Question #bb918

2 Answers
Stefan V.
Jan 30, 2015

Your solution's molality is #"0.712 molal"#.

Molality is defined as moles of solute divided by kilograms of solvent. In your case, urea is the solute and water is the solvent.

Determine the number of moles of urea you have by using its molar mass

#"8.84 g" * ("1 mole")/("60.0 g") = "0.147 moles"#

All you have to do now is plug in your values

#"b" = ("0.147 moles")/(206.5 * 10^(-3)"kg") = "0.712 molal"#

Don't forget to convert grams of water to kilograms of water, because the formula asks for kilograms of solvent.

Meave60
Jan 30, 2015

The molality is 0.712 m.

In order to determine the molality of a solution, you need to do the following:

Step 1. Calculate the number of moles of solute by converting the mass in grams of urea to moles using its molar mass, which is #"60.055 g/mol"#.

#"mol urea"# = #"8.84g urea"# x #"1 mol urea"/"60.055g urea"# = #"0.147 mol urea"#

Step 2. Calculate the number of kilograms of solvent. The mass of the solvent (water) is #"206.5 g"#. Use the relationship of 1000 g = 1 kg to convert from grams to kilograms.

#"206.5 g water"# x #"1 kg"/"1000 g"# = #"0.2065 kg water"#

Step 3. Divide moles of solute by kilograms of solvent.

#"molality"# = #"0.147 mol urea"/"0.2065 kg water"# = #"0.712 m"#

http://antoine.frostburg.edu/chem/senese/101/solutions/faq/computing-molality.shtml

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