00:01
Okay, so before we titrate, the only thing we have is nh3.
00:06
So let's use the information they've given us to find the moles of nh3.
00:10
Okay, we use the ideal gas law.
00:13
Pv equals nrt or moles is pv over rt.
00:18
So we've got 0 .25 atmospheres and one liter.
00:25
And ours 0 .0821.
00:28
And our temperature is 298.
00:31
That's in kelvin.
00:32
So they give us 0 .0378 moles of our nh3.
00:41
So we were told that that was going to be in 0 .815 liters, right? 815 milliliters.
00:49
So this will give us our concentration of our ammonia in 0464 molar.
00:56
So if we want to find the ph of that, we're just looking for the ph of a weak base.
01:06
So we'll go ahead and write our reaction, our hydrolysis reaction with water.
01:14
And we'll set up our icebox.
01:19
So 0 .0464.
01:22
Worry about our water minus x plus x plus x.
01:37
So this is a weak base, so this will be equal to our kb.
01:41
Well, that's the expression we want to use.
01:43
Our kb is 1 .8 times 10 and minus 5.
01:47
That's going to equal our products, x squared over reactants.
01:51
0 .0464 minus x.
01:56
And we'll do our usual assumption.
01:58
We'll go ahead and assume that x is much, much smaller than 0 .0464, so we can ignore this x right there.
02:06
Okay, so then when we solve for x, we're going to get 9 .14 times 10 of the minus 4 molar, and that's oh, h minus.
02:19
So first we'll see our poh minus the log of that.
02:23
We'll give us a ph of 3 .04.
02:26
And if we subtract that from 14, we'll get our ph, which is 10 .96.
02:35
So that was part a.
02:39
So in part b, we want to know the ph at the half equivalence point.
02:45
Well, we have a buffer, right? so that's going to be the concentration of our acid over base.
02:53
Or moles of acid overbase...