Question

Homework Assignment #6 Solutions (Due date: November 5th, 2019) (1) What is molarity, and what is molality? (1) Calculate the amount of water (in grams) that must be added to 5.00 g of urea (NH2)2CO in the preparation of a 16.2 percent by mass solution. (III) Calculate the number of g of solute that would be needed to make each of the following solutions: (a) 2.00 x 102 mL of 0.100 M NaBr (b) 2.50 x 10 mL of 0.200 M KOH

Answer 1

Molarity: Gives the measure of the concentration of a solution in terms of the number of moles of solute dissolved per litre of solution.

Molality: Measures the concentration of the solution in terms of the number of moles of solute per kilogram of solvent.

Mass percent = (mass of solute/mass of the solution) * 100%

Given: mass of solute = 5g

Mass percent = 16.2

Mass of solution =

16.2% = 5 / x * 100%

x = 30.864

mass of solvent = mass of solution - mass of solute = 30.864 - 5 = 25.864g

(i) molar mass of NaBr = 103

volume = 200ml= 0.2litre

molarity = 0.1M

Molarity = no of moles of solute / vol of solvent in litre

0.1 = (x/103)/0.2

x= 2.06g

(ii)molar mass of KOH = 56

volume = 250ml= 0.25litre

molarity = 0.2M

Molarity = no of moles of solute / vol of solvent in litre

0.2 = (x/56)/0.25

x= 2.8g