Learn about Senior Chemistry, Acids and Bases 5, in this comprehensive video by bannanaiscool.
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Rob Lederer: Calculation of pH of a strong acid, negative log of that concentration given. Calculating the pH of a strong base, you can go 14 minus the pOH, and be careful of those double hydroxides. But what if you're given a weak acid, an acid that doesn't dissociate 100% in solution? Well, then it doesn't make hydronium 100% of the time like a strong acid will. So what you need is a dissociation constant, a K value, that's the Ka. So what we need to do is when we are calculating the pH of say 0.1 mole per liter acidic acid, a weak acid, we need to take that acid and react it with water. Water will actually accept the proton to form acetate ion CH3COO-negative and hydronium. If we know that hydronium ion concentration, we know we can get the pH real easy, negative log of that number, but this doesn't break down 100% to make 0.1 here and 0.1 here, how much does it break down? Not as much, a little bit and an x value is how much will get of the acetate ion and importantly of the hydronium. So that's the changed line, right? Then the equilibrium line, you observed as 0.10 minus x and x and x here. I'm just taking for granted you remember everything about equilibrium chemistry in order to do this. If you don't, you got to review those other discs, right? Okay so if we write the expression for this, we don't include water liquid, so the K value which is called the Ka here as an acid dissociation constant for weak acids, strong acids have them too, but there essentially a 100% reaction, right? Ka equals the concentration of this times this over the acidic acid. Of course we know that in equilibrium, we have these concentrations and we substitute in. Now to continue with the math, bang, we got the answer. Now we substitute in the K value, you can find this on any data sheet in the back of your text book, you'll get the acidic acid's dissociation constant Ka equalling 1.8 times ten to the negative five equals x-square over 0.1 minus x. Hey, it will be really convenient if you get rid of that x. Well, if you take the initial concentration of 0.1, and divide it by the K value, you get a number that's greater than 1000, so you can essentially drop the x. If you don't like to, you'll have to do a quadratic formula, but you're going to get the same answer anyway, so drop it. This times this equals x-square, so x-square equals 1.8 times ten to the negative six. Then x equals the hydronium ion concentration, which equals when taking the square root of this number to find x, you get 1.316 times ten to the negative three. Keep all your numbers in your calculator till the very end. Now to get the pH, the negative log of that number and it equals 2.87. Notice two significant digits, two here, so I keep two after the decimal in my pH, does that make sense? Well, 0.1 mole per liter solution of the acidic acid is going to have a higher pH than a 0.1 mole per liter solution of a strong acid, which actually would have a pH of 1. So a pH of 2.87 means that it is a weaker acid in comparison to a stronger acid, although strong acids could have that pH if they were dilute, but that's complain. Strength vs. Concentration Okay so later turns into right now. Look, here is a 0.1 mole per liter solution of HCl. Here is a 0.1 mole per liter solution of acidic acid. So because this is a strong acid and this is a weak acid and at the same concentrations, we can compare relatively what their pHs will be. A strong acid will have a lower pH than the weak acid. So therefore you can make a kind of a comparison, but take a look at this. Here is a strong acid HCl, put as 1.4 times ten to the negative four moles per liters; 0.00014 moles per liter, 10 moles per liter acidic acid in this comparison, which one of these is the stronger acid? Don't get confused. A strong acid means you dissociate a 100%, that's still the strong acid. HCl is the strong acid. That is a weak acid; it'll never change, but this is a concentrated weak acid and this is a dilute strong acid. Wh