- 2015 AP Chemistry free response 1a
- 2015 AP Chemistry free response 1b and c
- 2015 AP Chemistry free response 1d
- 2015 AP Chemistry free response 1e
- 2015 AP Chemistry free response 2a (part 1 of 2)
- 2015 AP Chemistry free response 2a (part 2/2) and b
- 2015 AP Chemistry free response 2c
- 2015 AP Chemistry free response 2d and e
- 2015 AP Chemistry free response 2f
- 2015 AP Chemistry free response 3a
- 2015 AP Chemistry free response 3b
- 2015 AP Chemistry free response 3c
- 2015 AP Chemistry free response 3d
- 2015 AP Chemistry free response 3e
- 2015 AP Chemistry free response 3f
- 2015 AP Chemistry free response 4
- 2015 AP Chemistry free response 5
- 2015 AP Chemistry free response 5a: Finding order of reaction
- 2015 AP Chemistry free response 6
- 2015 AP Chemistry free response 7
2015 AP Chemistry free response 3a
Net-ionic equation for a weak base titrated with HCl. From 2015 AP Chemistry free response 3a.
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- At2:35, why didn't the K react with Cl to form Potassium Chloride?(22 votes)
- Yousef Sheikh, Potassium Chloride is a salt that readily dissociates into its respective ions. If it were to hypothetically react and form Potassium Chloride, it would break back down into K and Cl.(43 votes)
- at3:45why didn't Sal just write C6H8O2 in stead of HC6H7O2 for sorbic acid?(13 votes)
- Both formulas are correct for sorbic acid! Sal wrote it with one of the hydrogen atoms at the beginning to make it a little more clear that the underlying structure of sorbic acid has one proton (or hydrogen ion) that is reactive, since it can be donated to a base. By writing it as HC6H7O2 and knowing that is an acid, one can figure out that once sorbic acid donates a proton, you are left with H+ and C6H7O2-.(15 votes)
- At1:31, why did he write C6H702 as one ion ? why didn't he break it down ?(6 votes)
- Break it down into what? It is all one ion already. The whole of C6H7O2 is a polyatomic ion called sorbate.
- At3:53wouldn't the equilibrium arrow be used in the net ionic equation since sorbic acid is a weak acid?(5 votes)
- Yes it technically should have been but the fact is that since a strong acid is being mixed with a weak base, virtually all will react.(6 votes)
- @1:35how do we predict when something will dissociate and when it will not? Why doesn't the Carbon or any other part of the sorbate dissociate?(5 votes)
- This sort of thing will come to you with time doing practice problems.
In general though just remember that water is not strong enough to break C-C or C-H bonds.(3 votes)
- How do you know that the Hydrogen and the C6H702 come together to form sorbic acid, but the K and Cl don't come together to form Potassium Chloride?
I get that the K and Cl dissociate because of the water, but wouldn't that also happen for the sorbic acid?(4 votes)
- They want you to use some intuition here.
The conjugate base of many acids have the suffix -ate (sulfuric acid -> sulfate, nitric acid -> nitrate, citric acid -> citrate). You should have come across examples of these already in your studies.
They also give you the Ka of sorbic acid in the question. Sorbic acid...sorbate, they intend for this to click in your head that this is part of the reaction.
In ionic equations strong electrolytes (soluble salts, strong acids) are split up into the ions they make them up while weak electrolytes are not.
Since they have given a Ka value for sorbic acid, this tells you it is a weak acid, so it is a weak electrolyte.
Potassium chloride on the other hand is a very soluble salt so it is a strong electrolyte.(3 votes)
- Not sure if I missed anything, but how do you know that the H+ is going to bond with C6H7O2, and not with anything else? How do you know what is going to combine and what's not?
Please explain/link me to the appropriate video.(1 vote)
- The key word in this problem is "titrate." Usually titrations are done by adding to an acid to a base, or a base to an acid. In this case, we are adding an HCl solution, and HCl is an acid. The reaction that is happening during an acid-base titration is an acid-base reaction, where a proton (H+) reacts to form a bond with the base (defined as something that reacts with a proton). In this case, we know it reacts with the base C6H7O2 in part because there isn't anything else to react with. There is also K+ in solution, but potassium ions are usually spectator ions.
I hope that helps! You can learn more about identifying acids and bases in this video: https://www.khanacademy.org/science/chemistry/acids-and-bases-topic/acids-and-bases/v/bronsted-lowry-definition-of-acids-and-bases(8 votes)
- At2:28, why don't we split up the sorbic acid into its cation and anion parts. Isn't that what we do to aqueous compounds?(2 votes)
- Yes and no. Potassium Sorbate breaks up int K+ and C6H7O2- ions in solution. Then the H+ ions from HCl attach to the C6H7O2- to make sorbic acid. Since Sorbic Acid is a weak acid a tiny little amount will ionize but it will mostly stay together in water.(5 votes)
- At3:14, what is a net-ionic equation?(2 votes)
- A net ionic equation only shows ions that take part in the chemical reaction, any so called "spectator ions" are left out.(3 votes)
- wait if k and cl were spectator ions, shouldnt sorbic acid be dissassociated by water molecules and turn into ions finally?(3 votes)
- I'm not really sure what you're asking here.
Potassium sorbate is quite soluble in water so it's already split up into K^+ and C6H7O2^- ions. Does that help?(0 votes)
- [Voiceover] Potassium sorbate, and they give us its formula right over here, has a molar mass of 150 grams per mole. They put this decimal here to show us that these are actually three significant figures. Even the zero is a significant digit here. Is commonly added to diet soft drinks as a preservative. A stock solution of potassium sorbate, dissolvent's an aqueous solution here, of known concentration must be prepared. A student titrates 45 millileters of the stock solution with one point two five molar hydrochloric acid using both an indicator and a pH meter. The value of K A for sorbic acid is one point seven times ten to the negative fifth. All right, so let's tackle this piece by piece. Write the net ionic equation for the reaction between potassium sorbate and hydrochloric acid. All right, so first off, let's write the ionic equation, and then we'll, I'll write the net ionic equation, and hopefully you'll see the difference. So ionic, ionic equation. And the way we think about ionic equations is we think about well, if these are dissolved in water, it's an aqueous solution, these are going to disassociate into their, into ions. And so we would write that out on both of the, on both the reactant and the product side. So the potassium sorbate, we can write that as, it's gonna be a potassium ion dissolved in an aqueous solution, plus the C six H seven O two, this is also going to be an ion dissolved in the aqueous solution, plus the hydrochloric acid will dissolve, so you have the hydrogen proton dissolved in the aqueous solution plus the chloride ion, or anion I guess we could say it. so that's going to be in our aqueous solution, and then they react. What happens? Well, you're going to have the C six H seven O two react with the hydrogen proton to get to sorbic acid. So you're gonna have sorbic acid, H C six H seven O two, that's the sorbic acid. It's going to be in an aqueous solution. So I took care so far of that and that, and then you're going to have, and then you're going to have your potassium ions, your potassium ions and your chloride ions. It's going to be just like that. So this right over here is the ionic equation, not the net ionic equation. I have the ions on the reaction, on the reactant side, and then on the product side right over here. And did I, yep, I included everything. Now you do the net ionic reaction, you can imagine what's going to happen here. I have potassium ions on the left. I have potassium ions on the right. I could net them out on both sides. So let's net them out on both sides. I have chloride ions on the left. I have chloride ions on the right. I could net them out on both sides. And then I can write the net ionic equation. So the net ionic equation, net ionic equation, is going to be well, I have my C six H seven O two ion dissolved in an aqueous solution. You combine that with the hydrogen proton dissolved in the aqueous solution, and it's going to, it's going to give us sorbic acid: H C six H seven O two in our aqueous solution. So there you have it. That is the net ionic equation.