Question #def31

1 Answer
May 10, 2015

Your solution will have a molality of 0.2 molal.


To determine the molality of a solution, you need to know how many moles of solute are present per kilogram of solvent.

In your case, sodium hydroxide is the solute and water is the solvent.

Since you know how many grams of sodium hydroxide you must dissolve, you can use the compound's molar mass to see how many moles you'd have

#4cancel("g") * "1 mole NaOH"/(40.0cancel("g")) = "0.1 moles NaOH"#

Now you need to convert the volume of the solution to kilograms. This is done by using water's density, which you can assume to be equal to 1 g/mL.

#500cancel("mL") * (1cancel("g"))/(1cancel("mL")) * "1 kg"/(1000cancel("g"))= "0.5 kg"#

As a result, the molality of the solution will be

#b = n/m_"water" = "0.1 moles"/"0.5 kg" = "0.2 mol/kg" = color(green)("0.2 molal")#

So, keep in mind that you need moles of solute and kilograms of solvent in order to determine molality.