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February 13, 2016

# Homework Help: chemistry

Posted by Marie on Monday, April 29, 2013 at 11:40pm.

Consider 18.0 M H2SO4, if you need to make 250.0 mL of a 3.0 M solution of H2SO4, how would you do this?

18.0 M H2SO4 has a density of 1.84g/mL what is the molality of solution? Mass % of H2SO4 in the solution and the mol fraction of H2SO4 and water in the solution?
• chemistry - DrBob222, Tuesday, April 30, 2013 at 12:08am

#1.
mL1 x M1 = mL2 x M2
Substitute and solve.
250mL x 3.0M = mL2 x 18

#2.
18.0M means 18.0 mols/L
18.0 mols x (98g/mol) = 1764 grams H2SO4/L solution.
The mass of 1L solution is
1.84 g/mL x 1000 mL = 1840 grams H2SO4 + H2O
%H2SO4 = (mass H2SO4/total mass)*100 = (1764/1840)*100 = ?

mols H2SO4 = 1764/98 = ?
mols H2O = (1840-1764)/18 = ?
total mols = sum
XH2SO4 = mols H2SO4/total mols
XH2O = mols H2O/total mols.
molality = mols/kg solvent. Just substitute the proper numbers.

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• chemistry - Marie, Tuesday, April 30, 2013 at 12:27am

#1 250mL x 3.0 M/18.0M = 41.67mL?

#2 molality of solution???

Mass % 1764/1840x100= 95.87%

Mol fraction...H2SO4. 18/22.2=.81
H2O 4.2/22.2=.18

• chemistry - DrBob222, Tuesday, April 30, 2013 at 4:23pm

#1 looks ok.
#2.
mass % looks ok but if your prof is picky about the number of significant figures I think you need to round that number.
mol fractions look ok except I would have rounded the H2O to 0.19 and not 0.18. In fact, you dropped a 2 from 4.22 and 4.22/22.2 = 0.19

I don't see that you have calculated molality.
That's mols/kg solvent.
You have 18.0 mols and kg H2O = 0.076; therefore, m = 18.0/0.076 = ?

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