pH of Strong Acids & Bases - Online Tutor, Practice Problems & Exam Prep

Understanding the relationship between pH and pOH is crucial in chemistry, particularly with strong acids and bases, which are classified as strong electrolytes due to their complete ionization. For example, hydrochloric acid (HCl) and sodium hydroxide (NaOH) fully dissociate in solution, allowing for straightforward calculations of pH and pOH using the negative logarithm of their concentrations. This eliminates the need for ICE charts in these cases, streamlining the process of analyzing acid-base reactions.

pH of Strong Acids & Bases Concept 1

Hey guys, in this new video, we're going to take a look at the connection between pH and pOH and how they relate to strong acids and strong bases. Remember, we've talked about this before. We're going to say that strong acids and strong bases are considered to be strong electrolytes. Remember, strong electrolytes completely ionize. That means they break up 100%. You may even hear that these strong acids and bases are called highly electrolytic. That also means they break up completely. Highly electrolytic. If we take a look here, we have HCl and NaOH. Remember from the rules we've learned, we know HCl is a strong binary acid and we know NaOH is a strong base because we have a group 1a ion, Na⁺ connected to OH^-. What happens here, we have a single arrow going forward because they break up 100% to give us these ions. We'd say that the product side is highly favored. And just realize, if we have a strong acid or a strong base, I've said it before, we don't have to use an ICE chart in order to find pH or pOH. If you have a strong acid, take the negative log to find pH. If you have a strong base, take the negative log to find pOH. So let's take a look at example 1.

pH of Strong Acids & Bases Example 1

In example 1, calculate the pH of a 0.0782 molar solution of calcium hydride. Now, we know that calcium hydride is a strong base because we have calcium (Ca²⁺) connecting with hydride (H^-) ions. What happens here is that the 2 from Ca²⁺ and the 1 from H^- come together to form calcium hydride, CaH₂. What we should remember here is that there are 2 hydride ions in the formula. This is crucial when dealing with strong bases. For strong bases, you have to consider the actual number of basic ions such as OH^-, H^-, NH₂^-, and O₂^- to determine the real concentration of the strong base. In this question, we have 2 H^- ions. Multiplying the initial concentration by 2 gives us the correct strong base concentration.

For example, if we had Ca(OH)₂ with a 0.100 molar concentration, because it contains 2 OH^- ions, we would multiply the concentration by 2. If you had calcium amide, you would also multiply the concentration by 2 due to the 2 NH₂^- ions. By multiplying the initial concentration by 2 for CaH₂, we obtain a new OH^- concentration of 0.1564 molar. From here, the pOH is calculated as the negative logarithm of the OH^- concentration:

pOH = negative log ( 0.1564 )

This calculation gives a pOH of 0.81. It might seem confusing since we are dealing with H^- instead of OH^-, but all these ions contribute to the strong base's alkalinity. Using the relation pH + pOH = 14, we can find the pH:

pH = 14 - 0.81 = 13.19

Therefore, the pH for this solution of calcium hydride is 13.19. This example highlights the importance of determining the right concentration for strong bases using the number of basic ions in their Formula. In contrast, such multiplication is not necessary for strong acids like H₂SO₄, since the second hydrogen ion is quite weak compared to the first.

pH of Strong Acids & Bases Example 2

Now that we've done that one, let's look at example 2. In example 2, it says, "Calculate the pH of a 0.000 550 Molar HBr solution to the correct number of significant figures." Okay. So, we know that HBr is a strong acid. So we don't need to use an ICE chart. We can just take the negative log of that to find the pH. We're going to say since it's a strong acid, this is the concentration of our H⁺. Now, when we plug this into our calculator, it gives us 3.25964. But remember, we're asked to find the correct pH to the correct significant figures. How do we determine the correct significant figures? It's all based on our given concentration. We're going to say how many significant figures are in this concentration. We'd say that this particular compound, it has a decimal, so you count from left to right. You start counting once you get to your first nonzero number. You're going to skip these zeros. Our first nonzero number going from left to right is this 5. Once you start counting, you can't stop counting until you get to the very end. So we'd say 1, 2, 3. There are 3 significant figures in this concentration. So we know the correct number of significant figures in our concentration. That'll help us find the correct answer.

Now, there are 3 significant figures in this concentration. So my answer has to have 3 decimal places. So it has to have 3 decimal places. So again, the number of significant figures in your concentration determines the number of digits in your decimal places. So, 3 significant figures in our concentration means we need 3 decimal places. We need to keep 3 decimal places. Here, this 6 means that we have to round up. So the correct answer would be 3.260. Again, the number of significant figures in your concentration tells you the number of our pH needs 3 decimal places. Now, our pH requires 3 decimal places.

Now that we've seen these first two, I want you guys to attempt to do this last one here. Calculate the pH of 50 mL of 4.3x10^-7 Molar H₂SO₄. We know that this is a strong acid. And again, when it comes to acids, we don't have to worry about multiplying the concentration by 2. The concentration stays the same. Knowing that, you guys tell me what the new pH is going to be, what the correct answer will be to this question. Once you've done that, click on the explanation button and you'll see a video of me explaining what to do for this particular question.

Calculate the pH of 50.00 mL of 4.3 x 10^-7 M H₂SO₄.

• A. 6.37
• B. 6.07
• C. 7.67
• D. 7.37

Here’s what students ask on this topic:

What is the pH of a 0.01 M HCl solution?

To find the pH of a 0.01 M HCl solution, you use the formula:

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

Since HCl is a strong acid, it completely dissociates in water, so [H⁺] = 0.01 M. Therefore:

$\mathrm{pH}=-\log \left(0.01\right)$

Calculating this gives:

$\mathrm{pH}=2$

So, the pH of a 0.01 M HCl solution is 2.

How do you calculate the pOH of a strong base like NaOH?

To calculate the pOH of a strong base like NaOH, you use the formula:

$\mathrm{pOH}=-\log \left(\mathrm{[OH^-]}\right)$

Since NaOH is a strong base, it completely dissociates in water. For example, if you have a 0.01 M NaOH solution, [OH^-] = 0.01 M. Therefore:

$\mathrm{pOH}=-\log \left(0.01\right)$

Calculating this gives:

$\mathrm{pOH}=2$

So, the pOH of a 0.01 M NaOH solution is 2.

What is the relationship between pH and pOH?

The relationship between pH and pOH is given by the equation:

$\mathrm{pH}+\mathrm{pOH}=14$

This equation holds true at 25°C (298 K). It means that if you know either the pH or the pOH of a solution, you can easily find the other. For example, if the pH of a solution is 3, then the pOH is:

$\mathrm{pOH}=14-\mathrm{pH}=14-3=11$

This relationship is crucial for understanding the balance between hydrogen ions (H⁺) and hydroxide ions (OH^-) in a solution.

Why don't you need an ICE chart to find the pH of strong acids and bases?

You don't need an ICE chart to find the pH of strong acids and bases because they completely dissociate in water. This means that the concentration of hydrogen ions (H⁺) for strong acids or hydroxide ions (OH^-) for strong bases is equal to the initial concentration of the acid or base. For example, if you have a 0.1 M HCl solution, the [H⁺] is 0.1 M. You can directly use the formula:

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

to find the pH without needing to account for partial dissociation, which simplifies the calculation process.

How do you find the pH of a strong base solution?

To find the pH of a strong base solution, you first calculate the pOH using the formula:

$\mathrm{pOH}=-\log \left(\mathrm{[OH^-]}\right)$

Then, use the relationship between pH and pOH:

$\mathrm{pH}=14-\mathrm{pOH}$

For example, if you have a 0.01 M NaOH solution, the [OH^-] is 0.01 M. Therefore:

$\mathrm{pOH}=-\log \left(0.01\right)$

Calculating this gives:

$\mathrm{pOH}=2$

Then, find the pH:

$\mathrm{pH}=14-2=12$

So, the pH of a 0.01 M NaOH solution is 12.