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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: student8607 on December 01, 2008, 07:33:35 AM

Title: Missing Variables?
Post by: student8607 on December 01, 2008, 07:33:35 AM
This is all I am given:
Assume seawater is a 3.50% solution of NaCl. What is the molality of the solution?

Mass % = [mass component / total mass] x 100
molality = mol solute / kg solvent

Can I use NaCl's molar mass of 58.5g somewhere in the mass percent?

Where am I supposed to get the rest of the numbers from to calculate?
Title: Re: Missing Variables?
Post by: ARGOS++ on December 01, 2008, 07:56:13 AM
Dear student8607;

Yes, - “your” molar mass of NaCl is correct.
Think about how total mass is related to mass solute and mass solvent and mass%.

So you don’t need something else.
Good Luck!
                    ARGOS++

Title: Re: Missing Variables?
Post by: student8607 on December 01, 2008, 10:11:01 AM
total mass = mass% / 100?
Title: Re: Missing Variables?
Post by: ARGOS++ on December 01, 2008, 11:15:34 AM
Dear student8607;

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: Missing Variables?
Post by: student8607 on December 01, 2008, 12:53:14 PM
i think that is what i was trying to get at
so since the total 3.50%, does that mean the total mass is 3.50g?
Title: Re: Missing Variables?
Post by: ARGOS++ on December 01, 2008, 01:09:14 PM
Dear student8607;

No!, - Contrary you’re allowed to select the masstotal free! So select 1.00 kg for it to make it easy.

Good Luck!
                    ARGOS++

Title: Re: Missing Variables?
Post by: ARGOS++ on December 01, 2008, 01:32:35 PM
Dear student8607;

Try it this way:  How much is 3.50 %w/w of 1.00 kg, and how much mass is left for the Solvent.

Good Luck!
                    ARGOS++

Title: Re: Missing Variables?
Post by: student8607 on December 01, 2008, 01:52:06 PM
oh yes. dumb me.
so, 3.50% of 1k (1000g) is 35g of NaCl
there are 965g left over as the solvent

molality = mol solute / kg solvent
0.60mol / 0.965kg = 0.62!
Title: Re: Missing Variables?
Post by: ARGOS++ on December 01, 2008, 02:04:25 PM
Dear student8607;

Bravo!, - You did it!   Now you can try the other question.

Good Luck!
                    ARGOS++