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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: student8607 on December 01, 2008, 07:27:07 AM

Title: Molarity vs Molality
Post by: student8607 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
Title: Re: Molarity vs Molality
Post by: ARGOS++ 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!
                    ARGOS++
Title: Re: Molarity vs Molality
Post by: student8607 on December 01, 2008, 10:11:56 AM
yes
then there is a similar question below that asks for molality
Title: Re: Molarity vs Molality
Post by: ARGOS++ on December 01, 2008, 11:18:57 AM
Dear student8607;

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

Good Luck!
                    ARGOS++

Title: Re: Molarity vs Molality
Post by: student8607 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?
Title: Re: Molarity vs Molality
Post by: ARGOS++ on December 01, 2008, 01:04:29 PM
Dear student8607;

That’s not correct! (http://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!
                    ARGOS++

Title: Re: Molarity vs Molality
Post by: student8607 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?
Title: Re: Molarity vs Molality
Post by: ARGOS++ 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++
Title: Re: Molarity vs Molality
Post by: student8607 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?  ???
Title: Re: Molarity vs Molality
Post by: ARGOS++ 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!
                    ARGOS++
Title: Re: Molarity vs Molality
Post by: student8607 on December 01, 2008, 02:52:18 PM
Can we just assume 100mole or something?
Sorry, I'm not seeing the connection yet.
Title: Re: Molarity vs Molality
Post by: ARGOS++ 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++

Title: Re: Molarity vs Molality
Post by: student8607 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 %

Title: Re: Molarity vs Molality
Post by: ARGOS++ 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++

Title: Re: Molarity vs Molality
Post by: student8607 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???
Title: Re: Molarity vs Molality
Post by: ARGOS++ on December 01, 2008, 04:39:51 PM
Dear student8607;

Please control your last calculation; you did a BIG comma mistake!
Use the Units for control!

And if you have corrected it, then calculate the final molaRity from it.

Good Luck!
                    ARGOS++

Title: Re: Molarity vs Molality
Post by: student8607 on December 01, 2008, 04:48:56 PM
yeah i see that my calculations has way too many zeroes
but i doubled checked
density was given in g/mL

and we said 1kg for solvent. so that's 1000g + the 46.5g of solute from the molality
Title: Re: Molarity vs Molality
Post by: ARGOS++ on December 01, 2008, 04:56:52 PM
Dear student8607;

Please control again! (masstotal is ok!)

But I got:  1046.5g / 1.0173 g/ml = 1028.7 ml (= voltotal)
(If density is finally correct.)


Good Luck!
                    ARGOS++

Title: Re: Molarity vs Molality
Post by: student8607 on December 01, 2008, 05:49:01 PM
oh you are right
it is mass over density not the other way around

so then 0.2588mol/1.028L = 0.251M  ;)

Title: Re: Molarity vs Molality
Post by: ARGOS++ on December 01, 2008, 05:54:24 PM
Dear student8607;

You did it!  -  Hard but ultimately!

Repeat the small conversion once again, to “get” really behind it!

Good Luck!
                    ARGOS++

Title: Re: Molarity vs Molality
Post by: student8607 on December 01, 2008, 05:58:11 PM
YAY - thanks very much

so i got bold and tried another problem (molarity to molality)
and i failed  >:(

The density of a 0.500M solution of acetic acid in water is 1.0042g/mL. What is the molality?

Molar mass is 60.05g
Title: Re: Molarity vs Molality
Post by: student8607 on December 01, 2008, 05:59:33 PM
0.5mol acetic acid = 30.025g of acetic acid

D times V = total mass
(1.0042g/mL)(1000mL from the 0.500M) = 1004g total

so i figured 1004-30.025 would be grams of solvent (0.974kg)

0.5mol / 0.974kg = 0.513?
Title: Re: Molarity vs Molality
Post by: student8607 on December 01, 2008, 06:04:09 PM
oh wait, my fault, that WAS correct
0.513 is right
yay  :o
Title: Re: Molarity vs Molality
Post by: ARGOS++ on December 01, 2008, 06:06:41 PM

Dear student8607;

Correct!

Good Luck!
                    ARGOS++

Title: Re: Molarity vs Molality
Post by: Borek on December 01, 2008, 06:52:47 PM
Examples of general approach:

http://www.chembuddy.com/?left=concentration&right=percentage-to-molarity

http://www.chembuddy.com/?left=concentration&right=percentage-to-molality

And ultimate approach:

http://www.chembuddy.com/?left=CASC&right=concentration-cheat-sheet