 Let's do another one of these Calculate pH of the equilibrium problem. So this one says calculate the pH for the following solution a 0.10 molar Ammonia solution and we went and looked up the kb on the kb table that we had in our book and it ended up being 1.8 times 10 to the negative thing Okay So in order to do these particular problems you're going to have to write up a nice table first Okay, so in order to do that you have to figure out. Well, what's the balance chemical equation? Okay, and the other thing you want to think about is well, we're looking for the pH right So we're going to look for something that gives us a change in pH value like a H3O plus or OH minus Okay, so let's write down the chemical equation It's going to be NH3 aqueous plus water plus water liquid goes back and forth to the ammonium ion plus OH minus aqueous And that's what we're going to look for our pH with So this is a balanced chemical equation if it wasn't you'd have to balance it of course Remember with liquids Cancel them out They gave us the initial concentration of ammonia 0.10 We didn't have any ammonium ion or any hydroxide ions So we're going to subtract x from that Remember in this case, we're going to be using our 5% rule, so we'll get rid of that x there We're going to add x here and add x here It's going to give us x and x like that Does that make sense as far as we've done? To make it you got this one that makes sense to you. Okay, wonderful So remember what did I just say? We're going to use the 5% rule and you can check this later I'll let you do that on your own so we're going to effectively say that this because the kb is so small Not very much dissociated. Okay, so we're going to just erase this x So that reduces our kb equation, so let's solve our write out our kb equation So just like anything else It's going to be the products raised to their coefficients divided by the reactants raised to their coefficients, right? So what have we got? concentration of the ammonium ion times the concentration of hydroxide ion divided by the concentration of Ammonium So does everybody set with that one? So now let's just plug in our numbers Okay, so well This right. This is x and this is x right? So we're going to actually be solving for x square, which is what we'll be doing Okay, but let's just plug in our numbers. So we'll rewrite this equation to say 1.8 times 10 to the negative fifth Equals x Times x divided by 0.10 So now we're just going to algebraically manipulate this equation Get your calculators out so we can do this all together I'm going to get a pen and it starts writing a little darker, hopefully Hopefully I picked the right one So let's rearrange this equation to solve for x. So what are we going to get? We're going to get x squared equals 1.8 times 10 to the negative fifth times 0.10 Like that So that means that x equals the square root of 1.8 times 10 to the negative fifth times 0.10 Does that make sense there? So let's just do that together So the concentration of x is going to be 1.3 times 10 to the negative 3 molar like that So remember x is equivalent to the OH minus concentration Okay, and that's where we get the pH from Okay, so as everybody said with what we've got here. So let's just say we've got 1.3 times 10 to the negative third molar OH like that Okay, so I'm going to erase the top of this here so we can use that space to figure out the risk Okay, so remember kW equals the hydronium ion concentration times the hydroxide concentration like that Okay, kW It's going to be given to you 1.0 times 10 to the negative 14 So we could figure out from this the hydronium ion concentration, right? So we just say hydronium ion concentration equals kW divided by the concentration of OH minus like that, does that make sense? Okay So what do we got? 1.0 times 10 to the negative 14 divided by 1.3 times 10 to the negative 3rd I get 7.5 times 10 to the negative 12 molar Okay, so that's hydronium ions and we would expect that to be very small Because this one's relatively large. Okay, they're going to be inverses of each other So from that we should be able to figure out the pH, right? Because the pH remember is what? negative log of the guys hydronium ion concentration, okay? Yeah It's easy when you write it out. Okay, so what is it going to be? So we say the negative log times 7.5 times 10 to the negative 12 and As you would expect it to be, right? It's a very high number Okay, why because we have a weak base. We're putting a weak base, right? We would expect it to be over 7, right? And it is it's 11.1 13 Everybody understand why it should be Like just qualitatively higher than 7, right? Any questions on this one? Well, can you actually do it like my given the The POH? Yeah, you can do that. I just like to do it this way because this is the way I first learned how to do it You know, so I just keep doing it that way But yeah, you could get the POH from here say the negative log of the OH Concentration and then subtract that from 14. You should get the same answer. Did you get the same answer? Did you do it that way? Okay. Well, I'll bet you'll get the same answer. Any questions? Wonderful