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Topic: Mole fraction, Mass Percent, and Molality - Check my Work  (Read 6679 times)

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Offline ozzlomo

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Mole fraction, Mass Percent, and Molality - Check my Work
« on: May 04, 2009, 10:06:18 PM »
Sodium Fluoride has a solubility of 4.22g per 100g of water at 18 degrees C. Express this solute in terms of mole fraction, mass percent, and molality.

Sodium Fluoride (NaF) moles : 4.22g / 41.99 g/mol = .101 moles

H2O moles: 100g / (18.02 g/mol) = 5.55 moles

NaF Mole Fraction : .101 moles / 5.651 moles =

NaF Mass Percent : 4.22g / 104.22g X 100 = 4.05 %

Molality: 0.101 moles / 0.1 kg = 1.01 m

What are the mole fractions of ethanol (C2H5OH) and water in a solution prepared by mixing 60.0 ml of ethanol with 40.0 ml of water at 25 degrees C? The densities of ethanol and water at this temperature are 0.789 g/mol and 1.00 g/ml respectively.

Volume of ethanol is 60.0 ml.

60.0 ml X 7.89g/1ml = 473.4g

473.4g / 46.08 g/mol = 10.27 moles Ethanol

Volume of H2O is 40.0 ml.

40.0ml X 1.00g/1ml = 40.0g

40.0g / 18.02 g/mol = 2.22 moles H2O

Ethanol Mole Fraction: 0.178

H2O Mole Fraction: 0.822

Are my answers, units, and sig. figs correct?

Offline plankk

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Re: Mole fraction, Mass Percent, and Molality - Check my Work
« Reply #1 on: May 05, 2009, 02:02:28 AM »
The first one is correct.

In the task you wrote:
0.789 g/mol

And then you counted:
60.0 ml X 7.89g/1ml = 473.4g

You also mixed up unit.

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