A KCl solution containing 42 g of KCl per 100.0 g of water is cooled from 60 °C to 0 °C. What happens during cooling? (Use Figure 13.11.)

Video transcript

Welcome back everyone to another video. A potassium chloride solution containing 48 g of potassium chloride per 100 g of water is cooled from 90 degrees to 30 °C. What happens during cooling? We're given our solubility curve as well as for answer choices. A states that about 18 g of po assy chloride per 100 g of water will precipitate out of the solution. B states that about 13 g D about 5 g and D states that the composition of the solution will remain the same. So first of all, what we want to do in this problem is locate potassium chloride on our solubility chart. We noticed that it is given to us in brown. And now if we identify our two temperatures, 90 degrees and 30 °C, we essentially want to draw vertical lines intersecting with the curve. So we draw one vertical line and one horizontal one because we are looking for the solubility, we perform the same steps for 30 °C. So let's go ahead and do that right. So we have all of the information we need. So what we can say is that we start with a temperature of 90 °C. And now what do we see? Well, essentially we see that the solubility is about 53 g per 100 g of water. Now let's go ahead and identify the solubility at 30 °C. What we see there is that if we start with 30 degrees and essentially move to the left of the chart, we can clearly see that the solubility is about 35 g per 100 g of water. Now, we understand that the initial mass of potassium chloride is 48 g. So if we start with 90 °C, our solubility s is greater, right? Or basically, we can see the mass that can dissolve in 100 g of water at 90 °C is greater than the given mass. So essentially, this means that at 90 °C, all of our potassium chloride is dissolved in the solution because the solubility is greater than the mass of potassium chloride given in our solution, right? So essentially, we're given 48 g per 100 g of water. And now what can we say about our next step? Well, essentially, when we cool down our solution, we're reaching a lower solubility which is now lower than the mass of potassium chloride. So we cannot really dissolve all of the 48 g at 30 °C due to the fact that the maximum solubility is 35 g per 100 g of water. So the excess amount. In this case, if we take 48 g per 100 g of water, if we find the excess amount at 30 °C, we will get the mass of potassium chloride that will actually precipitate out of the solution. Because essentially that axis just cannot dissolve due to the fact that we have reached the maximum solubility. And when we find the difference, we notice that the amount of the precipitate formed will be 13 g per 100 g of water, which essentially corresponds to the answer choice. B it says that about 13 g of potassium chloride per 100 g of water will precipitate out of the solution. That would be it for today. And thank you for watching.