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10. Acids and Bases

The pH Scale

10. Acids and Bases

The pH Scale

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The pH and pOH of a compound helps to determine its acidity and basicity.

The pH Scale

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concept

The pH Scale Concept 1

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Hey, guys, In this new video, we're gonna take a close look at P H and P O H. So we're gonna say that when it comes to the concentrations of H positive and O H minus, they usually come as very small numbers. And what we need to realize here is that the pH scale was invented to basically turn these varying small numbers into more manageable ones. So we're gonna stay under normal conditions. The pH scale operates within the range of zero tau 14. But what we need to realize here is that the PH scale could be less than zero, or it could be greater than 14. All we would need to do is increase the concentration of our acid or base. Remember, 0. 14 are not the only numbers that your pH your pH could be that could be greater or less than that. It all again depends on upping the concentration of your compound. If you make your concentration very high, like over one Moeller, then I'll give you a very high or very low pH. Now we're gonna say, by taking the negative log of H positive and O H minus, we confined Ph and P O h. So ph just means negative log of H plus concentration or negative log of h +30 plus. Because remember, we said this before H positive H 30 plus Same exact thing. P O. H is the negative log of O H minus. So that means that p represents negative log.

pH stands for the negative logarithmic function of your hydronium concentration, whereas pOH stands for the negative logarithmic function of hydroxide concentration.

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The pH Scale Concept 2

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And what we should realize here is that by recognizing the relationship between h positive in O. H minus, we can find a new way to express them in terms of P H and P O H. So we come back here, we say that PH equals negative log of h positive. We could divide both sides by negative one to get rid of this negative. So negative pH equals log off h positive. Now we wanna just isolate th positive, so we're gonna divide out the log. When you divide anything by log, it becomes tend to that whatever the variable is. So we just find a new relationship. We can say that h positive. The concentration of H plus ions is equal to 10 to the negative pH. We could do the same thing with p. O. H. Divide both sides by negative one where negative p o. H equals log off O H minus divide both sides by log. So which minus equals 10 to the negative P o. H. So just remember the relationship we have here with these first two equations and how we have the relationship with their concentrations. Because sometimes you'll be asked to find HPE plus concentration. If they give you pH. Sometimes you may be asked to find O H minus concentration. If they give you p O H. You have to know the relationship both front words and backwards.

If we know the pH or pOH concentration then we can determine the concentration of hydronium ions or hydroxide ions.

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The pH Scale Concept 3

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Now we're gonna say a species with a pH greater than seven greater than seven is classified as basic. If your basic you're gonna say that your H plus concentration is less than you're O h minus concentration. And we can also say that the stronger the base than the higher the pH and the higher the pH, the greater you're hydroxide concentration. Okay, so we can understand this relationship as strongest base would have the lowest pH I mean highest pH and therefore have the highest, all H minus concentration. Remember, we already went over this several videos ago. How do we determine if something is weak or strong? You still need to know those in order to do the following questions, because again, I've said it so many times before. Why do we need to know the strength of things? Because eventually we're gonna have to use ice charts to find pH if we're given a weak acid or a week base. That's why it's important to know what our species is is a week or is it strong? Now we're gonna say a species with a pH less than seven is termed acidic. And if you're acidic, your H plus concentration is greater than Euro H minus concentration. Here we can say the stronger the acid than the lower the pH. So strongest acid has the lowest pH, which means you have the highest h plus. And then finally, we're gonna say species with a pH equal to seven is neutral. Remember, this is Onley true at 25 degrees Celsius. Because, remember, at 25 degrees Celsius, K w equals 1.0 times 10 to the negative 14. If they were to change the temperature, that would change our K W. And then neutral Ph would become a new number. It doesn't always equal seven. And we're gonna stay here if you're neutral and your age plus concentration is equal to your O H minus concentration.

If the pH is less than 7 then the solution is acidic, if the pH is equal to 7 then the solution is neutral and if the pH is greater than 7 then the solution is basic.

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The pH Scale Concept 4

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and we're going to stay here. We talked about P H and P O. H and just realized their relationship is connected together by this equation, we're gonna say ph plus p o h equals 14. This arises by the fact that we have k w equals h positive times O H minus. And if you take the negative log off H plus, it gives you pH if you take the negative log off. O H minus. It gives you P O H and K W was 1.0 times 10 to the negative 14. And if you took the negative log of K W, it would give you 14. So that's where this equation comes from. It comes from taking the negative log of all of this. And remember, if you have two things multiplying like H positive and O. H minus are multiplying and you take the negative log of that whole thing, then it becomes the addition between them. Okay, so these guys are related. These two equations that I circled are related because they're connected together by negative long. So just remember that relationship and you'll be able to remember this formula. Remember? Ph scale 0 14 you could be below zero or higher than depending on the concentration of the acid or base you're using.

Under normal conditions when the concentration is less than 1.0 M the pH scale is between 0 to 14. pH and pOH are connected by the following equation:

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The pH Scale Example 1

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Hey, guys, In this new video, we're gonna put to practice some of the concepts we learned about P. H and P O H. So let's take a look at the first example here it says, What is the hydroxide ion? So what's the O. H minus and the hydrogen ion concentration or hide rhodium ion concentration. Actually, that's H plus often acquis solution that has a pH equal to 6.12. So we have pH we need to find O h minus and h plus. Remember, we're able to derive a formula that connects us between H plus and pH. Remember that H plus equals 10 to the negative pH. So I would have to do now is just take that number and plug it in. When we do that, we get 7.59 times 10 to the negative seven. Now, we could find ohh now, and we confined Ohh in two different ways. Now, the first way we could find o. H minus is by simply using this equation. Kw equals H plus times. Ohh! Minus we know what K W is. We assume the temperature is 25 degrees Celsius. Since they don't tell us what it is. So this will be 1.0 times 10 to the negative 14. We just found out. What h plus is it? 7.59 times 10 to the negative seven. Now we have to find O H minus. Ohh! Mind this is our only missing variable. So just isolated. So here, o H minus would equal 1.32 times 10 to the negative eight. So that would be one way we confined O h minus. Now, the second way we could find it is if we know p o. H. We could make a connection between p o H and O H minus. So, first we'd say that pH plus p o H equals 14. We know Ph is 6.12. So let's just isolate r p o h subtract 6 12 from both sides. So p o. H equals 7.88 Remember, what's the connection between P. O. H and O H minus? You could just simply say that oh h minus equals 10 to the negative P o. H. And if you did 10 to the negative 7.88 you get the same exact answer for O. H minus. So two different methods to find the same exact missing variable. So it's basically your choice on deciding which way you want to go. Personally, I like to just find P o H and just do 10 to the negative p o. H. Because remember, if you don't put these two in brackets, your calculator may give you the incorrect answer. So it's probably always safer just to say oh h minus equals 10 to the negative P O. H.

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Problem

Problem

Which of the following statements about aqueous solutions is/are true?

A

For an basic solution the concentration of H₃O⁺ is greater than the concentration of OH^-.

B

The pH of a neutral aqueous solution is 7.00 at all temperatures.

C

An acidic solution under normal conditions has a pH value less than 7.00.

D

If the concentration of H₃O⁺ decreases then the concentration of OH^- will also decrease.

E

The pH of aqueous solutions is less than 7.

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The pH Scale Example 2

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Hey, guys, In this new video, we're gonna continue with our calculations dealing with P H and P O H. So, in this question, it says a solution is prepared by dissolving 0. moles off strong team hydroxide in water to produce a solution with the volume of 750 amounts. Now I'm asking you to find the concentration of O H minus. Remember these brackets being concentration? Another name for concentration is molar ity. And remember, mole arat e equals moles of solute over leaders of solution. So here we need the moles of O H minus, divided by the leaders of solution. Let's do the easier part first. The easier part is we have 750 ml of solution. So let's just change that to leaders. So we're gonna say we have 750 m. Els. You can say here for everyone Leader, there's 1000 mL or you could say for everyone ml there's 10 to the negative three leaders, so that gives me 0.750 leaders that will go on the bottom. Now the harder part we have toe isolate moles off just hydroxide ion So all we're gonna do here is we're gonna take these 0. moles of strong team hydroxide and convert them into moles of hydroxide ion on Lee. So we're gonna say we have 0.235 moles off strong team hydroxide and then just simply say, here for every one mole of strontium hydroxide how maney hydroxide zahren the formula, you can notice that there are two of them, so there's two hydroxide for every one mole off strong team hydroxide. So that's gonna give me 0.4 70 moles. Oh, h minus. So we're gonna take those molds and plug them up top. So 470 divided by 4700.750 will give me 0.6 to 7 Moeller Oh, h minus. So that'll be your answer here. Now, for part being, I want us to find the H plus concentration. So remember, all we need to realize here is if I know my o h minus concentration, then I know my age plus concentration because, remember, they're connected by this formula. K w equals H positive times O h minus. Plug in what we know for K W. And then fill in what we know for O H minus. So we got to do now is divide both sides by 6 to and we'll know what h plus is. H plus comes out to be 1.59 times 10 to the negative. 14. Hope you guys were able to follow along this question just how to do with a little bit off, uh, unit conversions and some little bit of stoke geometric relationships. For every one mole of strong team hydroxide, we have two moles of O H minus. So now that we've seen that one, I want you guys to attempt to do this practice question here. So remember, go back and see what the relationships are in order to find this correct answer and just realized you when I say pure water, just understand pure water means neutral water. So if your water is pure, you're neutral water. And remember, what? What does that mean in terms of H plus and O H minus, knowing that will be the key to solving this question? Good luck, guys.

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Problem

Problem

What is the Kw of pure water at 20.0°C, if the pH is 7.083?

A

8.26 × 10^-8

B

6.82 × 10^-15

C

7.23 × 10^-14

D

1.00 × 10^-14

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