00:01
For this problem, we are titrating hydrocyanic acid, a weak acid, with sodium hydroxide, a strong base, and we're asked to find the ph at the equivalence point.
00:10
So equivalence point definition is where our moles of acid and base are equal to each other, or our millimoles of acid and base are equal to each other.
00:18
And so that means that at the equivalence point, all we will have is this salt right here.
00:26
And so this sodium cyanide, the sodium part comes from the strong base.
00:31
The cyanide comes from that hydrocyanic acid, which is a weak acid, strong wins.
00:39
So this is going to be a basic salt, which means it will act like a weak base.
00:45
So we are going to need our kb value.
00:48
I looked at the ka value.
00:50
If you were given something different, please use what you have.
00:53
But to find the kb, you would take 1 times 10 to the negative 14th kw divided by the ka 6 .8 times 10 to the negative 10.
01:02
That would give you a kb of 1 .5 times 10 to the negative 5th.
01:09
Okay, so we need to figure out also, before we get going on this, at how many milliliters of base would be required to reach that equivalence point.
01:20
So to do that, we would take our molarity of acid times volume of acid, set that equal to molarity of base times volume of base.
01:29
So 0 .366 times 27 .2 equals 0 .406 times vb.
01:38
So vb is going to equal 0 .366 times 27 .2 divided by 0 .406 is 24 .5 milliliters.
01:51
So i will just put that up here because i'm going to need that.
01:56
Okay, so we need to know how many moles or millimoles, millimoles is just milliliters times molarity, how many millimoles of each of these we have at the equivalence point.
02:07
So 27 .2 times 0 .366 equals 9 .96...