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smonkey89 smonkey89
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12 years ago
This isn't the actual problem that I'm working, but I need an example.

Let's say I have 30 mL of pentane (d=.63 g/cm3)  and 45 mL of hexane (d=.66 g/cm3). If the volumes add on mixing, what is the molarity and molality of the pentane?
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wrote...
12 years ago
first find # moles of each component.

Grams   pentane =   30ml times   .63g/ml =  l8.9 grams of   pentane

grams   hexane.   45 ml times .66g/ml=  29.7 grams   hexane.

It appears that you are treating the hexane as the solvent and the pentane is the solute.

Now convert both masses into moles       l8.9g pentane  over   72g/mole =  .263 moles pentane

Moles  hexane =   29.7 grams over   86g/mole  =  .35 moles hexane.

Now lets see how many multiples of these two added volumes will make up one liter of solution.

        30ml plus  45 ml =   75 ml total

                l000ml/liter over 75 ml =  l3.33 multiples which will include

     l3.33 times  .35 moles Hexane =  4.66 moles hexane

and l3.33 times .263 moles pentane =  3.51 moles pentane in this liter of solution,   so the molarity of the pentane is   3.5l.

Now molality =  moles pentane per kilograms of hexane,  the solvent.

Looking back at our work it appears we have  29.7 grams hexane for each  .263 moles pentane.

so we scale up to a full kilograms of hexane.

                 .263 moles pentane over 29.7 grams hexane =     X moles pentane over l000 grams hexane                    

    Cross multiply and solve for X moles pentane/kg hexane

   X =  8.86 moles pentane per 1000 grams hexane or kg hexane

so  molality pentane =   8.86
wrote...
12 years ago
Molality:
First calculate moles of pentane in 1000g hexane
1) You have 45ml, d = 0.66, so you have 45*0.66 = 29.7g hexane
2) you have 30ml d = 0.63 = 30*0.63 = 18.9g pentane
1000/29.7*18.9 = 636.4g pentane in 1000g hexane
Molar mass pentane = C5H12 = 72.1493 g/mol
636.4g = 636.4/72.1493 = 8.82mol

Molality of pentane in hexane is 8.82m

Molarity:
You have 18.9g pentane in a final solution volume = 30+45 = 75ml
therefore you will have 1000/75*18.9 = 252g pentane in 1000ml solution
molar mass pentane = 72.1493g/mol
252g = 252/72.1493 = 3.493 mol
Molarity of pentane in solution  = 3.49M
wrote...
12 years ago
What would be the molality of the resulting solution obtained by mixing equal volume of 30% by weight H2SO4(density 1.208 g / ml) and 70% by weight H2SO4(density 1.61 g/ml)?
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