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Video transcript

water is amphoteric which means it can act as an acid or as a base and so let's say that this water molecule functions as a bronsted-lowry acid so it's going to be a proton donor and this water molecule functions as a bronsted-lowry base so it's going to be a proton acceptor so a lone pair of electrons on the oxygen take this proton and leave these electrons behind so we're going to form hydronium so we make h3o plus so let me go ahead and draw out h3o plus here so lone pair of electrons on this oxygen plus one formal charge and let's show these electrons here in red so these electrons in red are going to take this proton to form this bond here so we make hydronium and in the process these electrons over here in green come off onto this oxygen so let's go ahead and draw out what we would form so we have that oxygen right and then we had two lone pairs of electrons on this oxygen and we picked up another lone pair so the ones in green right here it's going to give this oxygen a negative one formal charge and this is of course is the hydroxide anion so this is the autoionization of water and we can write an equilibrium expression for this reaction all right so we would write ka but for this reaction it's special so we write kW instead of ka and kW is called the autoionization constant so this is the auto ionization auto ionization constant or you might hear different terms for this you might hear ion product constant so ion product constant so whatever term you want to use or whatever term your textbook uses and remember when we're writing an equilibrium expression you're going to put your products over your reactants your concentration so we go over here and we look at our products and we see h3o plus so we do the concentration of hydronium ions times the concentration of our hydroxide anions and then we don't worry about our reactants because we have pure water over here so we're done writing our equilibrium expression all right the concentration of hydroxide ion actually hydronium is what I underlined there so the concentration of hydronium ion and pure water at 25 degrees Celsius has been determined experimentally to be 1.0 times 10 to the negative 7 molar so this is the concentration of hydronium ions and the concentration of hydroxide again determined experimentally is also 1.0 times 10 to the negative 7 molar and into the concentration of hydroxide at 25 degrees Celsius of pure water turns out to be one point zero times 10 to the negative 7 so we can calculate the value for the autoionization constant we can calculate kW kW would just be equal to this would be 1 times 1.0 times 10 to the negative 14 so we can go up here and write it this is all this is equal to 1.0 times 10 to the negative 14 and with with an equilibrium constant much less than 1 you can think about the fact that the equilibrium lies far to the left right so you're not going to have that's why you have such low concentration of ions all right so this is this is the autoionization of water now let's think about the concentration of hydronium compared to the concentration of hydroxide for this example they're the exact same alright so the concentrations are the same the concentration of hydronium is equal to the concentration of hydroxide and so let me go ahead and write that so the concentration of hydronium ions is equal to the concentration of hydroxide and when that happens we say we're doing whistling that's neutral so water is neutral pure h2o is neutral and we use that as a as a comparison so if we had a solution where we had a concentration of hydronium ions that was greater than a concentration of hydroxide anions that's not a neutral solution we call this and a pick solution alright so this is an acidic solution and if we think about the opposite if we have a greater concentration of hydroxide anions so greater concentration of hydroxide anions then hydronium ions here then that's a basic solution so so that would don't refer to a basic solution here so let's go ahead and do a problem let's say that let's say we had some lemon juice and the concentration of hydronium ions of our lemon juice has been measured experimentally to be 2.2 times 10 to the negative third molar and let's say we're expected to find the concentration of hydroxide anions and also to classify our solution so are we dealing with a neutral solution and acidic solution or a basic solution so we can find the concentration of hydroxide anions by using the equation that we have up here so let me go back up here and let's look at this equation that we came up with alright so the concentration of hydronium ions times the concentration of hydroxide anions is equal to kW so let's go ahead and take take our equation and let's plug in our numbers let me go ahead and rewrite that down here so we have our concentration of hydronium ions times concentration of hydroxide ions is equal to 1.0 times 10 to the negative 14 so we can plug this into here alright so now we have 2.2 times 10 to the negative third we could just make the concentration of hydroxide anions X is equal to 1.0 times 10 to the negative 14 so we have a simple calculation so we'll just get out our calculator here and take 1.0 times 10 to the negative 14 and then we just divide that by 2.2 times 10 to the negative third and that will give us the concentration of hydroxide ions so X is equal to 4.5 times 10 to the negative 12 so X is equal to the concentration of hydroxide anions which is equal to four point five times ten to the negative twelve molar and so let's compare our concentrations here I'm going right molar here so let's compare the concentration of hydronium a to hydroxide the concentration of hydronium ions was 2.2 times 10 to the negative third and that number is larger than this number right 2.2 times seven negative third is larger than four point five times ten to the negative twelve so the concentration of hydronium ions is greater than the concentration of hydroxide so let me go ahead and use red for this the concentration of hydronium is greater than the concentration of hydroxide and so we're dealing with an acidic solution so lemon juice is acidic