Learn about Senior Chemistry, Acids and Bases 4, in this comprehensive video by bannanaiscool.
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The pH of a solution is arrived at by getting the hydronium ion concentration, taking the log of that and then making it a negative. So it’s on a logarithmic scale that we measure pH. And a lot of people say, pH goes from one to 14. No you can have pH less than one and even less than zero. You can have pH that is greater than 14. pH scale—there’s not really a scale so much as seven is right in the middle and anything less than seven is acid and anything greater than seven is a base. So, in a graphing calculator all you do is you type in negative then hit your log button then type in the hydronium and then press enter you’ve got yourself the pH. How do you do it if you’ve got a concentration of an acid that’s not the hydronium? Well if it’s a strong acid, now listen how many strong acids are there? Six. If somebody gives you the concentration of the strong acid it’s exactly the same as saying here’s the concentration of hydronium. So to find he pH here the pH = -log of the hydronium since HCl is hydronium it dissociate 100% again it’s the negative log ogf 0.2. When you type that into your calculator you get 0.70 and a lot of you are saying yeah that’s right because you've got two significant digits so you keep two significant digits. That’s not the way it works with pH. So stay with me and listen to this rule it’s very important. The number of significant digits in your hydronium ion concentration tells you how many numbers after the decimal to keep in your pH. It’s a logarithmic scale like I said. So all bets are off with significant digit comparisons. So look at this one. If you’ve got the pH of 0.3 m/l sodium hydroxide how do you find the pH of that? The first thing you need to do is you need to calculate the hydronium ion concentration because NaOH—well that has got OH in it and when you see that hydroxide, one hydroxide that’s the concentration of hydroxide in solution not hydronium. So then how could you do that? Well I’ll show you. Here it is. But it’s done in a very straight forward type of fashion. The pH equals the negative log of the hydronium, but I don’t have hydronium I have the concentration of the hydroxide here. But 1 x 10-14 divided by that hydroxide does equal to hydronium. Now, you're looking at this and you say well okay there are three significant data there so keep three. But a lot of people would then say 13.5—no, no 13.477 significant digits equal number of decimal places kept in the pH. Does that make sense? NaOH is a strong base, it has got a nice high pH. HCl has got a very low pH for an acid. So you see that pH scale I'm talking about doesn’t really come into play other than 7 means neutral. Ba(OH)2 is a strong base too, but look. Here’s the thing that will get you every time. Your teacher is not going to say to you Ba(OH)2 is the formula. He’ll just say barium hydroxide and you'll say oh that’s hydroxide so I’ll just take the concentration of 0.25 moles/liter and plug it in to my formula. No! Because Ba(OH)2 has two OH isn’t it? When it dissociates in the solution it forms two hydroxide ions and so 0.25 mole/liter Ba(OH)2 actually is 0.5 moles/liter hydroxide ion in it. So therefore you they have to calculate the pH with that concentration. Now, I want to show you something else. That pH the H stands for hydronium so the p stands for the negative log part. Well if you take the negative log of the hydroxide you get the pOH, and if you add a pH and pOH together of the concentration of hydronium and hydroxide that are in every solution of acid and base, you’ll get the number 14. And so if we wanted to calculate the pH here, the pH could equal rearrangement of this formula 14 minus the pOH. So that’s 14 – negative log of that concentration of hydroxide that will give you right there the pOH subtract it from 14 and that’s the pH. And in this case the pH is going to be 13.70. Notice that two significant digits mean two numbers after the decimal kept in the pH. So be careful of those multiple hydroxide on