pH of Strong Acids & Bases - Video Tutorials & Practice Problems

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concept

pH of Strong Acids & Bases Concept 1

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Hey, guys, In this new video, we're gonna take a look at the connection between Ph and P. O. H and how they relate to strong acids and strong bases. So remember, we've talked about this before. We're gonna say that strong acids and strong bases are considered to be strong electrolytes. Remember strong electrolytes, Completely ionized. That means they break up 100%. You may even hear that these strong acids and bases air called highly electrolytic. That also means they break up completely highly electro politic. So if we take a look here, we have HCL in N a o h. Remember, from the rules we've learned, we know HCL is a strong binary acid, and we know n a O. H is a strong base because we have a group one ai on and a positive connected to O. H minus. So what happens here? We have a single arrow going forward because they break up 100% to give us thes 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 a nice chart in order to find pH or P O. H. So 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 P O. H. So let's take a look. At example, one.

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example

pH of Strong Acids & Bases Example 1

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so an example? One, it says calculate the pH off a 0.782 Moeller solution off. Calcium hydride. No, we know that calcium hydride is a strong base. Because remember, we have calcium, which is C A two plus connecting with h minus are hydride ions. So what happens here? The two from here came here. The one from here came here. That's how we got calcium hydride. And what we should realize here is that there are two calcium hydride in the formula. Now, this is what you need to remember. When it comes to strong basis for strong bases. You have to be very careful. We're going to say that this is not the real concentration of our strong base. You're gonna say when you have strong bases, you have to look at the number off O H minus H minus and H two minus 02 minus. So you have to look to see how many of each you have in order to determine the rial concentration off that strong base. In this particular question, we have to h minus is and because we have two of them, we have to multiply this by two that will give us the correct concentration off our strong base. Okay, so you have to remember that. So, for example, if I had C a o. H. Two and there was a 0.100 Moeller of it again because you have 20 H is you'd have to multiply that by two. If you have calcium of mind, you would have to an h two. So again, you have to multiply the concentration times to to get the correct concentration. Now, what we're gonna do is we're gonna multiply the initial concentration times to to get the rial concentration. Okay, When we do that, we're gonna get our correct concentration. That's the number that we're gonna plug in. So when we do that, and I'm gonna remove myself guys from the image so we have more room to work with When we do that, we're gonna get a new number a new value for r O. H. Minus concentration. So here, we're gonna get 0.1564 Mueller, that becomes our new concentration. So we're going to say p o. H equals negative. Log off O H minus. So it becomes negative. Log off 0.1564 Moeller. Now, some of you may be asking, but Jules, this is saying the concentration of O. H minus. We don't have O. H minus. We have h minus. How do they relate? Just realized that all four of these ions can be treated as being the same. They all feed into the concentration of O H minus. Okay, so h minus. Just understood as a general term for the strong basic ion off a strong base. So if I have to h 20 h minus is that's equivalent to two h minus is equivalent to two NH two minuses. Same thing. So when we take the negative log of this, this will give us our p o h. It's gonna give us 0.81 for our p o h. But remember, let's pay attention. I didn't ask for the pH I asked for. I didn't ask for the p o. H. I asked for the pH. So we need to use the next equation. PH plus p o h equals 14. Plug in what we know. For P o h. 0.81 we need ph. So subtract 0.81 from both sides. So ph here equals 13.19 So that will be our pH for this first question. Hopefully, guys didn't get lost. But what I was saying Just remember for strong bases you have to look to see how Maney o H minus is You have h minus is you have an H two minuses. You have an 02 businesses you have their number, determines the correct concentration of our strong base. And right now, what you need to realize is we only do this for strong base. At this point. If we had a strong acid such as H two s, 04 that strong acid has too acidic. H is. But we wouldn't need to multiply the concentration times too, because when it comes to acids, the first age coming off extremely strong. But the second one that comes off is extremely weak. And because the second H plus is extremely weak, we wouldn't need to multiply the concentration times too. So we only do this multiplication of concentrations when we have strong basis. Later on, when we deal with acid based titrate, shins and buffers, we'll talk about how assets play a role

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example

pH of Strong Acids & Bases Example 2

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Now that we've done that one, let's look, at example, too. Now, an example to it says calculate the pH off a 0.550 Mueller hbr solution to the correct number of significant figures. Okay, so we know that HBR is a strong binary acid, so we don't need to find use a nice chart. We can just take the negative log of that to find Ph. And we're gonna say since it's a strong acid, this is the concentration of Rh Plus. Now, when we plug this into our calculator gives us 3. But remember, we looked were 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 gonna say how Maney Sig Figs are in this concentration. Remember Sig Figs? We talked about a long time ago when it came to significant figures. 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 non zero number. So you're gonna skip this because it's zero skip this. Skip this. Skip this. Our first non zero number going from left to right is this five. Once you start counting, you can't stop counting until you get to the very end. So we say 1233 Sig Figs in this concentration. So we know the correct number of significant figures in our concentration that will help us. Correct. Find the correct answer. Now, we're gonna say there are three sig figs in this concentration. So my answer has toe have three decimal places, so it has to have three decimal places. So again, the number of sick fix in your concentration determines the number of digits in your decimal places. So three sig figs in our concentration means we need three decimal places. So we need toe. Keep three decimal places. So here, this six means that we have to round up. So the correct answer would be 3. Again, the number of sick figs in your concentration tells you the number of decimal places. Your pH or your P O. H has tohave since the concentration at three sig figs are Ph needs three 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 off 50 miles off 4. times 10 to the negative seven Moeller h two s So 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 the number of H is that we have. We won't see something like that until we deal with titrate Asians. When we come to buffers and acids and bases reacting with each other for now, we don't have to multiply the concentration by to 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.

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Problem

Problem

Calculate the pH of 50.00 mL of 4.3 x 10^{-7 }M H_{2}SO_{4}.

A

6.37

B

6.07

C

7.67

D

7.37

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