00:01
So we're being asked to make a solution, a buffer solution, with a ph of 4 .5.
00:07
Well, what we want to do is we want to choose an acid whose pca is approximately equal to the ph that we're looking for.
00:18
Okay, so since we're needing 4 .5, we're going to want something with a ka to be about negative 4 .5, 10 to the negative 4 .5.
00:29
So i chose acetic acid for this one.
00:37
It's pca simply equals minus the log of its ka, which is 1 .8 times 10 to negative 5, and that gives us 4 .7.
00:51
So we want to use acetic acid.
00:56
And then to make a buffer, we need its conjugate base, which is acetate.
01:05
You could get acetate from something like sodium acetate, pretty easily.
01:13
And since our ph is a little higher than we want to, we can bring it down by simply using a little bit more of the acid than the base.
01:20
The second one, we want a ph of 9 .2.
01:26
So for this one, i chose hcn.
01:31
It's p -ka is minus the log of its ka, which is 5 .8 times 10 to negative 10.
01:42
So you would get a p -ka of 9 .23, almost exactly what we want...