Boiling Point Elevation of an Aqueous Solution
The boiling point of a one molal solution of a nonvolatile solute increases by a constant amount for a given solid. So basically we’re dealing with a solute and a solvent, and when the solute is added to the solvent, the entire boiling point of the solution increases. So what we’re looking for here is what is the boiling point of a 1.5 molal sodium chloride solution in water. So sodium chloride here is the solute and the water is going to be the solvent because the water is what’s dissolving sodium chloride. And then we call this an aqueous solution because it’s mainly made up of water because the sodium chloride is going to dissolve in the water so the solution consists of both, but basically what you have is just water with sodium chloride in it. So what we’re looking at here is what the new boiling point of the whole solution is. So first we’re going to find how much sodium chloride is raising the boiling point, and then we’re going to add that to the boiling point of water which is 100 degrees celsius. So we have this equation right here, and this side of the equation is the boiling point elevation, so that’s what we’re trying to find here. And then Kbm is the molal concentration of the solution, and they’re directly proportional to each other, so that’s why they equal each other. So we’re going to set this up as our variable over here and then we’re just going to plug in what we already know. So the first thing we know is the molal of the sodium chloride 1.5m, and then we know the Kb for water, and then we know the boiling point of water. So we’re going to go ahead and plug in these numbers, 0.51 degrees celsius over m multiplied by 1.5m and since this m right here is in the numerator and this m is in the denominator they can be crossed out, so the answer here is 0.77 degrees celsius. So that’s the boiling point elevation, that’s how much sodium chloride is raising the boiling point. But now we need to add that to 100 degrees celsius, which is the boiling point of water. And by adding that to the boiling point of water, we will find the boiling point of this new solution. So the answer is 100.77 degrees celsius. So that is the boiling point of this sodium chloride solution in water. And so that’s how you can find the boiling point elevation of an aqueous solution.
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Last updated: 07/25/2017
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