00:01
To calculate the initial ph of the buffer solution, we use the henderson -hasselbalch equation, where ph is equal to pka plus the log of the concentration of the base, that being ammonia, divided by the concentration of the acid, ammonium.
00:21
Pka is the negative log of the ka value for ammonium.
00:26
The ka value is kw, 1 .0 times 10 to the negative 14, divided by the kb value for ammonia.
00:36
You probably won't find the ka value for ammonium, but you should be able to find the kb value for ammonia, 1 .8 times 10 to the negative 5.
00:48
We divide kb for ammonia into kw, we get ka for ammonium.
00:53
We take the negative log and we get pka.
00:55
We then add to that the log of the concentration of ammonia.
01:01
They tell us that is 0 .30, divided by the concentration of ammonium, 0 .36, and we get a ph of 9 .176 or 9 .18.
01:19
Then we recognize that if we're going to add some sodium hydroxide, the hydroxide is going to react with the ammonium and in the process create more ammonia and water.
01:38
To determine the ph after adding 20 ml of 0 .050 molar to 80 ml of the buffer, we need to recognize that we haven't added enough of the sodium hydroxide to completely react with the ammonium, so we'll still have some ammonium left over...