Weak Acid Strong Base Titrations 1

by Jules Bruno
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So here it says, Consider the Thai trey shin of 75 MLS of 750.300 Mueller of H three c 303 with 12 miles of 120.450 Mueller K O. H. Here. We need to calculate the pH realize here that this is a weak acid based on the rules that we've learned about oxy acids, but also the fact that it has a k A less than one. So this is a weak acid. K O. H is a strong base. We have a weak species reacting with a strong species, so we know we need to do an I C. F chart. Remember, the strong species must always be reacted. So K o. H is a reactant. It is reacting with the weak acid because there's a strong species involved. We have a solid arrow going forward. Remember, what does the acid do? The acid donates in h plus and the base accepts it. As a result, we create the conjugate base que h two c 303 plus water. We have initial change final. Now remember, in a nice CF chart, we only care about three species we only care about the weak acid. We only care about its conjugate base. And we only care about the strong species. The fourth thing. The fourth compound, we ignore it. Next we need to use molds for a nice CF chart. Here were given volume of molar ity for both. Remember that mole Arat e equals moles over leaders. And if we multiply, both sides by leaders will see that moles equals leaders times molar ity. So what we need to do here is we need to convert these MLS in tow leaders and multiply them by the mole Aridjis to get moles. So divide your MLS by 1000 to get leaders, then multiply them by their mole Aridjis. So when we do that, we'll get these different moles. So we have 0.0 2 to 5 moles from my weak acid. And then for a strong base here. When I divide by 1000 mL, I get leaders multiplied by the molar ity that will give me my moles here for my strong base. We're not given any information on the conjugate base. So initially it zero now look at the react inside the smaller moles we'll subtract from the larger moles. So here are smaller moles which will subtract from my larger moles. I'll have left this much of my weak acid. I have zero left of my strong base. Remember, the conservation of mass will have the addition of this amount to the product side. So at the end of this tight, Trish in what do we have left? Well, at the end of this titrate shin, I have weak acid left and I have conjugate base left. What does this mean? This means that we have a buffer. So we are before the equivalents point, We have a buffer. So we use the Henderson Hasselbach equation. So ph equals P K, which is just the negative log. Okay. Plus log of conjugate base over weak acid. So can you get base over weak acid? Okay, so when you plug that into your calculator, negative log of the K gives me 2.82 point three 87 plus. Now, I take the log of everything in here. So when I do all that, that's gonna give me negative 0. So 2387 minus 501 gives me 1.89 as my final pH. So realize here we have a weak species with a strong species, so we need to set up a nice CF chart on the react inside the smaller moles of track from the larger moles. Look to see what we have left. We found that we had a buffer, so we used the Henderson Hasselbach equation to find our pH. Now that you've seen this example, take a look at the practice question that's left here. If you can approach it or don't know how to solve it, don't worry. Come back and see how I give the answer to this practice question. Good luck.