# Molality

Discussion
 Molality is the number of moles of solute per mass of solvent (in Kg) It is the number of moles of solute divided by weight of solvent in Kg – its symbol is m How is molality different from molarity? How is the symbol of molarity different from that of molality? In molality we talk of solvent while in molarity we talk of solution In molality we talk of weight of solvent Kg while in molarity we talk of volume of solution in litres

molality = no. of moles of solute/weight of solvent in Kg
molarity = no. of moles of solute/volume of solution in L

Activity
 0.1356 g of MgSO4 was dissolved in 200 ml of water (density of water is 1 g/ml) What is the molality of the solution? What is the molality of a solution of Cd(NO3)2 in 375.0 g of water? What is the volume of the water in the solution? How many grams of Sr(ClO4)2 are required to make a 0.3 m aqueous solution using 600 g of water?

Activity
 How many grams of AlCl3 are needed to make 500.0 g of 0.1 m aqueous solution?

NOTE:
Weight given is that of solution
Weight of solution is = to weight of solvent + weight of solute

# Molality and Molarity Conversions

Discussion
• Density of a solution is the mass of solution divided by volume of the solution
• Remember: Density is mass (g)/volume (ml) or mass (kg)/volume (L)
• Density of water may be taken as 1g per 1ml
• If you dissolve sodium chloride in water, will the mass of 1 ml of the solution be 1g, less than 1g or more than 1 g?
• When you dissolve more NaCl to the water, will that increase the mass of 1ml or not?

## In other words

• the density of the solution increases with increased concentration of the solution
• Density changes with concentration
• If you are given the concentration of a solution and its density, then you can calculate the molality of the solution
• Don't forget molarity is about moles per volume of solution
• While molality is about moles per weight of the solvent

Activity
 You have an aqueous solution of HCl which has a concentration of 0.123M, and density of 1.030 g/ml. Calculate the molality of the solution. NOTE: You know number of moles of HCl per litre of solution; so you can get mass of HCl in a litre of solution Density of 1.030 g/ml means that the mass of each ml is 1.030 g, and that is the mass of the solvent and solute in that 1 ml If you know how many g in 1 ml, you can get how many g in 1 L If you know the mass of HCl in 1L of solution, then you can get the mass of solvent alone in 1 L of solution

Activity
 A solution of KOH (aq), has molality of 3.21m, and its density is 1.163 g/ml. What is the concentration of the solution?