Male Speaker: Here are the real meat potatoes; here is what that acid base unit is all about reacting these chemicals together. Acids and bases and being able to determine or and find the answers to unknown volumes and concentrations like this question, which is asking you to find out what volume of the acid is needed to completely neutralize this base? So, I hope you realize that the acid HI is a strong acid and you just check them out in the chart, so it's hydronium and iodide, sodium hydroxide and water. You find the strongest acid's strongest base. Easy enough, its hyrodnium plus hydroxide makes 2H2O. 2H2O. There we go. Hey, now listen. You can't do stoichiometric unless you have a 100% arrow that's called the stoichiometric arrow for a reason that's because we can have proper ratios reacting together. Equilibrium arrows won't do that for you, so acid base stoich must have a 100% arrow. You know that means don't you? Remember, that means you have to have hydronium or hydroxide in a reaction. If you don't, you got weak reacting with weak and it ain't going to give you a 100% and you ain't going to be calculate anything. Eight, eight, eight. So, in this case, we were given 0.25 moles per liter hydroxide and here is the volume, I said 10.0 mls, so I changed that to liters, but look it, I didn't use my -- don't use the -- and then mole per liter hydronium or that's HI and what's the volume of that? So, again, what do we do here in order to calculate that volume? Well, remember the key is remembering that its moles that react with one another, right? Moles. So, if you have a concentration and a volume here, you know that moles per liter and liters that can you get the moles. You find the moles here. There is a one-to-one ratio so you low the moles at the point where these two react equivalently that's called the equivalence point. And then, if you know the moles here and the moles per liter, you can calculate volume. You can do it all in one line baby and here is one line. If you take that concentration and volume and multiply it together, but look it's 0.25. I know we have hydroxide here. I would like to go back to the original chemicals in the question in order to do this stoichiometry, it just looks better and it's more acceptable. So, we always want a net equation, but put the -- names into the stoichiometry. 0.25 moles of NaOH per liter, we never put a name of a compound next to liters. We put it next to moles, we put it extra grams, but we don't put it next to a volume. Never, because that volume, that liters not liters of NaOH, it's just liters of a solution made up of water in NaOH. So, don't -- because you won't get proper cancellation unless you do that so do it right. Now, 0.25 moles of NaOH per liter times the volume in liters that makes the liters cancel and you left with moles in NaOH. But we don't want both of NaOH while moles of the HI. So, we do a ration. Now, we got moles of HI where the moles of NaOH cancel, and then we want the liters of the HI, we wonder what the volume it requires to neutralize. So, we divide by the moles of HI per liters, it has moles per liter, but it's just inverted that's mole point on moles per liter and that's one liter over 0.1 moles of HI you can do that. Cancel out the moles of HI and now you are left with liters, which are 0.025 liters and 25 mls. Why did I say that? Do you think -- even though there is three here and three here, so you got to keep two in your answer. You can write that, and then just say or 25 mls because everybody can move the decimal three times to find that to knows what.