts: 95
Joined: Wed Sep 30, 2020 9:50 pm

Re: Determining pH

Postby Chinmayi Mutyala 3H » Sun Jan 24, 2021 11:33 pm

If you take the -log of the hydronium ion concentration (H3O+, or H+), you will get pH and if you take the -log of the hydroxide ion concentration (OH-), you will get pOH.

LovepreetSran_3H
Posts: 76
Joined: Wed Sep 30, 2020 9:33 pm

Re: Determining pH

Postby LovepreetSran_3H » Sun Jan 24, 2021 11:36 pm

The -log of [H+] gives pH and the -log of [OH-] gives the pOH

Gicelle Rubin 1E
Posts: 81
Joined: Fri Oct 02, 2020 12:16 am

Re: Determining pH

Postby Gicelle Rubin 1E » Sun Jan 24, 2021 11:46 pm

Jenaye Brelland 2I wrote:The -log[H+] will result in the pH. The -log[OH-] will result in the pOH. Remember that p in pH means -log. So pH is -logH+ and pOH is -logOH-.


Hi Jenaye! I'm glad to see you explanation written out! It serves as a little reminder :)

Sam Wentzel 1F 14B
Posts: 129
Joined: Wed Sep 30, 2020 9:50 pm
Been upvoted: 1 time

Re: Determining pH

Postby Sam Wentzel 1F 14B » Sun Jan 24, 2021 11:48 pm

PH = - log [H3O+] or -log[H+]
POH = -log[OH-]
14 = PH+POH (you can use this if you've found the PH of an Acid, or POH of a base, but not vice versa).

Lilly Catarozoli 1B
Posts: 109
Joined: Wed Sep 30, 2020 9:53 pm

Re: Determining pH

Postby Lilly Catarozoli 1B » Mon Jan 25, 2021 12:19 am

Use -log[H+] to get the pH and -log[OH-] for the pOH. You can also subtract from 14 to find one or the other!


Return to “Applying Le Chatelier's Principle to Changes in Chemical & Physical Conditions”

Who is online

Users browsing this forum: No registered users and 1 guest