TBR Gen Chem

[kambizzy]

Adding water to an aqueous solution of known
concentration alwavs decreases all of the following
EXCEPT:
A. density.
B. molarity.
C. molality.
D . mass percent of the solute.

What would have been your answer and why. Thank

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[gettheleadout]

This question was freakin' hard haha. I got it right by exhaustive POE but I didn't understand why the correct answer was actually true until I read the answer explanation.

I can post my thought process if you want, but that would also give away the answer, in case you don't want it posted yet.

 

[grburst]

Adding water to an aqueous solution of known
concentration alwavs decreases all of the following
EXCEPT:
A. density.
B. molarity.
C. molality.
D . mass percent of the solute.

What would have been your answer and why. Thank

I would say A because density is not a colligative property. Density of either the solute or of water is independent of the amount of each.

For B, the volume is increasing so molarity decreases (M = mol solute/total volume).
For C, molality also decreases because the mass of solvent is increasing (molality = mol(solute)/mass(solvent)).
The reasoning for why it's not D is similar. You are increasing the moles of water, therefore the grams of water, and therefore the total mass. But the mass of solute is the same.

 

[gettheleadout]

I would say A because density is not a colligative property. Density of either the solute or of water is independent of the amount of each.

For B, the volume is increasing so molarity decreases (M = mol solute/total volume).
For C, molality also decreases because the moles of solvent are increasing (molality = mol(solute)/mol(solvent)).
The reasoning for why it's not D is similar. You are increasing the moles of water, therefore the grams of water, and therefore the total mass. But the mass of solute is the same.

Exactly! This was my thought process as well.

 

[kambizzy]

Thanks guys, everything now makes sense, I knew that B, C and D were wrong i just did not know why A was the Answer because it first i thought that was right too. But now it clicks. Hehe

 

[Patassa]

Adding water to an aqueous solution of known
concentration alwavs decreases all of the following
EXCEPT:
A. density.
B. molarity.
C. molality.
D . mass percent of the solute.

What would have been your answer and why. Thank

That's a messed up question, plus what is the density of a solution? If you think about it, it's pretty complicated. Density of water is one thing, density of the solute is too, but density of the solution? Maybe I'm over thinking this.

 

[gettheleadout]

That's a messed up question, plus what is the density of a solution? If you think about it, it's pretty complicated. Density of water is one thing, density of the solute is too, but density of the solution? Maybe I'm over thinking this.

You might be overthinking it. An aqueous solution has the characteristic of being homogenous, so there is a uniform distribution of mass throughout. Thus, the density of the solution is simply the mass of the solution divided by the volume of the solution, and this value will hold for any sample removed from the stock.