 Okay, this video I have a feeling is going to be on the long side. I want to talk about concentration of acid in a solution, so in a liquid. This is different from strong versus weak acids. So we're going to be talking, let's just think, of strong acids in this case. Measuring how acidic a liquid is is the same thing as measuring the concentration of a liquid. A liquid is very acidic, that means the concentration of acid is very high. If you say a liquid is not very acidic, that means the concentration is very low. So we want you to know the difference between the word acidic and acid. Acid is a thing, acid is a type of chemical. Acidic means how concentrated that thing is in a liquid. So we're going to talk about acid concentration in this video. The example that I'm going to use is hydrochloric acid. So imagine that I have two containers, they both have the same amount of water in them. So this is almost all water molecules, but I'm not going to draw the water molecules. And in one container I throw in a bunch of HCl molecules. In the other container I throw in some HCl molecules, but not as many. So maybe over here it's a million HCl molecules, and maybe over here it's just a thousand. Because HCl is strong, almost all of these H pluses are going to get donated to water. And I'm going to make an awful lot of H3O pluses once those H pluses get donated to water. I'm going to make an awful lot of H3O pluses in this container over here on the left. The H pluses over here are also going to get donated to the water, but I'm not going to make as many of them because I didn't throw as many HCl's into this container over here. The container on the left has relatively high acid concentration because I threw in a lot of acid into the container on the left. The container on the right has relatively low acid concentration. And the way that we're measuring acid concentration, just to bring this back to the previous video, is we're measuring the concentration of H3O plus because that's the stuff that's going to get made once the H pluses get donated. So we would say the concentration of H3O plus in the container on the left is relatively high compared to the concentration of H3O plus in the container on the right. Now in the previous week's videos we talked about concentration and there's a whole bunch of different ways of measuring concentration. You can use percent by mass, percent mass over volume, et cetera, et cetera. The way that acid concentration is almost always measured is it's almost always measured using the molarity method. And if you remember the molarity method is number of moles of solute. In this case that would be the number of moles of acid divided by the number of liters of solution. So acid concentration is almost always measured using molarity. But it's molarity with a twist and the twist is coming up. Here's an example before we get to the twist. And the setup here is that even with very acidic solutions the concentration of acid is almost always a relatively small number, especially if you measure it in units of molarity. So in my pretend situation on the left let's say that I dissolved 0.002 moles of HCl into one liter of solution. Now if I want to measure the concentration of the H3O plus that gets made well it's going to be 0.002 moles of H3O plus dissolved in one liter of solution. And this is the molarity formula. So what is 0.002 divided by one liter? It's going to be 0.002 molar, capital M. This would be the acid concentration in the container on the left because we're just plugging moles and liters into the molarity equation and we're getting 0.002 molar. Now in the container on the right I didn't dissolve as much acid into the container on the right. This time I dissolved only 0.002. You can see there's an extra zero here. Moles of HCl dissolved in the liter of solution. So you can say what's the molar concentration of the H3O plus in this container on the right? It's 0.002 moles of H3O plus divided by one liter of solution and again this is the molarity equation. What's 0.002 moles divided by one? It's 0.0002 capital M. So if I said what's the concentration of the acid on the left, you'd say it's 0.002 molar, if I said what's the concentration of the acid on the right? You'd say it's 0.0002 molar and this is obviously a smaller number than this one. So the concentration of acid in this container over here, less acidic and this one is more acidic because the concentration of acid is higher in the container on the left. Now if you wanted to write this in scientific notation instead of saying 0.002 molar, you could say the concentration of acid in the container on the left is 2 times 10 to the negative 3 molar and if you wanted to write the concentration of acid in the container on the right instead of writing 0.0002 molar, you could write it as 2 times 10 to the negative 4 molar if you want. So here's the setup. This is the way that I usually present it to my students. Imagine that you're working in the lab and your research advisor says I want you to come up with a way of describing acid concentration to people and you say oh well we learned about molarity in class and I like the molarity method. So why don't I do that and I could say the container on the left I could describe the acid concentration or how acidic it is by using the molarity method and the container on the left has acid concentration of 2 times 10 to the negative 3 molar and the container on the right has concentration of 2 times 10 to the negative 4 molar and that makes sense because this number is bigger than this number so this one, this one is more acidic and this one is less acidic. You think that's a great idea and then you go to the bar after work and you start talking to your friend at the bar and you describe your new method for describing acid concentration and your friend says you know what I hate these numbers. These numbers are really weird. Why don't we fix your method? Why don't we turn these numbers into more reasonable numbers? And so your friend says why don't we do this? Why don't we take the logarithm of the acid concentration in molarity? So instead of telling people the concentration is 2 times 10 to the negative 3 molar why don't we tell them it's logarithm of 2 times 10 to the negative 3 and over here the acid concentration would be the logarithm of 2 times 10 to the negative 4. And that way we can get rid of this ugly scientific notation and we'll end up with more reasonable numbers. If you do that, if you take the logarithm of 2 times 10 to the negative 3 you'll come up with a number negative 2 point, about negative 2.7 and if you take the logarithm of 2 times 10 to the negative 4 you'll come up with a number of negative 3.7. So your friend at the bar says why don't we do this? We'll take the logarithm of the acid concentration in molarity units and in the container on the left we'll come up with a number of negative 2.7. I don't know what the frig that means but it sounds kind of cool and I don't have to look at scientific notation. And in the container on the right I would say the acid concentration is negative 3.7. Again I don't really know what that means but I still got rid of my scientific notation. And so you say oh yeah that sounds like a great idea. We'll describe the acid concentration over here as negative 2.7. We'll describe the acid concentration over here as negative 3.7. Negative 2.7 is still bigger than negative 3.7 so people will know that this concentration is higher than the concentration of the container on the right. So that's a great idea. But then another friend of yours comes into the bar and you and your friend describe your new method for telling people about acid concentration and your friend looks at this and they say you know what that's great but I hate those negative numbers. I just don't like negative numbers. So why don't we do this. Why don't we fix your method. We'll figure out the molarity of the acid concentration. We'll take the logarithm. But the logarithm seems to be giving us negative numbers. So why don't we multiply our answer times negative 1. And if we multiply the answer times negative 1 we'll get a positive number. So if we multiply this answer by negative 1 we'll get positive 3.7. So they say this is a really good way of describing acid concentration. We'll figure out the concentration of acid in molarity. We'll take the logarithm. Then we'll multiply the answer by negative 1. And so if I wanted to describe the concentration of acid in the container on the left I'd say it's positive 2.7. And if I wanted to describe the acid concentration in the container on the left I'd say it's positive 3.7. And your friend agrees this is going to screw everybody up because 2.7 is smaller than 3.7. But this one is more concentrated. But you all decide this is the very best way of describing acid concentration to other people. And believe it or not this is the way that everybody uses on this planet. This is the way that people describe acid concentration to each other. The acid concentration is almost always described to people using this formula. Negative 1 times the logarithm of the concentration of H3O plus in units of molarity. And you may know this. This is also known as pH. The definition of pH is minus 1 times the logarithm of H3O plus concentration in molarity units. To me this is the road to hell. It just seems like an incredibly complicated way of turning concentration numbers into more reasonable ones. But you end up with a logarithm, you end up with a negative 1, it's ugly. But this is the way that people use. So here is the formula for pH which you may have heard of. pH is just a way of describing how acidic a liquid is. And this negative 1 times the logarithm of the concentration of H3O plus. These square brackets here mean concentration of whatever is inside of the square brackets. And in this case I'm telling you it's in units of molarity. So almost all people on this planet describe acid concentration to each other using pH. Using this formula. It turns out not that you care and I'm never going to go beyond this. But even this equation is a little bit wrong. If you take more advanced chemistry you'll see that this equation gets modified. But almost everybody on the planet uses this simplified version of the pH equation. In case you care you can think about why it's called pH. The H stands for the H plus that gets donated from the acid. The P, when I was a student back in the Stone Ages they told me that it was, the P stood for the French word for power which was something like poivroir. So this is supposed to mean the power of how powerful the H pluses are in your liquid. I think these days they tell the students that it stands for potential or how much potential the H pluses have. But that's where the word pH comes from and that's why this is a lower case P and that's a capital H. Please don't memorize this formula. I will give this formula to you on a quiz or a test but you're going to have to be able to use it. So to summarize pH is a way of describing how concentrated acid is in a solution. In other words it's a way of putting a number on how acidic a liquid is. The weird thing about pH is because you're multiplying by negative one it means that the smaller the pH number is the more acidic your liquid is and the larger your pH number is the less acidic it is. This can be confusing but that's the way it is. So like I said if the pH number is relatively large that means your liquid is less acidic. Another way of saying that your liquid is less acidic is you can say that your liquid is more basic. So those basically mean the same thing. Or you can say more acidic means the same thing as less basic. Alright that's the end of this video. That is basically an introduction to acid concentration using pH. In the next video we're going to talk more about pH and pH numbers.