00:01
Okay, so we start with hydrofluoric acid, our acid, and this dissociates into h plus and f minus, and our ka is 3 .5 times 10 to the negative 4.
00:17
And this is just basically the ratio of h plus and f minus over hf, okay? so we have that.
00:28
Now, what we also need to realize is that when naoh, actually i need to write this off to the side more, naoh plus hf neutralizes that hydrofluoric acid and turns it into water and our salt.
00:47
That doesn't say together super well, but it is what it is.
00:51
So if we start with 100 ml of 0 .150 molar hydrofluoric acid, we get 0 .0150 moles of it.
01:07
0 .0150 moles.
01:12
Now we start with 0 of the h and the f, but that doesn't matter too much now because we're adding sodium hydroxide to this.
01:22
Our sodium hydroxide is 40 ml of 0 .250.
01:30
So that gives us 0 .01 mole of it.
01:41
And so when we subtract this much, because remember they react in a 1 to 1 ratio up here.
01:49
And so sodium hydroxide just basically neutralizes that hydrofluoric acid.
01:54
So we have to subtract the amount of naoh that just reacts with it.
02:01
And that leaves us with 0 .005 moles of hf.
02:07
And we have our total volume of 140 ml, right? because 100 plus 40.
02:14
So when we do them, 0 .005 divided by 0 .140, we end up with a concentration of 0 .0357 molar of hf.
02:28
And now remember this whole part.
02:31
Sorry, wrong button.
02:34
Hf would associate to produce h and f - and that's what we have to figure out, right? the ph of the solution is the h plus concentration.
02:46
So we start with 0 with both of these, but then this is going to dissociate and create more of this stuff.
02:57
So 0 .0357 minus x plus x plus x.
03:03
Ka is 3 .5 times 10 to the negative 4 is equal to our h, which is x.
03:12
Another x, so x squared over 0 .0357 minus x, which is our hf concentration.
03:21
So now what we have to do is basically solve this.
03:26
Now this gets a little bit annoying because this number here is very small.
03:32
So we can't really do the x's small approximation.
03:35
We're going to have to go through the whole quadratic formula type of thing.
03:39
So, you know, cross your fingers...