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Old 10-26-2009, 07:29 PM   #1
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Default Anyone good with Chemistry?

I got a chem test next week, and am currently taking some practice exams online. On one of them, I got this question wrong:


Quote:
The concentration of lead nitrate (Pb(NO3)2) in a 0.726 M solution is __________ molal. The density of the solution is 1.202 g/mL.
Any idea what to do here?? I'm lost...
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Old 10-26-2009, 07:32 PM   #2
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Default Re: Anyone good with Chemistry?

Quote:
Originally Posted by superman22x View Post
I got a chem test next week, and am currently taking some practice exams online. On one of them, I got this question wrong:




Any idea what to do here?? I'm lost...
Make myself sound smart here:
[Link] (I just C & P)
Quote:

0.755

Molarity is moles of solute (compound) in Litres of solution
M = mol / litres
Molality is moles of solute in kg of solvent (not solution, solvent only)
m = mol / kg

To work this out I first pretended I had 100 ml. Work out how many moles of Pb(NO3)2 are in 100 ml of 0.726 M Pb(NO3)2

M = mole / L
therefore mole = M x L
moles Pb(NO3)2 = 0.726 M x 0.1 L
= 0.0726 moles in 100 ml

You are told the density is 1.202 g/ml, therefore
weight of 100 ml of solution = 100 ml x 1.202 g/ml
=120.2 g

Some of this weight is attributted by the Pb(NO3)2, there are 0.0726 moles of Pb(NO3)2 in 100 ml, so work out the mass of this using

moles = mass / molecular weight

molecular weight Pb(NO3)2 = 207.2 + (2 x 14.01) + (6 x 16.00) = 331.22 g/mol
So mass of Pb(NO3)2 in 100 ml = 0.0726 mol x 331.22 g/mol
= 24.0466 g of Pb(NO3)2 in 100 ml of 0.726M solution

To find the mass of the water subtract mass of Pb(NO3)2 from total mass.
120.0 g - 24.0466 g = 96.1534 g

So in 100 ml of 0.726 M Pb(NO3)2 there are 0.0726 moles (24.0466 g) of Pb(NO3)2 and 96.1534 g of water.

Molality = moles / kg (remember moles Solute / kg SOLVENT (water))
= 0.0726 moles / 0.0961534 kg
= 0.755 m
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Old 10-26-2009, 07:43 PM   #3
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Default Re: Anyone good with Chemistry?

Oh, good deal. haha, thanks.
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Old 10-26-2009, 07:45 PM   #4
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Oh, good deal. haha, thanks.
*phew* Glad you knew what the hell that meant

I sure didn't..
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Old 10-26-2009, 07:49 PM   #5
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Default Re: Anyone good with Chemistry?

Molar is moles per liter of solution. Molal is moles per kilo of solvent.

So, lets assume one liter. That gives us .726 moles of Pb(NO3)2, but we don't know how much water was used. The mass of the solution is 1202 grams. Subtract 240 grams for the solute, and you get 962 grams, or .962 KG.

So now, we just divide. .726/.962 and you end up with .755 molal.
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Old 10-26-2009, 07:50 PM   #6
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Default Re: Anyone good with Chemistry?

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Originally Posted by Rubber314Chicken View Post
Molar is moles per liter of solution. Molal is moles per kilo of solvent.

So, lets assume one liter. That gives us .726 moles of Pb(NO3)2, but we don't know how much water was used. The mass of the solution is 1202 grams. Subtract 240 grams for the solute, and you get 962 grams, or .962 KG.

So now, we just divide. .726/.962 and you end up with .755 molal.
Pssh, of course. I mean only and idiot wouldn't know that. *scuffs*
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Old 10-26-2009, 07:51 PM   #7
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Default Re: Anyone good with Chemistry?

Quote:
Originally Posted by Rubber314Chicken View Post
Molar is moles per liter of solution. Molal is moles per kilo of solvent.

So, lets assume one liter. That gives us .726 moles of Pb(NO3)2, but we don't know how much water was used. The mass of the solution is 1202 grams. Subtract 240 grams for the solute, and you get 962 grams, or .962 KG.

So now, we just divide. .726/.962 and you end up with .755 molal.
Much simpler. lol, thanks. I am going to take another practice test in a few minutes. Hopefully a similar question will be on it, so I can practice this myself.
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