In this video, we're gonna do a quick review on pH. So I'm sure most of you guys remember from your previous chemistry courses, that the pH is directly related to the H+ ion or the proton concentration in a solution. But why do we even care about the proton concentration? Well, it turns out the proton concentration is super important to biochemists because many enzymes and biochemical processes are strongly affected by the concentration of protons. And so, if we change the concentration of protons, then we can potentially affect the enzymes activity, and that can affect the biochemical processes. So biochemists need a way to measure the concentration of protons in a solution. And that's exactly where the pH comes into play, because the pH is literally a logarithmic measurement of the H+ ion concentration in a solution. And the pH also indirectly measures the Oh minus ion concentration, and we can see that pretty clearly when we recall the ion constant of water, or k w, from our previous lesson videos, which we know is equal to the H+ ion concentration times the Oh minus ion concentration. And so if we use pH to calculate the H+ ion concentration, then we can use the k w to calculate the Oh minus ion concentration. And so you can see how calculating H+ through pH is also an indirect measurement of calculating the Oh- concentration. And so pH is also mathematically defined as the negative logarithm of the H+ ion concentration. And so down below in our equation, you can see that the pH is indeed equal to the negative logarithm of the H+ ion concentration. And the rules of logarithms say that the negative log is equal to the positive log of the reciprocal. And so it's good to be able to recognize both of these equations here in case your professor is leaning towards 1 or the other. And so one way that helps me remember the equation for pH is that I know that this p here really just represents the negative log. And so when I think of pH, I know that it's literally the negative log of H, or the negative log of the H+ ion concentration. And so hopefully, that'll help you guys remember it as well. Now, down below in our example, it's asking us to determine the pH of a solution with an H+ ion concentration of 0.04 molar. And so this is a pretty straightforward example. All we need to do is plug our H+ ion concentration into our equation up above. And so we know that the pH is literally equal to the negative log of the H+ ion concentration, which is given to us as 0 0.04. And so if you take your calculators and you do the negative log of 0.04, you'll get an answer of about 1.4. And so, this answer here matches with answer option D. And so we can go ahead and indicate that D here is correct, and all of the other answer options are incorrect. So this is a good review of pH, and we'll be able to get some practice utilizing these concepts of calculating pH in our next practice video. So I'll see you guys there.
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Problem
Problem
Determine the pH of a solution with a [H+] of 2 x 10-5 M.
A
pH = 2.2
B
pH = 4.7
C
pH = 10.9
D
pH = 6.1
3
concept
pH Scale
Video duration:
4m
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Video transcript
So I'm sure you remember from your previous chemistry courses that the pH scale ranges from 0 to 14. And, really, the pH scale has 3 sections to it: neutral, acidic, and basic. And so neutral solutions, recall, have a pH that is equal to 7. And so at a pH of 7, the concentration of hydrogen ions is equal to the concentration of hydroxide ions. And so acidic solutions have a pH solution, have a pH value that is less than 7. And so you can see that here, pH less than 7. And so when the pH is less than 7, the concentration of, H plus ions is greater than the concentration of Oh- ions. And then finally, basic solutions are solutions that have a pH greater than 7. And so you can see here pH greater than 7 indicates basic solutions. So basic solutions have a hydrogen ion concentration, less than the hydrogen, the hydroxide ion concentration. So let's take a look at our example below to refresh our memories on some of this. And notice that the pH scale here has a neutral, section here where the pH is equal to 7. And notice that the concentration of, H plus ions is equal to the concentration of Oh minus ions. And then you can see as the pH starts to decrease, the concentration of H plus increases and the concentration of Oh plus concentration of H plus starts to decrease and the concentration of Oh minus starts to increase. And so what's true here in every situation is k w. And recall that k w is equal to the concentration of H plus times the concentration of o h minus. And we know that at 25 degrees Celsius, which is the temperature that we assume biological systems are at, kw will always be equal to 1 times 10 to the negative 14th molar squared. So no matter, what you plug in for these values at any pH, you will always get 1 times 10 to the negative 14 at, temperature of 25 degrees Celsius. So here, we've put in some examples. I looked up the pH of lemon juice, and lemon juice has a pH of about, 2. And so you can see that here. Now neutral, pure water, of course, has a pH that is equal to 7, so it is neutral. And then, bleach, I looked up, has a pH that is equal to 12. And so, one thing that's interesting to note is that we see that, hydrogen concentration 0. And so the question is, can you have a concentration of hydrogen ions greater than 1? And the answer to that is, of course you can. However, what's gonna happen is, if you have a concentration of hydrogen ion concentration greater than 1, then what'll happen is you're gonna extend outside of the pH range, so you'll you'll have a negative pH value. And so the same applies for Oh minus. If you have a concentration of Oh minus greater than 1, you'll extend beyond the pH range here. And so, H plus or Oh minus concentrations greater than 1 will give you pH values that lie outside the normal one to 14 range. And these values are much harder to measure, So they're much more difficult to measure and on top of that, we don't really have to worry about that so much because in biological systems and biological solutions, the pH usually always ranges in that normal range from 0 to 14. So we don't have to worry about negative pHs or pHs greater than 14, which is nice. And so this, ends our lesson on the pH scale and I'll see you guys in some of our practice our next practice videos.
4
Problem
Problem
Determine the pH of a solution with a [OH-] of 3 x 10-4 M. Is the solution basic, acidic or neutral?