- Units & Notation
- Moles per litre
- Grams per litre
- Percent solutions
- Parts per million
- Practice problems
Moles per litre
Example 1
Find the molarity of a sodium chloride (NaCl) solution made when 0.2 moles NaCL is dissolved in 0.8 litres of water.
The video below shows the systematic approach to solving the above problem.
This video contains sound.
The process of solving problems like these can be simplified by following a series of steps.
The first step is to extract from the problem statement the information that has been provided and the information being asked for.
The information we are provided with is:
We have 0.2 moles of NaCl and 0.8 litres of water.
The information we are asked for is:
Molarity of the solution resulting from dissolving Sodium Chloride in water.
The second step is to identify the formula to find out the information being asked for.
In this problem we are being asked to find Molarity or concentration. Hence the formula to calculate Molarity is:
The third step is to match the information provided in the problem statement with the formula. As NaCl is dissolved in water, therefore NaCl is the solute i.e. 0.2 moles and water is the solution i.e. 0.8 litres.
The fourth step is to ensure that the information being used in the formula is in the correct units. Molarity is expressed in moles/litres so the solute should always be in moles and the solution should always be in litres. In this problem, both NaCl and water have the correct units.
Now we go ahead with the calculations. We get:
Do not forget to add unit to the answer. The unit of Molarity is M, hence the answer to the exercise is 0.25 M.
Besides the method shown above, there is an alternative approach to solving this problem as well. Remembering that molarity is the number of moles of substance needed for each litre of solution, use a proportion calculation:
0.8 litres contains 0.2 moles NaCl
1 litre contains 0.2 / 0.8 = 0.25 moles NaCl
Hence molarity is 0.25 M.
