If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:11:37

Video transcript

in the video on the autoionization of water we saw that you could have two neutral water molecules but one of them could swipe a hydrogen ion from the other and then you could be left with a hydronium ion and the one that had the hydro the hydrogen ion swipe from it remember a hydrogen ion is really just a proton this one actually gains an electron it gains an electron from that hydrogen it takes it takes the elect all the electrons from this covalent bond right over here and it forms another lone pair and it gets a negative charge and that becomes a hydroxide ion or we could also describe that auto-ionization like this and if I have one water molecule well the this the the oxygen grabs both of the electrons from this lone pair swipes hydrogen of the one electron it had and all of its all it's left with is its proton so a hydrogen ion is often taught you it's really just talking about a proton because the most common isotope of hydrogen only has a proton in its nucleus it does not have a neutron so you take away its electron you just have that proton left and the whole point of doing this to show look it could it could it's not going to be a typical thing most of the water molecules aren't going to be doing this but this this is going to happen if you have a large enough quantity of water molecules and we've even seen to what degree that happens how typical that is by looking at the concentration if we looked at the concentration so if we're talking about pure water we've seen that the concentration and we could think about it in two ways if we think about this top way of expressing the autoionization we could say the concentration of hydronium ions or if we think about this one over here we could think about the concentration of hydrogen ions and the reason why these two things are equivalent is because these hydrogen ions are really just going to associate themselves with a water molecule and become hydronium but in pure water at 25 degrees Celsius we've seen that these are going to be approximately the concentration whether you think of it as hydronium concentration or hydrogen ion concentration it's going to be approximately 1 times 10 to the negative 7 molar and what does that mean well molar this is just units for molarity that's the same thing as 1 times 10 to the negative 7 moles moles per liter and if you remembering what moles are first of all I encourage you to watch that video on Khan Academy on moles beset for a minor selves it's a quantity just as a dozen means 12 of something a mole of something means 6.022 times and roughly 6.022 times 10 to the 23rd of that thing so it's just a very very very large quantity so that's the concentration you could say the hydrogen ion concentration in pure water at 25 degrees Celsius but we could have asked ourselves that same question for other for other types of solutions for example orange juice so let's just say right over here so an orange juice orange juice orange juice the hydrogen ion concentration and once again I could also say this is a hydronium concentration depending on your glass of orange juice it wouldn't be a typical to find a glass that has a hydrogen ion concentration of say 1 times 10 to the negative 3.5 molar so once again this is this is actually a lot more than this you actually let me make it a round number you actually might be able to find something at 10 times 1 times 10 to the negative 4 molar I looked it up on the on the internet there's a there's actually a range kind of in the mid threes to low fours of hydrogen ion concentrations for orange juice and so you could find something like this and as you can tell and it's good to always just think about the numbers this is actually a much higher concentration and then this over here the exponent over here is less negative so this is a higher concentration so this is a higher concentration a higher concentration and we could look at something that has a lower concentration say something like bleach I actually looked up so bleach I'll write that right over here I actually looked it up on the Clorox website the Clorox bleach they say that their hydrogen ion concentration is approximately so their hydrogen ion concentration is approximately equal to equal to 1 times 10 to the negative 12 one times 10 to the negative 12 molar so this is a much lower concentration than you would have in just in just water right over in pure water 25 degrees Celsius so this is a much lower concentration this exponent over here is more negative is a lower concentration well this is all well and good but it can get a little bulky talking about concentrations in terms of you know scientific notation something times something to the negative whatever molar and so to help simplify that people have invented the idea of a pH a pH and let me introduce it in a color that I have not used yet before P and it you use lowercase P uppercase H and I've looked into where this this this notation comes from the H most people agree is referring to the hydrogen in some way but the P some people think is referring to potential some people just think it's a random letter that was just one of these historical artifacts but this is defined as the negative log of and we could say negative log if we if we don't write a base you assume it's base 10 but the negative log of the hydrogen ion concentration or it's equivalently the negative log because every the hydrogen ions are really hydronium ions h3o it's really the same thing as this as well so given this given this definition of pH let's calculate the pH is for pure water at 25 degrees Celsius or the pH of this glass of orange juice so the pH of this bleach well the pH of this pure water is going to be the pH let me see let me do it in the so the pH here so I have some space right over here it's going to be the negative log and I'll write the base 10 there just because that's assumed of it's hydrogen ion concentration which is 1 times 10 to the negative 7 molar and we want molarity right over here you want molarity right over here well 1 times 10 to the negative 7 is the same thing as 10 to the negative 7 and if you look at if you look at just the part where the logarithm before we think about the negative this is just saying to what power do we have to raise 10 to get 10 to the negative 7th power well that's just going to be negative 7 and if what I just did is confusing I encourage you to review logarithms on Khan Academy so we're going to have the negative of negative 7 which is going to be equal to which is going to be equal to positive 7 so the pH of pure water at 25 degrees Celsius and temperature would matter because it might affect out ionisation it but the pH at 25 degrees Celsius is 7 what about this orange juice well the pH the pH over here is going to be the negative log of instead of writing 1 times 10 to the negative 4 I can just write 10 to the negative 4 and of course that's base 10 well what do I have to raise 10 to to get 10 to the negative 4 well to raise that to the negative 4 power the negative of negative 4 is positive 4 by that same argument all right that's applying the same definition the pH the pH of this bleach is going to be equal to the negative log of 10 to the negative 12 I'm just ignoring this 1 times right over here I could write 1 times 10 to the negative 12 but that's not going to change its value and that of course is going to be equal to the negative of negative 12 which is positive which is positive 12 and so it might be fun to plot all of these so let me do that so let me plot these ph's let me plot these pH is so let me draw a line right over here and let's say that this is 0 1 2 3 4 actually I'm not gonna be able to get all the way to 12 so I have to make that little my scale a little bit smaller 0 1 2 3 4 5 6 7 8 9 10 11 12 and I could keep I could keep going over here so let me write this 1 2 3 4 5 6 7 8 9 10 11 12 so this is these are our pH is over here I could plot them that water at 25 degrees Celsius is going to be right over here and that is my water and that sometimes referred to as a neutral pH if I look at that orange juice with a pH of 4 that orange juice is going to be right over here and I've seen ranges for orange juice it's actually more in this range right over here so depending on your glass or whether it's gone bad or not so that's going to be orange juice could be found in that range so all right orange juice orange juice and then this bleach it depends on the concentration and all of that but this bleach in particular is right over here this bleach is right over here and I could plot other things if your your stomach the juices in your stomach are highly highly well I'll use I'll talk about the acidic a little bit more but your stomach juices are are going to be are going to be in let me just hit a new color are going to be in this range are going to be in this range right over here so stomach stomach and often recalled you know set of stomach juices stomach acid and if you had something like sea water it is it is going to be it's going to be in this range in this range right over here sea water now let's think about what this what this is telling us and remember this is a logarithmic scale if we as we go to the right as we go to the right our concentration is going down it's important to recognize and that's all because of this negative right over here in our definition but we saw that bleach has a much lower concentration than the water has so this is lower lower hydrogen ion concentration and as we go to the left we have a higher hydrogen ion concentration higher hydrogen ion concentration and in general if something has a pH below seven we tend to refer to it as acidic we say it is acidic so typically when you add acids to neutral water it's going your pH is going to go down it's going to get more and more acidic and as you go to the right if you have a pH above seven sometimes you will hear people say basic and sometimes you also might hear people say alkaline alkaline it is becoming more alkaline its hydrogen concentration is getting lower and lower and lower as we move to the right and I really want you to appreciate the logarithmic nature of it because if I handed you some seawater with a pH of eight you might say ok no big deal pH of 8 relative to a pH of seven but that's actually this is a logarithmic scale moving to the right we have 10 times we have a 10 times lower concentration of hydrogen ions if we were to move if we were to move three spaces to the right on this where is this it's 1/10 1/10 1/10 we have one thousandth the concentration of hydrogen ions so it's pretty and it's pretty interesting to think about but how how low how much lower the hydrogen concentration is and say bleach then in water and how much lower that is and say stomach acid or orange juice
Biology is brought to you with support from the Amgen Foundation