Below is the written transcript of my YouTube tutorial video H+ and pH Calculation for Strong Acids in MCAT Chemistry.
Leah here from here from leah4sci.com/MCAT and in this video we’ll take a look at strong acids as it’ll come out on your MCAT. You can find this entire video series along with practice quiz and cheat sheet by visiting my website leah4sci.com/MCATAcidBase.
When studying acids and bases, you’ll see some compounds that are strong acids and some that are weak. Think of the strong acid as very capable of doing what it’s trying to do. What does an acid try to do? If we have an acid dissociation reaction, we have HA where A is the molecule and the H is the proton. So HA is deprotonated acidic form and it will dissociate to give me an H+ and the conjugate A-. A strong acid is one that has a one way arrow, meaning the reaction goes to completion. If you take that strong acid and dissolve it in water, one hundred percent of the acid molecules will separate from their conjugate bases and dissolve independently in solution. The only way this can happen is if the conjugate base the A minus is stable in solution is comfortable with the resulting charge, in this case a negative charge and therefore has absolute no desire to reach out and grab that proton and hold on to it. A weak acid on the other hand has partial dissociation. So if I start out with HA, it will be in equilibrium with H plus and A minus. If a molecule of HA dissociates to give me H+ and A-, some of those A minus ions will reach out and grab that proton to reform HA. That means we don’t have a hundred percent dissociation and instead we have to look at the ratio that we see at equilibrium why would the A minus reach out and grab that H plus? Because it is not stable in solution and it is not happy being on its own. But on this video I wanna focus specifically on the strong acid.
You want to memorize the following strong acids for the MCAT. For Chloric acid, HClO4, Sulfuric Acid H2SO4, Nitric Acid, HNO3, Hydrochloric Acid HCl, Hydrobromic Acid HBr, and Hydroiodic Acid HI. H3OO+ is also a strong acid but we’ll mention that separately. There are two categories of strong acid that you want to memorize for the MCAT. The first category are the acids made my associating a proton with an Oxyanion. Oxyanion comes from Oxy which is Oxygen and negative ion, something that has a charge. This means a molecule that has Oxygens and a negative charge. If you take away a proton from each of these acids, they’ll get a negative charge. The reason the acids of these specific Oxyanions are considered so strong meaning they’ll dissociate a hundred percent in solution is because the conjugate bases are very very stable. Let’s take a look at HClO4 which will dissociate to give H+ and ClO4- in solution. If I draw the lewis structure for Cl minus, I start with the Chlorine and then put four oxygens around it. We’ll gonna use my lewis structure cheat which is atoms, octets, electrons, and formal charge. All the atoms are present, I’m gonna give every atom a complete octet. Count the electrons, chlorine has 7 valence, Oxygen has 6 times 4 is 24 plus one for the negative charge, for a total of 32 electrons. Oxygen has six visible electrons and then two more in the bond for a total of 8. I have 4 of these, 8 times 4 is 32 so that means my atoms are good, my octets are filled, I have all my electrons present but now we’ll do a quick formal charge using my shortcut of should, minus, has. Oxygen should have 6, directly attached to each one I have 7, 6 minus 7 is negative one. We have a negative one charge on each of the Oxygen atoms which doesn’t appear to be very stable. In fact if we continue we have an even worse charge than chlorine. Chlorine should have a total of 7. Only four present, 7 minus 4 is positive 3. That doesn’t look right. But negative is attracted to positive and you’ll actually have an internal attack where three of the negative oxygens will attack chlorine, that cancel out their charges. And the structure is a chlorine with one double bound, two double bound, three double bound oxygens, one single bound oxygen that still has a negative one. The remaining three oxygens and Chlorine now have no charge, we got rid of the charge. Chlorine is an exception to the octet rule so it’s perfectly fine having all these extra bonds. So we reduced the charge of the net to negative one. How does this explain why this is so stable making its conjugate acid so acidic? Well I chose to leave this one negative. But what if instead I put a pi bond here and chose to make this one negative, or this one, or this one. And the answer is they’re all correct. This ion has a total of four resonance structures. Resonance equals stability and that makes it so stable when it can share it’s negative charge among 4 different oxygen atoms. You can find videos explaining this and lewis structure, and formal charge, on my website leah4sci.com/MCATAcidBase.
Going back to the Oxyanion acid series, you can see resonance for sulfate, N for nitrate which explains why these are such strong acids. Now let’s move on the Hydrogen Halides. Hydrogen because of the H and Halides for the Halogen. You should be familiar with the halogens on the Periodic Table, starting at the top we have Fluorine, then Chlorine, then Bromine, and Iodine. And you should also remember that size increases as you go down the periodic table. Fluorine, because it’s such a tiny atom, when negative will desperately try to grab that Hydrogen, it’s so small and clingy making it a bad acid because an acid has to let go of its proton. But as you get to the larger halogens that are much larger and much more capable of holding that negative charge because they distribute it over their big size, that makes them very stable. And if they’re stable they don’t mind letting go of that hydrogen and if they don’t mind letting go of that hydrogen, that makes them a strong acid. And finally we have H3O+ which is not only a strong acid, but also the only acid you’re going to see in solution for other strong acids because of the leveling effect. I like to think of the leveling effect as the equalizer effect. If I take a really really strong acid, HA and i dissolve it in water, that acid will dissociate so quickly to give me H plus and A minus. But don’t forget that I still have H2O in solution and the water will use its lone electron to grab that H+ giving me an H3O+ in each place. So any strong acid that is dissolved in water will level off and the strongest acid you see ones it dissociates is hydronium or H3O+. A stronger acid cannot exist because as soon it touches water, it dissociates, it levels off to the strength of H3O+.
Now we know that a strong acid dissociates a hundred percent, so how does that help us understand the pH in solution? Recall from your logs studies that p of anything is equal to negative log of that thing. So if we want to measure the H+ concentration of the strong acid, knowing that the concentration will increase ten-fold as it gets stronger and decrease ten-folds as it gets weaker, we wanna use a logarithm scale and therefore we take the negative log of the H+ concentration and that’ll give us the pH. Now you’re not allow to calculate on the MCAT so make you’re very comfortable with your log and antilog calculations. And if you’re not, my video series is linked on the acid base page. So for example, say you’re told that you have zero point zero three (0.03) moles of HCl and you dissolve it in 1 liter of solution and you’re ask to find the pH. We know that pH is equal to negative log the H plus concentration. So the first thing we need is an H+ concentration. Concentrational molarity is equals to moles over volume, in this case 0.03 moles of HCl divided by 1 liter of solution. This gives me 0.03 molar or you can write it in scientific notation, that’s to the minus two, three times ten to the minus two.
Next we wanna find the pH. ph is equal to negative log of 3 times 10 to the minus 2. I put this to the scientific notation to remind you that we’re using the trick and the pH is going to be around 2. If you to be more specific, use the log numbers that you have memorized. If the numbers starts with a three, the negative log will give you a 5. So the question is, is our pH 1.5 or 2.5? Well let’s see. 1.5 is between the numbers 1 and 2. 2.5 is between the numbers 2 and 3. Just 3 times ten to the minus 2 goes between 1 and 2 or 2 and 3. The easiest way to do this is to think of 1 times ten to the minus the number which is equal to that number. So I have to take my three times ten to the minus 2, round it down to one and up to 10. Right because we want to have a one and a ten. Ten is really one times tens, we need that one. That means the pH is going to be somewhere between one times ten to the minus two which is three rounded down to one and ten times ten to the minus two which is three rounded up to ten. The pH of one times ten to the minus two, pH is equal to negative log one times ten to the minus two is equal is two. The pH of ten times ten to the minus two we can’t do coz it’s not in proper scientific notation. Ten times ten to the minus two is the same thing as saying, one times ten to the minus one. Because we’re doing a times ten divided by ten, we’re dividing ten by ten which is one, we’re multiplying to the power of negative two times ten which is to the power of negative one and pH is equal to negative log of one times ten to the minus one which is equal to one. Meaning our pH is somewhere between one and two, in this case it’s 1.5 because the three gave us the five after we determine it’s between one and two. Now this is a little too in depth for MCAT calculations, what I would rather you do especially if the choices are far apart is for you to say well, three times ten to the minus two is very close to one times ten to the minus two, just round it down, give your answer as two, it’s not that close between 1.5 and 2, but if your answers are varied enough, this is close enough. Now notice I used HCl concentration as my H+ concentration, how was I able to do that? If I have 0.003 moles of HCl and I know that one hundred percent is going to dissociate, I’m going to get 0.003 moles of H+, 0.03 moles of Cl minus. Cl minus is a spectator, it’s such a weak base it’s not attacking anything we ignore it and that’s how we found our H+.
Two more things you wanna look out for, the second thing is the dehyprotic series. If I have H2SO4 we set as diprotic because it has two protons. If I have a complete dissociation then I have to account for both hydrogens in solution. Now the strong acid here is not the complete dissociation, it’s just one proton coming off HSO4 minus plus H+ and this right here is less likely to dissociate making it a weaker acid. So when would you have to do a complete dissociation ,meaning break off that second proton, when you’re doing something like a buffer or titration when a base is attacking it. But if that comes up and you want to know how many protons are there in solution, you want to account for both. But if you’re simply trying to find the pH, then H2SO4 is only giving up one H+ easily and then it gets a little greedy it doesn’t want to give up that second one so make sure that when you’re doing a calculation, you’re only accounting for one in a simple solution, so for example if I tell you that I have 0.1 molar H2SO4 and I want you to find the pH. Then all you do here is pH equals negative log of 0.1 which is one times ten to the minus one and that answer is one.
The final trick I want to show you is if the answer does not make sense logically, it is not the answer. Many students fall for the question that goes something like this. Find the of the solution when one times ten to the minus moles of HNO3 are added to one liter of H2O. Looks pretty straight forward. HNO3 is a strong acid means a hundred percent dissociation and that means I’m going to add one times ten to the minus nine moles of H+ in one liter of solution, that would be my molarity. So you set it up pH is equal to negative log of the H+ concentration of one times ten to the minus nine. Take that number right there and your pH is nine. pH is 9 for an acid? How is that possible? pH of a less than 7 is an acid, 7 is neutral, greater than 7 is a base. How can you dissolve Nitric Acid in water and get a basic pH? And the answer is, it’s a trick question. Neutral water has a pH of 7, if you’re adding H+ to the solution in this case we’re adding one times ten to the minus nine, it’s not enough to change the pH and so the pH is just going to be 7 or maybe 7.0 something but it’t not enough to actually change it so ask yourself, does this make sense logically? and if not, what is a better solution? So in this case, the pH of the solution that has a tiny tiny bit of acid added to it is going to have the same pH it started with and that’ll be a pH of 7.
Let’s break it this down a little more in case you’re not confident with it. Neutral water has a pH of 7 which means it has an H+ concentration of one times ten to the minus seven. Let’s say this is one liter of water and we’re adding a teeny tiny bit of acid without changing the volume. So we’re adding one times ten to the minus nine H+. What happens when you add one times ten to the minus seven and one times ten to the minus nine? one times ten to the minus seven equals this thing plus one times ten to the minus nine which is this thing. Now, please don’t draw this out on the MCAT, you are wasting time, I’m just drawing this to prove a point. The answer is going to look like this: If I put this into a scientific notation, I get 1.01 x 10 to the minus seven. To find the pH for this solution, pH is equal to negative log of the H+ in solution which is negative log of one point zero one times ten to the minus seven, I’m not given a calculator, I am not wasting my time. One point zero one rounds to one, One times ten to the minus seven is seven and we’re right back to our neutral solution.
Be sure to join me in the next video where we look at strong bases and how we can use that to find the pOH and the pH of the solution. You can find this entire video series along with my practice quiz and cheat sheet and all the links mentioned on this video by visiting my website leah4sci.com/MCATAcidBase.