Ap Chem Help When Given Molarity And Need To Find Molality?

I am given the information that the density of potassium hydroxide is 1.05g/ml and the molarity of the potassium hydroxide is 1.13. How do I find the molality? I know that molality is moles solute over kg solvent and that I can assume that I have 1000g of the solvent or 1kg.

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2 Responses to “Ap Chem Help When Given Molarity And Need To Find Molality?”

  1. If the molarity is 1.13, then there are 1.13 moles KOH in every liter of solution. Keep that as your numerator and change your denominator to mass units. The density tells you that 1 mL of solution is 1.05 grams. Then 1 liter is 1050 grams. Subtract the mass of solute (1.13 moles times the molar mass of KOH) from the 1050 and you have grams of solvent. Divide by 1000 to get to kg, and that is your denominator.

  2. So I assume that you are working with a solution of KOH here.
    The molarity is 1.13 moles KOH per liter of solution. 1.13 moles KOH x 56.105 g/mol = 63.399 g KOH per liter of solution. Be careful! This is grams per liter of solution, not grams per liter (kg) of water (which would equal molality). You must account for the volume of KOH in the solution before molality can be calculated.
    So, find the volume of KOH per liter solution (this is why you are given density). 63.399 g / 1.05 g/mL = 60.380 mL KOH.
    Now find the volume of water per liter solution. 60.380 mL KOH per liter of solution leaves 1000 – 60.380 = 939.620 mL H2O.
    So molality = moles solute per kg solvent, which is 1.13 / 0.93962 = 1.203.