00:01
To calculate each of these ph values, we first need to know what the equivalence point volume is.
00:07
We recognize the reaction of hypochlorous acid with potassium hydroxide is one -to -one.
00:15
Therefore, if we have 50 milliliters of 0 .22 molar hypochloric acid, then if potassium hydroxide has the same concentration, which it does, 0 .2 .0 molar, it will require, because it's a 1 -1 reaction, 50 -mylase, millimeters of potassium hydroxide.
00:36
So before adding any potassium hydroxide at zero milliliters, all we have is the weak -based hypochloric acid.
00:44
So we perform a weak -based ph calculation where the hydronium concentration is equal to the square root of k -a for hypochloric acid multiplied by its concentration.
00:56
K -a was not provided, so we'll have to look it up.
00:59
It might be slightly different than what you have in your table, but in my table it's 3 .5.
01:03
Times 10 to the negative 8.
01:06
We multiply that by the concentration of hypochloric acid, and we get a hydronium concentration of 8 .77 times 10 to the negative 5 molar.
01:15
So we take the negative log of this to get a ph, and we get 4 .06.
01:21
After adding 25 milliliters, 50 milliliters being the equivalence point volume, we are at half equivalents.
01:29
For any weak acid titration, at half equivalence, ph equals p .k .a.
01:34
P -k -a is just the negative log of the k -a value, so ph is 7 .46.
01:41
At 30 -milliliter's, we're still pre -equivalence.
01:45
We haven't reached the equivalence point.
01:47
So we've created some hypochlorite, but we still have some hypoklorous acid.
01:53
So we have a buffer solution.
01:56
For a buffer solution, ph can be calculated using the henderson -hassel -baltz equation, where ph is equal to p -ka, again the negative log of the k -a value.
02:06
Plus the log of the moles of base over the moles of acid, or it could be the molarity of the base over the molarity of the acid, but it's typically easier to just use moles.
02:18
The moles of the base, the base being the weak base, to hypochlorous acid, that being hypochlorite, the moles of it we create from the balanced chemical reaction will be equal to the moles of potassium hydroxide we add.
02:34
Every mole of this we add creates a mole of the weak base.
02:38
So how many moles of potassium hydroxide did we add? well, we added 30 mill liters, which is 0 .030 liters at a concentration of 0 .220 moles per liter.
02:49
Multiplying the volume by the molarity gives us the moles of k -o -h we added, which is equal to the moles of the weak base formed.
02:59
We then divide that by the moles of the weak acid we have left, which will be the moles of the weak acid we start...