pH - Online Tutor, Practice Problems & Exam Prep

The pH scale, ranging from 0 to 14, categorizes solutions as acidic (pH < 7), neutral (pH = 7), or basic (pH > 7). The pH is defined as the negative logarithm of the hydrogen ion concentration, pH=-log([H⁺]). At 25°C, the ion product constant of water, K=[H⁺]⁢[OH^-]=1⨉10^-14, remains constant. Understanding pH is crucial for biochemical processes, as enzyme activity is influenced by proton concentration.

Calculating pH

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.

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

pH Scale

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, k_w, is equal to the concentration of H⁺ times the concentration of OH^-. At 25 degrees Celsius, which is the assumed temperature for biological systems, k_w 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?

• A. pH = 12.9
• B. pH = 10.5
• C. pH = 5.7
• D. pH = 7

Here’s what students ask on this topic:

What is the pH scale and how is it used to categorize solutions?

The pH scale ranges from 0 to 14 and is used to categorize solutions as acidic, neutral, or basic. A pH of 7 is neutral, meaning the concentration of hydrogen ions (H⁺) is equal to the concentration of hydroxide ions (OH^-). Solutions with a pH less than 7 are acidic, indicating a higher concentration of H⁺ ions. Solutions with a pH greater than 7 are basic, indicating a higher concentration of OH^- ions. This scale is crucial in biochemistry because enzyme activity and many biochemical processes are influenced by the proton concentration in a solution.

How is pH mathematically defined?

pH is mathematically defined as the negative logarithm of the hydrogen ion concentration. The equation is:

$\mathrm{pH}=-\log \left({\mathrm{[H}}_{+}\right)$

This means that if you know the concentration of H⁺ ions in a solution, you can calculate the pH by taking the negative logarithm of that concentration. For example, if the H⁺ ion concentration is 0.04 M, the pH would be:

$\mathrm{pH}=-\log \left(0.04\right)=1.4$

What is the ion product constant of water (K_w) and its significance at 25°C?

The ion product constant of water (K_w) is the product of the concentrations of hydrogen ions (H⁺) and hydroxide ions (OH^-) in water. At 25°C, K_w is always equal to 1 × 10^-14 M². The equation is:

$K_w={\mathrm{[H}}_{+}]\times {\mathrm{[OH}}_{-}]=1\times 10^-14$

This constant is significant because it allows us to calculate the concentration of OH^- ions if we know the concentration of H⁺ ions, and vice versa. This relationship is crucial for understanding the pH and pOH of solutions.

Why is understanding pH important in biochemistry?

Understanding pH is crucial in biochemistry because many enzymes and biochemical processes are highly sensitive to the concentration of protons (H⁺ ions) in a solution. Changes in pH can alter the structure and function of enzymes, affecting their activity and, consequently, the biochemical pathways they regulate. For example, a slight change in pH can lead to enzyme denaturation or changes in substrate binding, which can significantly impact metabolic processes. Therefore, maintaining an optimal pH is essential for proper cellular function and overall biological homeostasis.

Can pH values be negative or greater than 14?

Yes, pH values can be negative or greater than 14, although these are rare and typically occur in highly concentrated solutions. A pH less than 0 indicates a very high concentration of H⁺ ions, while a pH greater than 14 indicates a very high concentration of OH^- ions. However, in most biological systems, the pH usually ranges from 0 to 14, making extreme pH values uncommon in natural biological contexts.