10. Acid-Base Titrations
Weak Acid-Strong Base Titrations
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So when we reach the equivalence point between a weak species and a strong species, we're actually gonna have to utilize 2 charts. We'd start off with an ICF chart because, again, we have an acid and a base titrating, but at the end, we'll have a product, which is weak in itself and would need an ICE chart in order to determine pH and p or pOH. So if we take a look here at the equivalence point, we're gonna say at the equivalence point of a weak acid strong base titration, whatever is stronger will dictate what kind of pH I have. Since the base is what's strong, it's gonna create a solution that is basic. And if the solution is basic, that means it'll be greater than 7 when we have our standard temperature of 25 degrees Celsius. So here we divide our ml's by a1000 to get liters of each, and when you multiply them together you'd see that we have actually the same moles for both, which makes sense because at the equivalence point, we have equal moles of our acid and our base. They would subtract from one another, so at the end we'll have both of them being 0. Through the law of conservation of mass, we'd be gaining this amount on the product side. So at the end, what we have is conjugate base. This conjugate base, it comes from a weak acid, so it's a little bit stronger, but in actuality, it's still a weak base. Okay? So it's still a weak base. And remember with weak acids and weak bases, we need to use an ICE chart in order to find our pH. So what we do here is we'd have to now set up an ICE chart with this NO2-. Remember, in an ICE chart, the units have to be in molarity. So we take the moles of what's left of our conjugate base and divide it by the total liters used within the titration, which would be the 0.300 liters of the weak acid plus the 0 point 100 liters of a strong base. When we do that, we get a new concentration of 0.075 molar of our nitrite ion. So we plug that here. Now remember, in an ICE chart, our weak acid or weak base reacts with water. Since it's a base, it's going to be a proton acceptor, so water will donate an H+ to it, giving us nitrous acid+OH- as products. Now we ignore water because liquids and solids are ignored in ICE charts. Initially, both of these products are 0. We lose reactants to make products. We bring down everything for the equilibrium line. So that's our filled in ICE chart. Now because we're dealing with a base, bases, weak bases utilize Kb. We know what the Ka of Nitrous acid is, and we're gonna use that to find Kb. Kb equals k w divided by k a. So remember k w is 1.0 times 10 to negative 14, divided now by the 7.1 times 10 to the negative 4, that gives me a value of 1.41 times 10 to the negative 11. That's our k b, which we'll plug into here. At this point, in order to isolate x, we're gonna say k b equals x squared divided by the concentration of our conjugate base, which is our weak base. So we're gonna do 1.41 times 10 to the negative 11 equals x squared divided by 0.075. Multiply these 2 together, that gives me x squared equals 1.0575 times 10 to the negative 12. To get just x, we take the square root of both sides, so x equals 1.0283 times 10 to the negative 6. Now remember, when we find x, x here will give us either H3O plus or Oh minus based on the equation in the ICE chart. Here, x is giving us Oh minus. Which means that if I take the negative log of that, I'll get pOH. So that comes out to 5.99. And if I know pOH, I know pH, because pH is 14 minus pOH, which is 8.01. So again, we're seeing that we have the gradual increasing of our pH as we start adding more and more of our strong base titrate. So at the equivalence point, we found out that our pH is 8.01. Now that we've seen this, click on the next video, and let's see what happens when we go a little bit beyond the equivalence volume. So we're gonna go a little bit above a 100 ml of our potassium hydroxide.
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