00:01
In this question we're asked to look at a buffer solution.
00:04
It's a tris formula.
00:07
I always like to start with a balanced reaction, but i've simplified this one quite a bit because the formulas for the tris base and its conjugate are pretty complex and we don't need to see the whole formula to see the stoichiometry of the balanced reaction.
00:21
We have this weak base, a tris base, that is going to react with hydrochloric acid and it's going to form its conjugate acid which we call tris hcl.
00:31
This is the basic balanced reaction.
00:34
Our job is to find out how much of the base and accompanying conjugate acid we need to make a buffer at the ph that we're given here.
00:43
So we need to use the henderson -hasselbalch equation.
00:49
So the henderson -hasselbalch equation says we can find the ph of a buffer solution if we know the pka of the acid component, the tris hcl here, and that i looked up and its value for the tris hcl is 8 .1 and we know the ratio of the base to acid components.
01:09
That is something we're going to need to find and we're going to find that information because they tell us the ultimate ph you want to have for the buffer solution.
01:21
So we can plug in that ph value.
01:23
They tell us we want the ph of the buffer to be 8 .9.
01:27
So using the pka value and then solving for this ratio by first subtracting that 8 .1 value from both sides gives us a 0 .8 and now we're going to take the inverse log of that 0 .8 value to find the ratio of the base to acid we need to give us the ph of the buffer indicated here.
01:52
So when we complete that step we find that the ratio of base to acid is 6 .3...