Chem Question

[TheRealAngeleno]

Which of the following solutions has the GREATEST salt concentration?

A. 10 g NaCl mixed with 100 grams water (volume = 102.5 mL)
B. 10 g NaCl mixed with enough water to make an aqueous solution of rho = 1.06g/mL
C. 10 g KCl mixed with 100 grams of water (volume = 103.1 mL)
D. 10 g KCl mixed with enough water to make an aqueous solution of rho = 1.03 g/mL

So it can't be C or D b/c you would get more moles per gram of NaCl. From there I'm pretty much stuck...

Thanks a lot guys.

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[Rabolisk]

For a, the density of the solution would be 110 grams / 102.5 mL.

 

[Rolling]

^^^ Winner

 

[TheRealAngeleno]

For a, the density of the solution would be 110 grams / 102.5 mL.

Ah thanks a lot. This problem is actually pretty easy don't know what up with me

 

[TheRealAngeleno]

110g/102.5ml > 106g/100mL so A would be the answer.

 

[cheesier]

I have a feeling this is a dumb question, but why is it not b?

Doesn't A have ~0.1 g/mL while B has 1.06 g/mL? Does that not mean that A has a lower concentration than B?

 

[Long Way to Go]

I have a feeling this is a dumb question, but why is it not b?

Doesn't A have ~0.1 g/mL while B has 1.06 g/mL? Does that not mean that A has a lower concentration than B?

When you are evaluating B, remember that you have to subtract out the mass of water.

 

[cheesier]

When you are evaluating B, remember that you have to subtract out the mass of water.

Subtract it out from what?

 

[TheRealAngeleno]

I have a feeling this is a dumb question, but why is it not b?

Doesn't A have ~0.1 g/mL while B has 1.06 g/mL? Does that not mean that A has a lower concentration than B?

It would actually be ~1.10g/mL which is > 1.06g/mL

 

[cheesier]

It would actually be ~1.10g/mL which is > 1.06g/mL

How do you figure? It says 10 g dissolved in 102.5 mL. 10/102.5 = ~0.1

 

[Rabolisk]

Density of solution is not the same thing as the mass of NaCl divided by volume of the solution.

 

[cheesier]

Density of solution is not the same thing as the mass of NaCl divided by volume of the solution.

But it's asking which has the greatest concentration of salt, not the greatest density. Right?

 

[TheRealAngeleno]

But it's asking which has the greatest concentration of salt, not the greatest density. Right?

B/c both solutions have the same salt dissolved into the same solvent, the solution wit the greater concentration will have a greater density, a greater molality, and a greater molarity.

 

[cheesier]

B/c both solutions have the same salt dissolved into the same solvent, the solution wit the greater concentration will have a greater density, a greater molality, and a greater molarity.

Man, this question is making me feel much dumber than I think I should. A solution with 1.06 g in 1 mL of water has a greater molarity than a solution with 0.1 g in 1 mL. Why doesn't that mean that the salt concentration is greater in B?

 

[Rabolisk]

There is not 1.06 g of salt per mL of water in B.

 

[cheesier]

Nvm, I got it.

 

[BerkReviewTeach]

Which of the following solutions has the GREATEST salt concentration?

A. 10 g NaCl mixed with 100 grams water (volume = 102.5 mL)
B. 10 g NaCl mixed with enough water to make an aqueous solution of rho = 1.06g/mL
C. 10 g KCl mixed with 100 grams of water (volume = 103.1 mL)
D. 10 g KCl mixed with enough water to make an aqueous solution of rho = 1.03 g/mL

So it can't be C or D b/c you would get more moles per gram of NaCl. From there I'm pretty much stuck...

Thanks a lot guys.

This is such a great question. I love BR's explanation. This is typical of how helpful a great explanation can be. Doing those four passages and slowly reading through their explanations will do wonders for your understanding of that section.

I hope your teacher explained what the "LQ" at the start of a BR answer explanation means (in the class handouts). Whenever you see that at the start of the explanation, you should read the answer explanation thoroughly, because there's a test strategy in the explanation. In this case, the math setup is really helpful and shows that you don't need to take a number to completion to get the right answer, even for a question that looks math heavy.

The point they were trying to make is that this question is easy when you realize that all you need is a common unit for comparison, and density is the one they are handing you. Too many times, we are programmed into converting to the standard unit of molarity. For salt in water solutions, the greater molarty correlates with greater molality, greater density, higher boiling point, lower vapor pressure, and so on. They can hide simple questions in tricky looking wording.