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2. Water

1

Calculating pH

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in this video, we're going to 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 plus ions or the proton concentration and 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 biochemist 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 log arrhythmic measurement of the H plus ion concentration and a solution, and the pH also indirectly measures the O. H. 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 plus ion concentration times, the O. H. Minus ion concentration. And so, if we use pH to calculate the H plus ion concentration. Then we can use the K W to calculate the O. H minus ion concentration. And so you can see how calculating H plus through pH is also an indirect measurement of calculating the O. H minus concentration. And so pH is also mathematically defined as the negative log, a rhythm of the H plus ion concentration and so down below. In our equation, you can see that the pH is indeed equal to the negative log. A rhythm of the H plus 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 professors leaning towards one 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 plus ion concentration. And so hopefully that will help you guys remember it as well. Now down below, in our example is asking us to determine the pH of a solution with an H plus ion concentration of 0.4 Mueller. And so this is a pretty straightforward example. All we need to dio is plug our H plus ion concentration into our equation up above. And so we know that the pH is literally equal to the negative log of the H plus ion concentration, which is given to us as 0. And so, if you take your calculators and you do the negative log of 0.4 you'll get an answer of about 1.4. And so this answer here matches with answer Option D, and so we could 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

2

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

pH Scale

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so I'm sure you remember from your previous chemistry courses at the pH scale ranges from 0 to 14 and really, the pH scale has three sections to it. Neutral, acidic and basic. And so neutral. Solutions Recall Have a pH that is equal to seven. And so, at a Ph of seven, 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 seven, and so you can see that here. Ph less than seven. And so when the pH is less than seven, the concentration of H plus ions is greater than the concentration of O. H minus ions. And then finally, basic solutions are solutions that have a pH greater than seven. And so you can see here pH greater than seven indicates basic solutions, so basic solutions have ah, 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 P H is equal to seven and notice that the concentration of H plus ions is equal to the concentration of O H minus ions, and then you can see as the pH starts to decrease the concentration of H plus increases and the concentration of O H minus decreases. And then, as the pH increases, the reverse will happen. The concentration of H plus starts to decrease, and the concentration of O H 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, air at K W will always be equal to one times 10 to the negative. 14th Moeller squared. So no matter what you plug in for these values at any pH, uh, you will always get one times 10 to the negative 14 at a temperature of 25 degrees Celsius. So here we put in some examples, I looked up the pH of lemon juice and lemon juice has a pH of about two, and so you can see that here now. Neutral. Uh, pure water, of course, has a pH. That's equal Thio seven. 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 of one is equal to zero a ph of zero. And so the question is, can you have a concentration of hydrogen ions greater than one? 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 one, then what will happen is you're gonna extend outside of the ph range, so you you'll have a negative pH value. And so the same applies for O H minus. If you have a concentration of O. H minus greater than one, you'll extend beyond the pH range here. And so H plus R O H. Minus concentrations greater than one will give you ah pH values that lie outside the normal 1 to 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. Ph S or pH is 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

Determine the pH of a solution with a [OH^{-}] of 3 x 10^{-4} M. Is the solution basic, acidic or neutral?

A

pH = 12.9

B

pH = 10.5

C

pH = 5.7

D

pH = 7

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