Molarity
- A solution may be described qualitatively or quantitatively
- Qualitatively, the terms dilute and concentrated are used to describe the amount of solute in a given quantity of the solvent
- A solution with a relatively small concentration of solute is said to be dilute
- A solution with a large concentration of solute is said to be concentrated
- Quantitatively, a solution is described in terms of its concentration
- Qualitatively, the terms dilute and concentrated are used to describe the amount of solute in a given quantity of the solvent
Molarity
- Molarity (M) expresses the concentration of a solution as the number of moles of solute in a litre of solution:
M = n / V
- A 1.00 molar solution (1.00 M) contains 1.00 mol of solute in every 1.0 L of solution
- A solution can be prepared to a specified molarity by weighing out the calculated mass of solute and dissolving it in enough solvent to form the desired volume of solution
- For example, to prepare 250.0 mL of a 1.00 M solution of CuSO4:
- Determine the number of moles of the solute (CuSO4) required
- In this case, this is given as n = Molarity × Volume
- n = 1.00 × 0.250 = 0.250 mol
- In this case, this is given as n = Molarity × Volume
- Determine and weigh the corresponding mass of CuSO4 required
- Using the the expression: Mass = n × Mr
- The molar mass (Mr) of CuSO4 is calculated as:
- Mr = (Mr)cu + (Mr)S + 4(Mr)O
- Mr = 63.6 + 32.0 + 4 (16) = 159.6 g/mol
- Mass = 0.250 mol × 159.6 g/mol = 39.9 g
- The molar mass (Mr) of CuSO4 is calculated as:
- Transfer 39.9 g of CuSO4 into a 250 mL flask and add some quantity of water to dissolve the solute
- Add more water until the solution reaches the calibrated mark of the flask
- Using the the expression: Mass = n × Mr
- Determine the number of moles of the solute (CuSO4) required
Preparing a Solution
How to prepare a 250mL of 1.00 M solution of CuSO4
Dilution
- Alternatively, you can start with a more concentrated solution, called the stock solution, and dilute it with water to give a solution of the desired molarity
- The calculations are straightforward if you keep a simple point in mind: Adding solvent cannot change the number of moles of solute
- That is:
- The calculations are straightforward if you keep a simple point in mind: Adding solvent cannot change the number of moles of solute
nsolute (stock solution) = nsolute (dilute solution)
- In both solutions, n can be found by multiplying the molarity, M, by the volume in litres, V
- Hence, a dilution expression can be obtained:
Ms × Vs = Md × Vd
-
- Where the subscripts s and d stand for stock and dilute solutions, respectively
Worked example
Calculate the molarity of a solution made by dissolving 5.00 g of glucose (C6H12O6) in sufficient water to form exactly 100 mL of solution. (Mr glucose = 180.2 g/mol)
Answer:
Analyse: We are provided with the mass of glucose and the volume of solution and asked to determine the molarity of the solution
Plan: To determine the molarity of the solution the following steps are required:
Step 1: Convert the mass of glucose to moles and the volume of the solution to litres
- n = Mass/Molar Mass
- n = 5.00/180.2
- n = 0.0277 moles
- Volume = 100/1000
- Volume = 0.1L
Step 2: Substitute the values of the moles of glucose and the volume of the solution
- Molarity (M) = number of moles/volume of solution
- Molarity (M) = 0.0277/0.1
- Molarity (M) = 0.277 M
Worked example
- How many grams of Na2SO4 are there in 15 mL of 0.50 M Na2SO4?
- How many millilitres of 0.50 M Na2SO4 solution are needed to provide 0.038 mol of this salt?
Analyse:
- In part a, we are given the volume of the solution and the concentration (Molarity)
- We are then asked to determine the mass of the solute(Na2SO4) required
- In part b, we are given the number of moles of solute and the molarity of the solution
- We are then asked to determine the volume of solution in millilitres
Plan:
For part a:
- Use the molarity expression to determine the number of moles of the solute (n)
- Convert this number of moles to mass
For part b:
- Use the molarity expression to determine the volume of solution in litres
- Convert to millilitres using the appropriate conversion factor
Solution
- Part a:
- Step 1: Convert volume in mL to L:
- Volume of solution = 15/1000 = 0.015L
- Step 2: Rearrange the concentration expression in terms of the number of Na2SO4 (n):
- n = M × V
- n = 0.50 × 0.015
- n = 0.0075 moles
- Step 3: Using the mole-molar mass expression, convert the moles of solute to mass:
- MNa2SO4 = n × Mr
- MNa2SO4 = 0.0075 × 142.04
- MNa2SO4 = 1.1g
- Step 1: Convert volume in mL to L:
- Part b:
- Step 1: Rearrange the concentration expression in terms of the volume of solution:
- V = n/M
- V = 0.038/0.50
- V = 0.076 L
- Step 2: Convert volume in L to mL:
- V = 0.076 × 1000
- V = 76 mL
- Step 1: Rearrange the concentration expression in terms of the volume of solution:
Worked example
How many millilitres of 5.0 M Na2Cr2O7 solution must be diluted to prepare 250 mL of 0.10 M solution? How much water needs to be added to this concentrated solution?
Analyse:
- We are given the volume and concentration of the diluted solution and asked to determine the volume of the concentrated solution required to prepare
- We are then asked the volume of water required to make the dilute solution
Plan:
- Using the dilution formula, we can calculate the volume of the concentrated solution required
- Then subtract that volume from the volume of the dilute solution to obtain the volume of water required
Answer:
- Step 1: Using M1V1 = M2V2
- Rearrange the equation to V1 = (M2V2)/ M1
- V1 = ?
- M1 = 5.0 M
- V2 = 250 mL
- M2 = 0.10 M
- So, V1 = (0.10 × 250)/5.0
- V1 = 5.0 mL
- Rearrange the equation to V1 = (M2V2)/ M1
- Step 2: The volume of water added will be:
- VH2O = 250 - 5.0
- VH2O = 245 mL