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pH and pOH 18,271 views

Teacher/Instructor Jacqueline Spivey
Jacqueline Spivey

Ph.D.,U.C.Santa Cruz
Teaching at a top-ranked high school in SF

She teaches general and chemistry at a top-ranked high school in San Francisco. Prior to that, she lead and published a number of research studies and lectured at SF State University.

pH and pOH denote the negative log of the concentration of hydrogen or hydroxide ions. High pH means that a solution is basic while high pOH means that a solution is acidic. Neutral solutions have pH and pOH of 7.

Hi guys. So lets go ahead and talk about pH and pOH so the most simple thing that you need to know first is that p just means the negative log of a quantity so here we're saying p is equal to negative log of hydrogen ion concentration and here with pOH, p is equal to negative log of the hydroxide ion concentration so let's talk about acids first and so remember that acids increase the concentration of protons in a solution. Okay so something else to know is that in solutions the molar concentration so the concentration of moles per liter in an aqueous solution is really small so that's why we actually use the logs scale so that we can discuss orders of magnitude, changes between various solutions.

So here the basic definition of pH is that it's equal to negative log base 10 of the concentration of protons in your solution. You can also describe the acidity or basicity of your solution based on the concentration of the hydroxide ions so then in that case it would be the pOH is equal to negative log of the concentration of hydroxide ions and you would use on of the other base on which information is given to you in a problem that you're trying to solve. So let's do the most basic example so let's calculate the pH of a neutral solution at 25 degrees C say so if something is neutral what does that mean? So we'll see in a second it means that you have a pH of 7.

So something else that you should refer back to the equilibrium section in order to give the information for is that at pH 7 the concentration of protons is equal to 1.0 times 10 to the -7 molar or moles per liter. And that comes from a fact that the disassociation the equilibrium concept for water is equal to 1.0 times 10 to the -14 and that's equal to the concentration of protons times the concentration of hydroxide ions so that's where the 1 times 10 to the minus seventh value comes from. So knowing that this is the concentration of protons in solution for a neutral solution, we can plug it into our simple equation and get the, the pH is equal to the minus log of 1 times 10 to the minus seventh which means we have negative of a negative 7 which gives us a positive 7 so pH of a neutral solution is pH 7.

So then what happens to the pH of a solution as it's made more acidic? So we just calculate it that a neutral solution has a concentration of protons that is 1.0 times 10 to the minus seventh so if something is being made more acidic meaning that the concentration of protons is increasing that means when the concentration of protons increases past 1.0 times 10 to the minus seventh that since the minus pH is equal to minus the log of a concentration of protons that as the pH goes down the concentration of protons will go up or vice versa. I think I said it the first way before so basically your pH will go down so since neutral pH is pH 7 once you increase the concentration of protons you can expect your pH to go down so here is a very simple chart outlining what it means for something to be acidic, neutral and basic based on the proton concentration in molars concentration or moles per liter the same thing for hydroxide concentrations and then the corresponding pH value so if something is acidic that means that the proton concentration is higher than 1.0 times 10 to the minus seventh or that the hydroxide concentration is lower than 1.0 times 10 to the minus seventh and that the pH value is less than 7 so between 1 and 7. If its neutral basically the concentration of both protons and hydroxide ions are equal like they balance each other out so they're both that 1.0 times 10 to the minus seventh and your pH value is equal to 7.

If you have a basic solution it means that your concentration of protons is less than 1.0 times 10 to the minus seventh and hydroxide concentration is greater than 1.0 times 10 to the minus seventh and your pH value is greater than 7 usually between 7 and 14, the higher the value, the more basic your solution.

Again in the beginning I noted that we use a log scale because of the changes in magnitude by protons and so it's important to know that a change in concentration of protons by a factor of 10 only causes the pH to change by 1 so that a solution with a pH of 6 has 10 times the proton concentration as a solution of pH 7 and so that's why we use the log base. And that is pH and pOH.