Which of the following aqueous solutions has the lowest freezing point?
(A) 0.2 m NaCl (B) 0.2 m CaCl2
(C) 0.2 m H2SO4
(D) 0.2 m NH3 (E) 0.2 m Al(NO3)3
Answer and Explanation:
Freezing point of a pure solvent depends on the amount of solute that gets dissolved in it. In order to determine which solution has the lowest freezing point, we need to look at the molality as well as whether the solute is ionic or covalent. Since the molality is equal for all of the answer
choices, we need to look at what kind of solutes are present.
Choice A: NaCl is ionic since it has a metal and a nonmetal. NaCl dissociates into 2 ions: Na+ and Cl -
Choice B: CaCl2 is ionic since it has a metal and a nonmetal. CaCl2 dissociates into 3 ions: one Ca 2+ and two Cl- ions
Choice C: H2SO4 is a strong acid, and dissociates into 3
ions: two H+ and one SO4 2-
Choice D: NH3 is a covalent compound since it is composed of 2 nonmetals, and therefore doesn't dissociate when it is in water
Choice E: Al(NO3)3 is an ionic compound and will dissociate into 4 ions when it dissolves in water: one Al3+ and 3 NO3 - ions.
Thus,
since Al(NO3)3 dissociates into the most ions, the solution with it will have lowest freezing point.
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#APChemistryquestions
1.5 M glucose 0.3 M `Na_2SO_4`1 M NaCl`H_2O`
Answer : C
Solution : Freezing point depression in water depends only on the number of solute particles per unit volume <br> 1 MnaCl = 2 x 1 x 6.02 x `10^23` particles per liter 0.3 `NaSO_4` = 3 x 0.3 x 6.02 x `10^23` particles per liter. <br> 1.5 M glucose = 1 x 1.5 x 6.02 x `10^23` particles per liter. <br> 0.5 M `BaSO_4` = 3 x 0.5 x 6.02 x `10^23` particles per liter . <br> Dividing by `6.02xx10^23` we can see that the comparative figures are NaCl, 2 , `Na_2SO_4` ,0.9 , glucose , 1.5 , and `BaSO_4`.1.5 Thus , the NaCl solution has the greatest number of particles per unit volume (considering the ionization of NaCl , `Na_2SO_4 and BaSO_4`) and its will have the lowest freezing point.
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Problem 78 Hard Difficulty
Answer
KBr will have the lowest freezing point
Video Transcript
This is problem 78 in chapter 11 of chemistry. The science in context. And it is asking which of the following Equis Solutions. Um, we'll have the lowest of their freezing points. Okay. And so, um, to figure out lowest freezing point. Remember, when we add a soul you to a solution or to a solvent that decreases the free point. Right? Um, we call this the freezing, freezing point Depression, Depression, because it gets decreased. When you add Saul you to assault, it's OK. And the equation to figure that out is a delta t f equals I k f times em where I is your event called Ah Hoff. Ah factor K f is a constant for your soul soul event. And, um is your morality okay? And so we're talking about it a gui a solution. So everyone is in water. That's what that means is that everyone is suspended in water. So the KF for everyone is gonna be the same. Whatever the chaos for water is, I'm and so it's good if it's gonna be the same for everyone. And we're trying to compare freezing point depressions. We can ignore k off. We don't need, Teoh, Take that into account. Everyone is okay. So the only things we need to take into account to determine who will have the, um you know, the greatest freezing point depression, cause we're trying to get to the lowest freezing point. Um, we just have to compare our event cough fact Hoff factor and Armel allergies. Okay, so what we can do is just multiply these two together, um, and see which one has the highest high times M and so for glucose. If we look at it, this is glucose C six, age 12. 06 eso. That's I factor is gonna be one. This doesn't associates, not an electrolyte. Okay, maybe our isn't electrolytes. That's gonna associate two K plus and br minus. So I in this case is too, Um, And for any to s 04 sodium sulfate. It's going to do any a two of them and s 04 So in this case, I equals three. So we had to buy these out. So in the case of glucose, we're gonna have, um, 0.5 times one, which is just 0.5 So this one was the C six, Age 12. 02 age 12. 02 So this is your eye Times M for Ah, you're glucose. Okay, um, for KBR we would have 0.3 I'm just removing the zeros at the end. It's not necessary. So we get 0.6 in this case for KBR. Okay? And finally, for zero point for sodium sulfate, it's your visor. 15 times three, which is zero point. Ah, zero for five. OK, and that's for sodium soul fate. Okay, So even though we don't know k f, we already multiplied out. This is I and this is m that we multiplied out on the highest. I am waas, our potassium bromide, which was 0.6 hot. Been known that k off when we multiply it out, huh? Potassium bromine will have the greatest raising point depression, which means that it would have the lowest freezing point because it would depress the freezing point lower than the other ones. Okay, more than the other ones. So our answer in this case is potassium bromide.