Question from Topscore

[kevinrho]

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How did they get this number I circled in red? I'm not sure how they got this number. Help is appreciated.

Also I wanted to know if it mattered having n in the equation. I know that in this problem it is 1 therefore it didn't affect the answer but what if n wasn't 1.

I also found something similar in Destroyer: the equation for osmosis also included vant hoffs number, but chad did not include it in his equation for osmosis. does it matter?

anyways I'm taking my DAT on the 22nd of July. I don't feel ready at all. PAT I am but not anything else.

 

[RING12]

How did they get this number I circled in red? I'm not sure how they got this number. Help is appreciated.

Also I wanted to know if it mattered having n in the equation. I know that in this problem it is 1 therefore it didn't affect the answer but what if n wasn't 1.

I also found something similar in Destroyer: the equation for osmosis also included vant hoffs number, but chad did not include it in his equation for osmosis. does it matter?

anyways I'm taking my DAT on the 22nd of July. I don't feel ready at all. PAT I am but not anything else.

.

 

[UndergradGuy7]

1st one.

They said that 40% of it reacted.
so 14.250 moles x 0.60 (60% is left) = moles left
moles / L = concentration

2nd one
if n = 2 then Tb= 2 * Kb * m
so twice as big.

We use n in the osmotic pressure, freezing point lowering, and boiling point elevation equations, even though most equation sheets don't include it in the formula.

 

[kevinrho]

Thanks Undergradguy, however, I tried what said about 14.250 moles x 0.60, but that doesn't come out so I used 4.75 M x 0.60 to get 2.854. Thanks you got me in the right direction.

Thanks again for the equations part about n. I was a little confused there, but will included it in my equation for now in because the DAT will want us to include it correct?

 

[Pennpaki]

I had the same issue...

I did 60% of no. of moles since 40% are used up but they did 60% of molarity... can anyone explain why the second is correct but the first is wrong??

 

[UndergradGuy7]

I had the same issue...

I did 60% of no. of moles since 40% are used up but they did 60% of molarity... can anyone explain why the second is correct but the first is wrong??

They are both right... I think you guys mis read what I wrote...
I said...
so 14.250 moles x 0.60 (60% is left) = moles left
then moles / 3 L(given in the problem) = concentration

So after you multiply 0.6 by the moles you have to divide by the 3 liters given in the problem to get 2.85. You guys forgot to divide by the 3 liters.

 

[kevinrho]

thanks undergradguy