So here we're dealing with the calculation at the equivalence point here, it says, consider the determination of 50 mls of 500.1 50 moller Hydrofluoric acid with 500.100 moller sodium hydroxide at the equivalence point, what would be the ph of the solution at the equivalence point we're told here that R. K. Value for Hydrofluoric acid is a 3.5 times 10 to negative four. Now we have our weak acid reacting with a strong base. Remember the weak acid is a proton donor. So H plus will combine with O. H minus to give us water. The N A plus will combine with the F minus to give us an A. F. We have here because we're mixing acid and base. We know we're utilizing an I. C. F. Chart. And now I'm writing this pretty small because remember we said that at the equivalence point between a weak acid and a strong base, we first have to do an ice I. C. F. Chart, followed by an ice chart. Alright, so here water is ignored. Now we divide these on these mls here by a 1000 to get the most And to get the leaders which are multiplied by malaria T to get the moles. So it's gonna give me .0075 moles of HF. Since we're at the equivalence point we have equal moles of our weak acid and strong base. So even though I don't have the volume of N A. O. H. Which I'll need eventually though, I don't need it because I know at the equivalence point, both of these have the same moles. We don't have any initial information on any F. So initially it's zero. Now remember on the react inside the smaller mold subtract from the larger moles. Since they're both the same, we're going to get zero at the end for both. They've completely neutralized each other. But remember on this side we have the creation of this many moles of the conjugate base and a F. Now remember this is our conjugate base. A conjugate base here is a weak base. And because it's a weak base in order to find its ph we'd have to use an ice chart. Now, in order to figure out in order to use an ice chart, we need to use the units of polarity. So we need the polarity of this conjugate base. Now we don't really need the N. A. There it's just a spectator ion. So it's just F minus. That's important. We need its concentration. So we need the moles of its that's left which is the same number, but we need the total volume, the volume of both. The H. F. And the N. A. O. H. Now to figure out the volume of the base. Remember at the equivalence point, their moles are equal because they're at the equivalence point where the molds are equal. You can say M acid times V acid equals M base times V base plug in the concentration of the acid times its mls equals polarity of the base And we're looking for its volume again, we need the volume of the base because we need the total volume in order to figure out the final the new concentration of F minus. So polarities cancel out here. So I'll have my Volume of my base which comes out to be 75 mm. So now I'm gonna come over here and say that my total volume is 0.50 liters of the weak acid, 0.75 liters of the strong base Together. That gives me a concentration of .06 Mohler for my fluoride ion. Now we can bring it down and utilize an ice chart. Remember weak acids and weak bases react with water within an ice chart. Remember also that bases are proton except ear's. So water is going to act as the acid and donate an H plus f minus that gives us back H F. And the creation of minus. So we have initial change equilibrium in a nice chart. We don't care about solids and liquids so water is ignored. Bring in the initial concentration of the F minus Our products initially are zero. We lose react ints to make product now because we have a base here weak base. Remember weak bases utilize KB. So there's a lot of conversions that are going on in this problem. We have the K. A. We need KB. Remember KB equals KW divided by K. A. So if you plug that incorrectly in your calculator, make sure you put parenthesis around it, you get 2.9 times 10 to the negative 11. So that is our K. B. Value for f minus. Bring that down. So KB equals products. Overreact ints so 2.9 times 10 to the negative 11 equals X squared divided by 0.6. Remember at the equivalence point we can ignore the minus X here because it's just KB equals X squared over the concentration of our conjugate base which is our weak base. Alright now multiply these two together. So that gives me X squared equals 1.74 times 10 to the negative 12. Take the square root of both sides. So x equals 1.3. 2 times 10 to the -6. Now remember at this point when we find X X will either give me a 30 plus or minus. Go back and look at your ice chart. In the ice chart we see here that X gives us O H minus. So since I know the concentration of O H minus, that means if I take its negative log that gives me P O. H. And if I know P O H. I know ph because ph equals 14 minus P O H. So that comes out to be 8.120. So here are answers reasonable because we know that when it comes to a weak acid and and strong based filtration at the equivalence point, our solution should be basic, so we should expect to get a ph above seven. Alright, so just remember within the different points of our titrate nation between a weak acid a strong base. You have to remember when to utilize an ice chart when utilizing I cf chart or in this case utilizing first in i c f chart, followed by an ice chart.
