00:01
This is kind of a fun problem because it asks you to tackle this from the other direction.
00:05
It's asking how many grams of hf would you need if you wanted to prepare 672 milliliters of a solution with a ph of 2 .91.
00:15
You're given the ka for hf and we want our answer to two decimal places.
00:24
Okay, so this is our end goal right here.
00:28
We're going to start by changing our ph to an h plus concentration.
00:32
So in order to do that, it's simply 10 to the negative ph.
00:38
So second log negative 2 .91 is 0 .00123 molar.
00:49
Okay, that's our h plus concentration.
00:51
So now thinking about this, we're trying to find the concentration of our whole acid.
00:56
H plus squared over concentration of hf.
01:02
So we have two of these three pieces of information.
01:06
So 7 .1 times 10 to the negative fourth equals, we now know this is 0 .00123 squared.
01:14
And that is over the concentration of hf, which is what we're trying to find.
01:19
So doing your algebra, 0 .00123 squared divided by 7 .1 times 10 to the negative fourth equals, that gives us an hf concentration equal to 0 .00213 molar...