00:02
If we have a weak acid that dissociates or, more specifically, reacts with water, producing a -minus and the hydronium ion, because the stoichiometry is one -to -one here, these two concentrations equal each other.
00:20
So that means the a -minus concentration will be equal to the hydronium concentration, and the hydrogenum concentration is simply 10 to the negative ph.
00:30
So that'll be 10 to the negative 2 .62 which is provided and we get a value of 2 .40 times 10 to the negative 3 molar.
00:47
Then we go to the percent dissociation equation that percent is equal to the a minus concentration divided by the h a concentration times 100.
01:05
So now knowing the a minus concentration and knowing the percentage we can calculate the ha concentration, rearranging this equation, h .a is going to be equal to the a minus concentration divided by the percent dissociation times 100.
01:27
So this gives us 2 .40 times 10 to the negative 3 divided by 8 .29 % times 100 and we get a value of 2 .89 times 10 to the negative molar.
01:57
Now to determine the pca that'll be the negative log of the ka value and the ka value is going to be equal to the a minus constant.
02:19
Multiplied by the hydronium concentration, divided by the ha concentration.
02:27
But these two are equal, both being that value there.
02:32
So we'll take 2 .40 times 10 to the negative 3, divide that by, this is going to be the ha concentration at equilibrium.
02:45
This is the initial ha concentration.
02:50
Okay, they want the concentration of h .a.
02:53
At equilibrium...