"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.

- Skill level:
- Easy

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### Things you need

- Copper sulphate pentahydrate
- Water
- Balance
- Volumetric flask
- Pencil
- Paper

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## Instructions

- 1
Measure and record the mass of copper sulphate pentahydrate to be added to the solution using a balance.

- 2
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).

- 3
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.

- 4
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).

- 1
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).

- 2
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.

- 3
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.