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
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molality = no. of moles of solute/weight of solvent in Kg
molarity = no. of moles of solute/volume of solution in L
Activity
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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?
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Activity
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How many grams of AlCl3 are needed to make 500.0 g of 0.1 m aqueous solution?
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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
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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
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Activity
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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?
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