00:01
I instruct my students that for the polyatomic ion, the species that has two hydrogens removed, has its concentration equal to k -a -2, and that's always the case with the common assumptions that are made.
00:16
To show you that this is true, we'll look at the first k -a -1 reaction, where h3 -p -o -4 reacts with water to produce hydronium ion and h -2 -p -o -4 -1.
00:28
This is the larger of the k -a values, so this is the one that produces the most amount of hydronium, and we only focus on this to determine the hydronium ion concentration.
00:41
Because the stoichiometry is one -to -one, the hydrogenium concentration will equal the h -2 -p -o -4 -minus concentration.
00:54
Then when we move on to k -a -2, the k -a -2 reaction, this acid that was created, the conjugate base to phosphoric acid reacts with water to make h .p .o .4 -2 -minus and a little bit more hydronium.
01:12
The k -a -2 expression is h -p -o -4 -2 -1 concentration, multiplied by hydronium, divided by h -2 -o -4 -9.
01:25
But we've already established up here that these two concentrations are equal to each other.
01:30
If they're equal to each other, they cancel from the k -a -2 -e -2...