# TBR Gen Chem

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#### kambizzy

##### Full Member
10+ Year Member

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.

##### MD
Moderator Emeritus
10+ Year Member
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

##### Full Member
5+ Year Member
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.

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.

Last edited:

##### MD
Moderator Emeritus
10+ Year Member
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

##### Full Member
10+ Year Member
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

##### Full Member
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.