00:01
The reaction is going to be sodium hydroxide with hypochlorous acid producing sodium hypochlorite and water.
00:09
Before adding any sodium hydroxide we have a 0 .1 molar solution of hypochlorous acid.
00:16
Because this is a solution that contains a weak acid we can calculate the hydronium concentration by simply taking the square root of ka multiplied by the concentration of the weak acid and we get 6 .16 times 10 to the negative 5 molar.
00:34
This equation works so long as this value we obtain is less than 5 percent of the original concentration which it is.
00:41
Ph then initial is just the negative log of that hydronium concentration giving us 4 .21.
00:52
Next to calculate the ph midway or not midway but three quarters of the way to the equivalence point we can use the henderson hasselbalch equation as we will have a buffer solution containing some unreacted hypochlorous acid and some formed hypochlorite.
01:14
According to the henderson hasselbalch equation ph equals pka the negative log of the ka value for hypochlorous acid plus the log of the moles of the base the hypochlorite.
01:31
Every mole of sodium hydroxide we add forms a mole of hypochlorite so it'll be the volume of sodium hydroxide added multiplied by its molarity which will give us the moles of hypochlorite formed.
01:43
We then divide that by the moles of the weak acid which will be the moles we start with 100 milliliters at 0 .1 molar gives us the initial moles.
01:54
We then subtract off the moles that reacted which will be the moles of sodium hydroxide added and we get a ph of 7 .90...