TBR - gen chem molarity

[premedicine555]

Ugh, so I think i'm overthinking easy problems.

For TBR, Passage VI of stoich, Which of the ollowing solutions has the GREATEST molarity?
a) 4 percent by mass KBr in water
b) 4 percent by mass Kcl in water
c) 4 percent by mass NaBr in water
and d) 4 percent by mass Nacl in water

Okay, so firstly, is it saying 4% of the mass of the compounds? Secondly, I understand the answer is the one with the MOST moles of salt, so shouldn't it be KBr since it has .86 moles (39+80)/(39+80+18)?

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[Pisiform]

Ugh, so I think i'm overthinking easy problems.

For TBR, Passage VI of stoich, Which of the ollowing solutions has the GREATEST molarity?
a) 4 percent by mass KBr in water
b) 4 percent by mass Kcl in water
c) 4 percent by mass NaBr in water
and d) 4 percent by mass Nacl in water

Okay, so firstly, is it saying 4% of the mass of the compounds? Secondly, I understand the answer is the one with the MOST moles of salt, so shouldn't it be KBr since it has .86 moles (39+80)/(39+80+18)?

They are asking for greatest Molarity which is M = Moles / Volume

Don't worry about 4% since all of the answer choice has 4%. They also have the same volume so don't worry about volume either. Since Molarity = Moles/Volume, in this case volume is constant for all as we said so it only depends on Moles.

Moles = Mass / Mr. Mass is constant too. So only thing left is Mr. Moles and Mr has inverse relationship. So the one that has less Mr would have greatest Moles and hence greates Molarity.

So answer should be NaCl I think

EDIT: Mr means Molecular Mass

 

[premedicine555]

so molarity = (moles of SOLUTE)/Liter.. and to find moles of solute, I would do: (grams of solute)/molecweight of solute;

however don't we need the amount of solute for each salt to find which has the greatest moles?

 

[Pisiform]

so molarity = (moles of SOLUTE)/Liter.. and to find moles of solute, I would do: (grams of solute)/molecweight of solute; however we weren't given a gram of solute....? Don't we need the amount of solute for each to find which has the greatest moles?

thats my point !!! its 4% by mass ...same for all. You should only worry abt Molecular mass

 

[premedicine555]

@psiform: So, because they're all 4% by mass, that means they're all "4 grams" of solute, meaning since the sample is the same amount, we can ignore it? I'm just confused as to what "4% by mass means.."-- 4 grams of solute?

 

[Ssina]

@psiform: So, because they're all 4% by mass, that means they're all "4 grams" of solute, meaning since the sample is the same amount, we can ignore it? I'm just confused as to what "4% by mass means.."-- 4 grams of solute?

I think it mean in a i.e. 100g solution, 4g is the salt (solute) and 96g the water (solvent), since it says in water. and yeah since they all are 4% by mass, then you have the same grams of each... so as mentioned above, we just care about the moles, which would be grams (same for all)/MW, and the smallest MW gives us more moles....

good question!! I mixed up the MW and thought it would be B also

 

[Erythropoietin]

Same volume;
so the math becomes: g / molecular weight = g / (g/mol)

= 4 / (g/mol) .....4 is the same for both
NaCl is 58 g/mol
KBr is 119 g/mol

you are looking for the largest number since "moles" are in the numerator in Molarity
and therefore 4/58 > 4/119