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Tips for Calculating pH and pOH and More - Concept

Teacher/Instructor Jonathan Fong
Jonathan Fong

M.Ed.,San Francisco State Univ.

Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.

Here are tips and tricks for calculating pH, and pOH and more. So there are 6 equations or 6 formulas that will make your life easier for simple calculations of pH, and pOH.

So equation number 1; pH equals negative log the concentration of H+. So the negative log means the concentration of H+. It could be Hydronium or H+ that you have there. So that's number 1, remember P is negative log.

Number two; pOH equals negative log of OH-. So that should make it pretty easy. Number 3; this is actually the reverse of number 1. I want to get the Hydrogen ion concentration from the pH. All I do is I take 10 to -pH power and that's how I get the Hydrogen ion concentration. Remember, the units would be in Molar for their concentration, don't forget. Remember pH has no units.

Number 4 is actually the opposite of number 2. So if I want the Hydroxide ion concentration, 10 to the -pOH. Remember Hydroxide ion concentration would have a unit molar. pOH no units.

Number 5 is related to this. Hydrogen ion concentration times Hydroxide ion concentration equals 1 times 10 to the -14 at 25 degrees Celsius. Now this is also known as Kw, or W for water. This would be the product constant for water. So [H+] times [OH-] equals 1 times 10 to the -14.

Then number 6; pH plus pOH will equal 14. That's why at pH 7, pOH would be equal to 7. So with these six equations, it will make your life easier for calculating pH, pOH and anything else that your teacher might throw at you, or the book.

Then, a couple of other things. So question; acidic, neutral, basic. This simple chart will help you figure out if something is acidic, neutral or basic. So let's make a little chart. PH, pOH. Neutral pH 7. pOH has to be 7, because 7 equals 14. So that's easy. pH basic, anything greater than 7 is basic. So like the number 9 could be bigger than 7, so that's basic pH.

Now basic pOH needs to be less than 7, because remember, if I want to get 14 from 9, then it be like 5. So 5 is less than 7. So don't get tripped up. So pH greater than 7, basic. pOH less than 7 would be basic also, because all those would add up to 14.

Last but not least acidic. Acidic pH less than 7. So like maybe here 1. pOH would have to be greater than 7. So if this is 1 for pH, pOH would have to equal 13, so that's greater than 7. So 1 plus 13 is 14.

Also a couple of things. pH is less than 7. So for acidic it's not between 0 and 7 non-inclusive, it's actually less than 7. So you can actually have -pH, and that's okay. For basic pH, you can have above 14, where something is very basic, highly concentrated with Hydroxide. So you might have a pH of 15, or something higher than 14. So that's also possible, just in case.

So just to summarize, hopefully this chart; pH less than 7, neutral 7, and then pH greater than 7 is basic. For pOH it's just the opposite. It's flipped, because remember, they need to add up to 14.

So hopefully these 6 equations, and this little mini-chart about acidic, neutral or basic will help you when you're calculating pH, and pOH. So hopefully this helps you understand that. Have a good one.