00:01
So benzoic acid is h .c7h502.
00:10
It is a weak acid, so it does not completely dissociate, but an h -plus comes off plus the c -7h -502 minus.
00:23
Now, you might also see this written as the benzoic acid plus water, because acid -based reactions are done in water.
00:36
And then the h -2o and the h -plus come together and form h -3o -plus plus c -7h -5 -0 -2 -2 -mines.
00:46
Now we can look up the value of the equilibrium constant for the acid.
00:51
It is 6 .3 times 10 to the minus 5.
00:56
So having this reaction, knowing that the acid is aqueous, water is a liquid, the hydronium ion is aqueous, and the benzate ion is aqueous, we can set up in ice table to figure out the concentration of h -plus.
01:22
And since water is a liquid, it is not going to participate in the equilibrium.
01:27
And we have a 0 .1 molar solution of the acid, which means we start out with none of our products.
01:36
And if we start out with none of our products, the reaction has to shift to the right to make some product.
01:41
How much? the thing is we don't know.
01:43
And if we don't know, we call it x.
01:46
But we're making it so it's positive.
01:50
We do know that there's a 1 -to -1 ratio between the h3o plus and the benz -8 ion, because our coefficients here are 1.
01:59
So the amount of h3o plus i make is the same amount as the benzoyate ion that i make.
02:06
And if we come over to the other side, if i made the stuff on the right, i must have used the stuff on the left.
02:13
So that's going to be negative.
02:15
How much? i don't know.
02:17
But i know for every 1 h3o plus i made, i used a benzoic acid.
02:23
So it's minus x.
02:24
So at equilibrium, we started with point one.
02:27
It went down by x.
02:29
For h3o plus, we started with none, it went up by x.
02:35
And for the benzodiae and we started with none, it went up by x.
02:39
And we can use this ice table and especially the e or the equilibrium line because we can set that equal to our ka, because we can look at our law of mass action.
02:52
So if you come up with the law of mass action expression, the equilibrium expression, you put the products in the numerator, raised to the power of their coefficient, these are both one, divided by the reactants.
03:18
But we don't include water because we don't include pure solids or pure liquids in the expression.
03:25
And how did we represent each of the things in the numerator? x and x...