Hey guys, in this new video, we're going to continue with our calculations dealing with pH and pOH. In this question, it says, a solution is prepared by dissolving 0.235 moles of strontium hydroxide in water to produce a solution with a volume of 750 ml. Now, I'm asking you to find the concentration of OH^{-}. Now remember, these brackets mean concentration. Another name for concentration is molarity. And remember, molarity equals moles of solute over liters of solution. Here we need the moles of OH^{-} divided by the liters of solution. Let's do the easier part first. The easier part is we have 750 ml of solution. Let's just change that to liters. We're going to say we have 750 ml. You can say here, for every 1 liter, there's 1000 milliliters or you could say for every 1 ml, there's 10^{-3} liters. That gives me 0.750 liters. That'll go on the bottom. Now, the harder part, we have to isolate moles of just hydroxide ion.

All we're going to do here is we're going to take these 0.235 moles of strontium hydroxide and convert them into moles of hydroxide ion only. We're going to say we have 0.235 moles of strontium hydroxide. And then just simply say here, for every one mole of strontium hydroxide, how many hydroxides are in the formula? You're going to notice that there are 2 of them, so there's 2 hydroxides for every one mole of strontium hydroxide. So that's going to give me 0.470 moles, OH^{-}. So we're going to take those moles and plug them up top. So 0.470 divided by 0.750 will give me 0.627 molar OH^{-}. That'll be our answer here.

Now, for part b, I want us to find the H^{+} concentration. Remember, all we need to realize here is if I know my OH^{-} concentration, then I know my H^{+} concentration because remember, they're connected by this formula. Kw=H+OH-. Plug in what we know for Kw and then fill in what we know for OH^{-}. So all we got to do now is divide both sides by 0.6 27 and we'll know what H^{+} is. H^{+} comes out to be 1.59x10^{-14}. Hope you guys were able to follow along.

This question just to do with a little bit of unit conversions and some little bit of stoichiometric relationships. For every 1 mole of strontium hydroxide, we have 2 moles of OH^{-}. So now that we've seen that one, I want you guys to attempt to do this practice question here. So remember, go back and see what the relationships are in order to find this correct answer. And just realize here when I say pure water, just understand pure water means neutral water. So if your water is pure, it's neutral water. And remember, what does that mean in terms of H^{+}, OH^{-}? Knowing that will be the key to solving this question. Good luck, guys.