"Molarity" refers to the concentration of a solute in a solution and is defined as the general formula of moles of solute divided by the litres of solution. Recall that "solute" is the substance being dissolved into a solvent to form a solution. Calculating the molarity of a solution of copper sulphate pentahydrate simply requires dividing the moles of copper sulphate pentahydrate by the volume of solution; this same method is used to determine the molarity of any solution.
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Things you need
- Copper sulphate pentahydrate
- Volumetric flask
Measure and record the mass of copper sulphate pentahydrate to be added to the solution using a balance.
Convert the mass to number of moles using the molar mass of copper sulphate pentahydrate. Since the molar mass of copper sulphate pentahydrate is 249.70g/mol, the conversion formula is: number of grams x (1 mol/249.70g) = number of moles. For example, 25 grams of copper sulphate pentahydrate equals 0.1 moles (25g x (1 mol/249.70g) = 0.1 mol).
Dissolve the copper sulphate pentahydrate in water using the volumetric flask to create the desired volume. Measure the volume of the final solution in litres.
Divide the number of moles by the volume of the solution. For example, if 25 grams of copper sulphate pentahydrate are used to create a one litre solution, the molarity is equal to 0.1 M (0.1 mol/1 L = 0.1 M). If, instead of a one-liter solution, the same amount of copper sulphate pentahydrate is used to create a two-liter solution, the molarity is 0.05 M (0.1 M/2 L = 0.05 M).
Determine the precise amount of copper sulphate pentahydrate needed to create a specific volume with a specific molarity using the previous formulas. For example, assume a 600ml solution of 0.5 M copper sulphate pentahydrate is desired. Since the molarity of a substance is the number of moles per litre of solution, we can predict that 0.5 M copper sulphate pentahydrate equals 0.5 moles of copper sulphate pentahydrate in one litre of solution (0.5 moles/1 L = 0.5 M).
Convert to the desired volume by multiplying the number of moles per litre of solution by the desired volume. Continuing the example, multiply 0.600 litres (600 mL = 0.600 L) by the ratio of 0.5 moles per litre to produce 0.3 moles of copper sulphate pentahydrate to make 600ml of 0.5 M solution.
Convert the number of moles into grams using molar mass. Finishing the example, 0.3 moles of copper sulphate pentahydrate equals 75g (0.3 mol x 249.7g/mol = 75g). This is the amount of copper sulphate pentahydrate needed to make 600ml of 0.5 M copper sulphate pentahydrate solution.
Tips and warnings
- Molarity should not be confused with "molality," which is determined by dividing the number of moles of solute by kilograms of solvent.
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