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 \( H^+ \) 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 enzyme's 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 \( OH^- \) 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 \( OH^- \) ion concentration. And so if we use pH to calculate the \( H^+ \) ion concentration, then we can use the \( K_w \) to calculate the \( OH^- \) 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 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 \( H^+ \) 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 0.04. 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.
pH - Online Tutor, Practice Problems & Exam Prep
Calculating pH
Video transcript
Determine the pH of a solution with a [H+] of 2 x 10-5 M.
pH Scale
Video transcript
So I'm sure you remember from your previous chemistry courses that the pH scale ranges from 0 to 14. The pH scale has three sections: neutral, acidic, and basic. Neutral solutions, recall, have a pH equal to 7. At a pH of 7, the concentration of hydrogen ions is equal to the concentration of hydroxide ions. Acidic solutions have a pH value less than 7. When the pH is less than 7, the concentration of H+ ions is greater than the concentration of OH- ions. Basic solutions are solutions that have a pH greater than 7, indicating that the hydrogen ion concentration is less than the hydroxide ion concentration. Let's take a look at our example below to refresh our memories on some of this.
Notice that in the neutral section of the pH scale the pH is equal to 7, and the concentration of H+ ions is equal to the concentration of OH- ions. As the pH starts to decrease, the concentration of H+ increases, and as the pH starts to increase, the concentration of OH- starts to increase. The constant value, kw, is equal to the concentration of H+ times the concentration of OH-. At 25 degrees Celsius, which is the assumed temperature for biological systems, kw is always equal to 110-14M2.
I looked up the pH of lemon juice, and it has a pH of about 2. Neutral pure water has a pH equal to 7, so it is neutral. Bleach has a pH equal to 12. It's interesting to note that hydrogen ion concentrations can be greater than 1, which would extend outside of the normal pH range, resulting in negative pH values. Similarly, if the concentration of OH- is greater than 1, it will extend beyond the typical pH range. These values are much harder to measure, and in biological systems and solutions, the pH usually always remains in the normal range from 0 to 14, so there’s no need to worry about negative pHs or pHs greater than 14.
This ends our lesson on the pH scale, and I'll see you guys in our next practice videos.
Determine the pH of a solution with a [OH-] of 3 x 10-4 M. Is the solution basic, acidic or neutral?