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Topic: Molarity vs Molality  (Read 12812 times)

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Offline student8607

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Molarity vs Molality
« on: December 01, 2008, 07:27:07 AM »
I'm a little confused at how I am supposed to set this up:
The density of a 0.258m solution of glucose in water is 1.073g/mL. What is molarity?

molarity = mol solute / L solution
molality = mol solute / kg solvent
sucrose's molar mass is 180.2g

density = mass/volume


so would the molarity have a volume of 0.001L?

then just convert grams to moles? 0.00564mol

Offline ARGOS++

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Re: Molarity vs Molality
« Reply #1 on: December 01, 2008, 08:05:20 AM »
Dear student8607;

Are you sure, that the question asks for molaRity and not molaLity?
(I think for molaLity.)

Good Luck!
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Offline student8607

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Re: Molarity vs Molality
« Reply #2 on: December 01, 2008, 10:11:56 AM »
yes
then there is a similar question below that asks for molality

Offline ARGOS++

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Re: Molarity vs Molality
« Reply #3 on: December 01, 2008, 11:18:57 AM »
Dear student8607;

In this case for molaRity: What is 0.258m solution meaning?

Good Luck!
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Offline student8607

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Re: Molarity vs Molality
« Reply #4 on: December 01, 2008, 12:49:21 PM »
perhaps that there are 0.258moles of solute for every 1 kg of solvent?

so now we have moles for molarity  :D
just need volume...can we use the 0.01L?

Offline ARGOS++

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Re: Molarity vs Molality
« Reply #5 on: December 01, 2008, 01:04:29 PM »
Dear student8607;

That’s not correct!

0.258m solution means contrary, 0.258 molaR (= moles / Liter) !!
Do you see now, that the question for results in molaRity makes no sense!
(But for molaLity!)

Good Luck!
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Offline student8607

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Re: Molarity vs Molality
« Reply #6 on: December 01, 2008, 01:06:25 PM »
but i thought little m was for molality (moles/kg)
big m (M) is for the molarity?

Offline ARGOS++

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Re: Molarity vs Molality
« Reply #7 on: December 01, 2008, 01:13:11 PM »
Dear ;

Sorry!, - so it’s my mistake for that!
But then it’s similar to your other question! Solve it first.

Sorry again!
Good Luck!
                    ARGOS++

Offline student8607

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Re: Molarity vs Molality
« Reply #8 on: December 01, 2008, 02:09:42 PM »
hmm
D = m/v

0.258m = 0.258mol solute per 1 kg of solvent
molar mass for glucose is 180.2g/mol

0.258mol = 46.5g

D = m/v
1.073g/mL = (46.5g) / (?L)
43L?  ???

Offline ARGOS++

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Re: Molarity vs Molality
« Reply #9 on: December 01, 2008, 02:39:13 PM »
Dear student8607;

The first part is ok.

But to turn to the opposite you have to ask in how much masstotal are the 0.258 moles contained, and second: what is the volume of the masstotal?
Then you should be able to answer the question for molaRity.

Good Luck!
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Offline student8607

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Re: Molarity vs Molality
« Reply #10 on: December 01, 2008, 02:52:18 PM »
Can we just assume 100mole or something?
Sorry, I'm not seeing the connection yet.

Offline ARGOS++

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Re: Molarity vs Molality
« Reply #11 on: December 01, 2008, 02:57:25 PM »
Dear student8607;

Do you remember?:
More important for you is:  masstotal = mass Solute + mass Solvent!
(I think you know how to use it in mass%.)

Good Luck!
                    ARGOS++


Offline student8607

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Re: Molarity vs Molality
« Reply #12 on: December 01, 2008, 04:01:31 PM »
ok so if i assume 0.258molality means 0.258moles (per every 1 kg of solvent)
then i can convert that to grams (mass) = 46.5g of SOLUTE

but how am i to get mass of water for mass %


Offline ARGOS++

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Re: Molarity vs Molality
« Reply #13 on: December 01, 2008, 04:09:35 PM »
Dear student8607;

I told you already that mass of Solute (46.5g = 0.258 moles) is correct.

But what is the mass of the Solvent? – You wrote it in the parentheses from the definition.
(You don't need any %!)

Good Luck!
                    ARGOS++


Offline student8607

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Re: Molarity vs Molality
« Reply #14 on: December 01, 2008, 04:31:29 PM »
46.5g - solute
1000g(aka 1kg) - solvent
1046.5g - total for solution

M = mol solute / L of solution
M = 0.258mol / ?L

D=m/v
1.0173g/mL = (1046.5g) (?mL)
0.000972mL???
« Last Edit: December 01, 2008, 04:48:06 PM by student8607 »

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