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Choose the best conjugate pair that can be used to make a pH =8.5 buffer (choose only one)A. Propionic acid / propionate [Ka = 1.3 x 10-5]B. Carbonic acid / carbona...

Question

Choose the best conjugate pair that can be used to make a pH =8.5 buffer (choose only one)A. Propionic acid / propionate [Ka = 1.3 x 10-5]B. Carbonic acid / carbonate [Ka = 4.3 x 10-7]C. Hypochlorous acid / hypochlorite [Ka = 3.0 x 10-8]D. Hypobromous acid / hypobromite [Ka = 2.0 x 10-9]E. Phenol / phenolate [Ka = 1.6 x 10-10]

Choose the best conjugate pair that can be used to make a pH = 8.5 buffer (choose only one) A. Propionic acid / propionate [Ka = 1.3 x 10-5] B. Carbonic acid / carbonate [Ka = 4.3 x 10-7] C. Hypochlorous acid / hypochlorite [Ka = 3.0 x 10-8] D. Hypobromous acid / hypobromite [Ka = 2.0 x 10-9] E. Phenol / phenolate [Ka = 1.6 x 10-10]



Answers

Which of the following conjugate acid-base pairs should you choose to prepare a buffer solution that has
${pH}=4.50 ?$ Explain.
(a)${HSO}_{4}^{-}$ and ${SO}_{4}^{2-}$



(b) ${HOCl}$ and ${OCl}^{-}$
(c) ${C}_{6} {H}_{5} {CO}_{2} {H}$ and ${C}_{6} {H}_{5} {CO}_{2}$

To choose the correct weak acid with the conjugate base. For a desired ph we need to find an acid, a weak acid with a PK value close to the desired ph. However, in the back of the book, PK values are not given just K values. So as an alternative, we could find an acid that has a K. A value close to the hydro knee um concentration that is present at a desired ph. So if we have a ph of 3.5, the hydro knee um concentration is 10 to the negative 3.5 Or 3.16 times 10 to the -4. There are a lot of weak acids with K values close to 3.16 times 10 to the negative four. I'll just name a few lactic acid Where K. is 1.4 times 10 to the -4 or HF where ca is 6.8 times 10 to the -4. If our desired buffer ph is 55. The hydro knee um concentration is 3.16 times 10 to the negative six Mueller. So we could use a cynic acid with a K. Two value of 23 times 10 to the negative six. Or moronic acid With the K. two value of two times 10 to the -6.

This question is pretty straightforward. If we know the two concentrations, the weak acid and the weak base, then pH will be equal to PK of formic acid. We can look that up in the back of the book today is 1.8 times tend the negative four. So we'll take the negative log of it to get the P K A plus the log of the base concentration, which is thesis odium form eight, divided by the acid concentration, which is formic acid, and we get 3.59 So if we're gonna have a ratio of acid to base that gives us a pH 0.5 units higher than this on the Ph is going to be 4.9 That will be equal to the PK a plus the log of the ratio of base toe acid. Do a little bit of algebra, and we get a base toe acid ratio of 2.21 or take the reciprocal. The question is asking for it the acid to base ratio and we get 0.45

Remember that buffers are most effective when the pH equals the PK. That means if we're finding if we're looking for a buffer to use for a solution that has a certain page weaken, try to find a buffer that has a similar pH that ph. This also means that the pH is equal to the negative Long. I think A or K A is 10 to the negative peach. So if we have a pH. Uh, 4.5, then 10 to the negative 4.5 is 3.16 times 10 to the negative five. So this is the k A that we're looking for for a, um we guess it or weak base ethanol amine or H O C H two C h two and H two has a okay, a value, uh, 3.2 times 10 2 magnificent and a country get asset. H o C H two, ch two and h three. Plus, I did pick acid or h O C. Ch. Two for c o. H has a contra get base of H won't go see ch two for C O minus and a K A value of 3.8 times 10 to the negative IQ because these values air relatively close. This conjugal pair is an appropriate buffer for a ph of 4.5. If we have a pH seven 10 to the negative seven is just one times 10 to the negative seven. So we're looking for, um, a weak acid with a K A value of approximately one times 10 to the negative seven h two and he s four minus, which is the country. Get base of arsenic acid, uh, can lose another hydrogen atom to become h s. So or four to minus arsenic acid has a k A to value of, um 1.1 times 10 to the night or seven. So this country get pair makes an appropriate appropriate buffer for a pH of seven.

Okay, so this question consists of two parts. Part A is asking us to calculate the pH of a buffer solution of formic acid. Um, so the way we can do that is we can use the general equation for a buffer solution which I have written here is the hydrogen ion concentration or the hydro Nia. My on concentration, I believe Thea, the book has a written as hydro me. Um, but essentially, this is the same thing equals the ratio of the concentration of the acid to its country base times the equilibrium constant of the acid. So the acid we're dealing with it within this problem is formic acid. So I have the equilibrium constant written here, and so we can start plugging the values that were given in the problem into this equation. So our hydrogen ion concentration is going to be equal. Is going to equal the concentration of formic acid, which is 0.50 Mueller 0.5 zero. Well there, divided by the concentration of the contra bass, which in this case is given by sodium for mate, which is assault of sodium and the form eight ill. So that is going to be 0.35 is what we're given. 035 Moeller. No. And we can multiply this by the K A of formic acid and that is gonna give us a hydrogen ion concentration. So do you, H plus, that is equal to two point five seven times 10 to the negative fourth. And to get Ph. What we do is we take the negative log of this. So if we take the negative log of both sides negative log of, I'll just write it like this. So this is just saying we're taking the negative log of both sides of it. So ph this guarantee equal three boy five nine. And so that is the answer to part A. So what part B asks us is what the ratio of acid to congregate base needs to be if we increase the pH by 0.5. So I'm gonna erase some of this to make some space. So I'm gonna race the calculation we just did. And so our new ph since we want to increase it by 0.5 is going to be for oh, and oh, no. And so 4.9 is our Ph and we want to get one way we can do this is to get hydrogen im ion concentration and essentially work backwards. So we want to know what the ratio of acid to bases. So if we take the hydrogen concentration and divide it by que it by the K a, then we'll get the ratio of passive to contradict base. So the hydrogen ion concentration, if we take, is gonna be if we take 10 to the negative four point. Oh, no. When? Oh, no. And that's gonna be our hydrogen concentration. And that's essentially doing what we're doing is a negative log in reverse to get hydrogen concentration. So we took the negative log of hydrogen concentration to get pH. And now we're raising 10 to the negative pH to get the hydrogen ion concentration. And so that works out to be about eight 0.13 8.13 times 10 Teoh and negative. This right? So what we can see from this equation is that if we divide both sides by K A, then we get hydrogen ion concentration divided by K A. Equals the ratio of acid to country base So from here, it's All we have to do is take this and divide it by this. So we'll take 8.13 three signs, 10 to the negative fifth. And that's Muller because it is a concentration. I'll put units here. Just so we're clear that this is a value of concentration and not not something like unequal of their constant. And then what we can do is divide that by the K. So 1.8 times 10 it's the negative fourth. And what we get for the ratio of acid to congregate base is 0.45 So that is our answer to party. Gonna rewrite that zero. Okay, there it ISS So 0.45 So this is our answer to part B. And this is our answer to part a So part A. The pH was 3.59 and then we raised that pH by 0.5 to 4.9 and begat that the ratio of they asked it to the contra get base has to be 0.45 for this particular acid in order to give a pH of 4.9


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