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Strong acids and strong bases

Video transcript
- [Voiceover] Here we have some strong acids which ionize 100% in solution. HClO4 is perchloric acid. HCl is hydrochloric acid. HBr is hydrobromic acid. HI is hydriodic acid. H2SO4 is sulfuric acid and HNO3 is nitric acid. Let's do a calculation using nitric acid here. Calculate the pH of a 0.030 molar solution of nitric acid. Well nitric acid is a strong acid which means it ionizes 100% in solution. If we have HNO3 and H2O here, the nitric acid is gonna donate a proton to water and if water accepts a proton, water turns into H3O plus, the hydronium ion. If HNO3 loses a proton, we're left with NO3 minus or nitrate. Since nitric acid is strong, we get 100% ionization here and everything turns into our products. If this is our concentration of HNO3, this would also be our concentration of hydronium ions. The concentration of hydronium ions is .030 molar since we're dealing with a strong acid. To calculate the pH, all we have to do is plug in to our definition of pH. The pH is equal to the negative log of the concentration of hydronium ions. All we have to do is plug this number into here so the pH is equal to the negative log of 0.030. Let's get out the calculator and do that. All right, so we have the negative log of .030. We get 1.52. Remember when we're rounding, all right, so the pH is equal to 1.52. For our significant figures, we have two significant figures here so we have two to the right of our decimal point. Instead of writing it this way, all right, so instead of writing this, you could have written a shortened version. You could have just written HNO3, nitric acid ionizes 100% and so it turns into H plus and NO3 minus, so that's just a shortened version of the same thing we have above. If you're working with this, you could have said that the pH is equal to the negative log of the concentration of H+. That's the same thing as we did above. All right, next let's look at strong bases. We're only gonna talk about metal hydroxides as being strong bases in this video. Some common metal hydroxides, sodium hydroxide, potassium hydroxide, calcium hydroxide and magnesium hydroxide. Let's do this problem. Calculate the pH of a 0.20 molar solution of sodium hydroxide. Let's go ahead and write sodium hydroxide here is a strong base. You get 100% dissociation in water, so 100% dissociation. Sodium hydroxide consist of Na plus and OH minus so you get dissociation and you get the sodium cation and the hydroxide anion, so Na plus and OH minus in water. If we have .2 molar of NaOH, we're also going to get .20 molar of hydroxide ions in solutions and sodium hydroxide is a strong base and we have 100% dissociation. If the concentration of hydroxide ion is 0.2, all right, we need to go for the pH and so one way to solve this problem is to use this equation. The concentration of hydronium times the concentration of hydroxide is equal to 1.0 times 10 to the negative 14 from an earlier video. We can plug in our concentration for hydroxide and we can say the concentration of hydronium is x. We have x times 0.20 is equal to 1.0 times 10 to the negative 14. If you do that math, all right, so if you do that math, x is equal to the concentration of hydronium ions. This would be five, right. This would be 5.0 times 10 to the negative 14. Now that we have the concentration of hydronium ions, we can now calculate the pH of our solution because pH is equal to negative log of the concentration of hydronium. We can do this on our calculator here. Let's get out the calculator to find the pH. The pH is equal to the negative log of, this is 5.0 times 10 to the negative 14. We get a pH of 13.30. All right, so let's go ahead and write that down here. pH is equal to 13.30. There's another way to do this problem, all right, and that is when you find, let me use a different color for this. When you find the concentration of hydroxides, you can immediately find the pOH, right. The pOH is equal to the negative log of the concentration of hydroxide ions. All right, so that would be .2, so we could plug that into here to find the pOH. pOH is equal to the negative log of the concentration of our hydroxide and ion. Let's go ahead and do that math. All right, so we have the negative log of .2 and we get .70. All right, so the pOH is equal to .70 here, 0.70 and to find the pH all we have to do is remember another formula. pH plus pOH is equal to 14, so if we plug in the pOH as .7 right here. 14 minus .7 obviously is gonna give you 13.3. We get the same answer for the pH no matter how we do this problem. All right, let's do one more calculation here. Let's look at what we're given for our problem. Calculate the pH of an aqueous solution that contains 0.11 grams of calcium hydroxide in a total volume of 250 milliliters. All right, so if we have calcium hydroxide, if we're going to have an aqueous solution of calcium hydroxide, let's look at what we're gonna get here. We're going to get calcium ions in solution. All right, and we're also gonna get hydroxide and ions in solution but if you balance this you need a two here so for every one mole of calcium hydroxides dissolved in water, you're gonna get two moles of hydroxide ions. Let's first figure out how many moles of calcium hydroxide we have. Over here we have .11 grams of calcium hydroxide, let me write that down here, .11 grams of calcium hydroxide. One way to think about finding the moles is to divide by the molar mass. The molar mass of calcium hydroxide is 74 grams per mole. If we divide .11 grams by 74 grams per mole, the grams would cancel and we would get moles here. Let's do that on the calculator, so .11 divided by 74 is equal to .0015. So we get .0015 here, so 0.0015 is how many moles, that's how many moles of calcium hydroxide, so let me emphasize this here. This would be the, if we have these many moles of calcium hydroxide, how many moles of hydroxide ions would we have in solution? All right, we would have twice that number because we have this ratio here of one to two. We need to multiply that number by two. By two to figure out how many moles of hydroxide ions we have. That number times two would be 0.0030 moles and now we're talking about moles of hydroxide ions. Over here we're talking about moles of calcium hydroxide and here we have moles of hydroxide. All right, so let's figure out the concentration of hydroxide ions in our solution and concentration would be moles over leaders, all right, leaders of solution here. Our total volume is 250 milliliters. We convert 250 milliliters into liters. Right now just be one, two, three, so .25 liters. 0.0030 divided by .250 liters, that's gonna give us the concentration of hydroxide ions. This gives us concentration of hydroxide and let's do that calculation here. Let's get some room, we have .0030 divided by .250 and we get .012 molar. All right, so this is .012 molar for the concentration of hydroxide. Now that you have the concentration of hydroxide ions and solution, you can find the pH using one of the two ways that we talked about above. All right, so you could go for this equation, hydronium ion concentration times hydroxide ion concentration is equal to 1.0 times 10 to the negative 14. All right, and you could take this and plug it into here. You get x times 0.012 is equal to 1.0 times 10 to the negative 14 and let's get a little bit more room here and let's do this calculation really quickly. We have 1.0 times 10 to the negative 14. We're gonna divide that by .0. Actually let me do that again. We're going to divide that by .012 and this is going to give us 8.3 times 10 to the negative 13. All right, so let's write this down, x is equal to 8.3 times 10 to the negative 13. Remember what x referred to, x referred to the concentration of hydronium ions so this is the concentration of hydronium ions which means we can now calculate the pH. All right, the pH be equal to the negative log of that concentration. The negative log of 8.3 times 10 to the negative 13. Let's do one more final calculation here. We're gonna do negative log of 8.3 times 10 to the negative 13, and we're going to get 12.08. All right, so the pH is equal to 12.08 and we're finally done with our problem here. Notice the pH is in the base range, so calcium hydroxide is a base.