 So, this one says a chemist prepares a copper etching solution by diluting concentrated nitric acid, which is HNO3. So, 2, a concentration of .0063 mole, and it wants us to calculate the hydronium ion concentration, the pH, the hydroxide ion concentration, and the POH. So, let's go ahead and do that. So, the first thing we have to remember is that nitric acid is a strong acid. If you don't remember that, it's going to be very difficult to do this problem. So, I'm going to write down a chemical equation that helps me out. This is H plus is the same thing as H3O plus, okay? H plus is the same thing as hydronium. So, this thing is going to be equivalent to whatever concentration is there. Is everybody okay with that? Everybody understand? So, the concentration of this thing here is .0063 mole, right? Let's figure out what is the concentration of H plus, which is going to be the concentration of H3O plus, which equals .0063 moles of nitric acid, so per liter of solution, right? And then we have a 1 to 1 ratio there. Is everybody okay with that? So, we've got 1 mole nitric acid for every 1 mole H plus, and remember H3O plus comes from that. So, what we can do, cancel, cancel, and we should get 0.0063 mole or H plus. Is everybody okay with doing that? Remember you get that conversion factor from that chemical equation. So, now we should be able to write down, well, what's the concentration of H3O plus, right? It's the same thing as H plus, so let's write it down. 0.0063 mole. Is everybody okay with that? Wonderful. Let's do the pH now. pH equals what? Negative log of the what? Yeah, concentration of hydronium. So, we have the concentration of hydronium. So, we're going to put the pH. Remember unit list. Is everybody okay with this point? So, you can do this next part one of two ways. You can subtract it from 14 or you can go back to this concentration and use KW. Okay, I'm going to do that second one because it's a little, I don't know, hard. So, can I erase this kind of part down here? Everybody's got that face? So, do you guys remember what the KW expression was? KW equals, yeah. So, we're looking for the hydroxide concentration. So, OH minus equals KW divided by H3O plus concentration, which we got up here. So, remember KW is 1.0 times 10 to the negative 14. I did a very small number, 1.6 times 10 to the negative 12 molar. Okay, so hopefully everybody got that. So, are there any questions on how to do that portion? Remember units here. So, don't go with the units that you would have got if I didn't put the molar down here. Okay, so POH, well how do we do that? Negative log of the OH concentration. Do we have the OH concentration? Yes. So, we're just not putting those units in there so I won't confuse myself. 11.80. So, you should be able to do all of these with a strong acid or strong base for that matter, okay? By using this technique. So, practice these guys, practice on the other ones they give you there.