Video Transcript : pH pOH ka and kb Calculations for Weak Bases in MCAT Chemistry

Weak Base pH pOH ka and kb Calculations in MCAT Chemistry by Leah FischBelow is the written transcript of my YouTube tutorial video pH pOH ka and kb Calculations for Weak Bases in MCAT Chemistry.

If you prefer to watch it, see Video HERE, or catch the entire MCAT Acid Base Series

[Start Transcript]

Leah here from leah4sci.com/MCAT and in this video we’ll take a look at strong bases. You can find this entire video series along with the practice quiz and cheat sheet by visiting my website leah4sci.com/MCATAcidBase.

Just like a strong acid, a strong base is one that will dissociate or react a hundred percent when dissolved in solution. If we place a strong base like NAOH in water it will dissociate one hundred percent to give me NA+ and OH-. We have a one way arrow to show that this is a reaction that goes to completion and this is a strong base. NAOH qualifies as Arrhenius base because we get the OH minus in solution if the solution is water. But the strong base doesn’t just have to have an OH in it, it can also react with water or other solution to give that OH minus. For example the methide CH3 minus when dissolved in water will react with water to give you methane which is CH4. But now that fourth Hydrogen comes from H2O and what you are left with in solution is OH minus ones again an Arrhenius base. But this can also happen in a non-water solution that means it falls under the Bronsted-Lowry category. For example if I take the same CH3 minus but this time I dissolve it in solution of Ammonia NH3, it’ll still react to give me CH4 but this time base that forms is the amide NH2 minus instead of OH minus.

A weak base on the other hand is one that does not go to completion. For example, if I dissolve NH3 ammonia in water, it’ll react with water very similarly to the CH3 minus. We’ll show equilibrium arrows and then our product of NH4 plus and OH minus. Because Ammonia is a weak base, we have to show the equilibrium arrows because the reaction doesn’t go to completion. When ammonia is dissolved in water you’ll have some ammonia and then some ammonium but they’ll keep going back and forth in equilibrium so that we’ll have some ratio of ammonia to ammonium. The strong base that you want to memorize for your MCAT are the hydroxides that are pair with alkaline metals, group one on the periodic table and alkaline earth metals group 2 on the periodic table.

Here’s a list of some of the strong bases you want to memorize for your MCAT. Our hydroxides that are pair with your alkaline metals which are group 1 in the periodic table and the alkaline earth metals which are group 2 on the periodic table. But the alkaline earth have two hydroxides so don’t forget to account for them in your calculations. Also recognize that some of the heavier alkaline earth metal bases will be more soluble in an acidic solution and less soluble in a basic solution to the common ion effect.

When it comes to deforming the strength of the base, you can measure in pH or pOH. Remember that a p of anything is equal to negative log of that thing. Then pH is equal to negative log of the H plus concentration something that we looked at in the acid video and pOH is equal to the negative log of the OH minus concentration. However, on the MCAT you can expect to have to measure bases in both pH and pOH. More so than acids will be measured in pOH. That means you have to be comfortable going between them. The two important equations to remember are that kw is equal to the H plus concentration times the OH minus concentration which is equal to one times ten to the minus fourteen at 25 degrees Celsius taking the p of this entire equation. We get that pkw is equal to pH. Multiplication becomes plus, pOH and that is equal to just fourteen. Again, at 25 degrees Celsius.

So let’s try a few quick practice problems. Say you’re given a point o one solution of pOH and asked to find the pH. First thing we want to recognize is the kOH is a strong base and will give us one hundred percent dissociation. So the molarity of kOH is automatically equal to the molarity of OH minus because it dissociates one hundred percent to OH minus ions. And if the concentration of OH minus is equal to zero point zero one, we have two ways to find the pH. The first thing we can do is use a kw is equal to H plus times OH minus, but I don’t wanna deal with exponents and scientific notation if I don’t have to so I prefer to go the second route. Find the pOH from the OH minus concentration and then use the pOH to find the pH much faster and honestly way less stressful. The pOH is equal to negative log the OH minus concentration of zero point zero one which we’ll first turn into scientific notation of one times ten to minus one two, one times ten to the minus two which means our pOH is equal to two. If you’re not familiar with this trick, go back to my MCAT Math video series which you can find on my website leah4sci.com/MCATMath.

If pkw of fourteen is equal to pH plus pOH, to find the pH we simply do fourteen minus two which is equal to twelve and that’s our pH. Let’s do another example. This time you’re give zero point zero five moles of calcium hydroxide dissolved in 500 milliliters of water. And you’re asked to find the pH. First thing we’ll want to do is figure out our path. If we’re going from a base to pH, I prefer to find from base to pOH to pH. And in order to find the pOH I need the OH minus concentration. To find the concentration, we’ll use the equation of molarity is equal to moles per liter. And in this case we’re given zero point zero five moles divided by a volume of 500 milliliters which is not liters. To go from milliliters to liters you have to divide by a thousand or use the simple trick to move the decimal back three spaces which gives us point five liters. Great setup but there’s way too much going on! Decimals and fractions at the same time this is too much for the MCAT so let’s simplify as much as we can. If we divide the 5 out for the top and bottom, replace with a one since 5 divided by 5 is one for both top and bottom. This gives me 0.01 over 0.1 but I wanna get rid of the decimal especially in the denominator so I’ll move the decimal one space to the right, top and bottom which is the same thing as multiplying by ten over ten or one. And that gives 0.1 mols. A liter or 0.1 mol. But this is not my OH minus concentration.0.1 molar is the concentration of calcium hydroxide and you can’t forget that for every one molecule of calcium hydroxide we have two ions of OH minus so every one mol of calcium hydroxide give us two moles of hydroxide. You should recognize that all you have to do is multiply by two and if you’re not sure here’s the setup. 0.1 molar calcium Hydroxide times the ratio of one calcium hydroxide for every two OH minus. This shows you that we multiply by two. Two times one is two with one decimal space gives us point two molar and that’s our OH minus concentration.

So the pOH is equal to negative log of 0.2 which is the same thing as saying negative log of 2 times 10 to the minus 1. Two times ten to the minus one is very close to one times ten to the minus one, so in this case if you forgot to double it your answer would still be close enough. We’ll round it down to one because on the MCAT close enough is good enough and get a pOH of 1. If pOH is around 1 then pH is around 13. Now say you have two choices on the MCAT where one is slightly greater than 13 and one is less than 13, how do you know which one to choose? When we round it, we round it towards pure hydroxide ions. That means we made it more acidic than it should be. But in reality we have more hydroxide so it’s more basic. So the pOH should really be something less than 1 coz it’s much stronger and therefore the pH should be something greater than 13. So all you have to look for is pH slightly greater than 13. I punch this into the calculator just to show you how close we are. And the calculator says that the pH is equal to 0.699 or 0.7 and the pH is equal to 13.3. 13.3 is slightly greater than 13 but doing this on the MCAT without a calculator would have taken you so much time. Now there’s one more thing I want to talk about when it comes to strong bases and that’s a leveling effect. Just like with acids, if you take a very strong base and dissolve it in water, the absolute strongest base that exist in water will be OH minus. So how is that possible? If you started with the stronger base, shouldn’t you have a stronger base in solution? We’ll use the example of methide CH3 minus ion knowing that it is a much stronger base than water. If we place this into water, the strong basic electrons on the carbon will reach out and grab one of the hydrogens and pluck it right out of the water molecule to give you a CH4 gas which bubbles out of solution plus an OH minus ion. The moment the methide ion touches water, instead of only strongest base, is only Hydroxide.

The idea of the leveling effect is that any really really strong base that enters water levels up so it’s only as strong as the hydroxide in water. Be sure to join me in upcoming videos where we look at calculations for weak acids and weak bases and specifically focus on MCAT appropriate shortcuts. You can find this entire series along with my practice quiz and cheat sheet by visiting my website leah4sci.com/MCATAcidBase.

Are you stuck on a specific MCAT topic? I offer Private Online Tutoring where I focus on your needs to strengthen your individual weaknesses. Tutoring details can be found using the link below or by visiting my website leah4sci.com/MCATTutor.

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