Concentration Calculations
Simple Explanation
Concentration measures how much solute is dissolved in a given volume of solution. In moles, it is calculated as c = n/V, where n is the amount in moles and V is the volume in litres (dmĀ³), giving units of mol/dmĀ³ (also called molarity).
Why Do We Need It?
Concentration tells you exactly how 'strong' a solution is, which is essential for accurately measuring out reactants in solution rather than as solids.
Formula
Concentration Formula
c = n/V
The concentration of a solution, from the moles of solute dissolved and the volume of solution.
- c
- ā concentration, in moles per cubic decimetre (mol/dmĀ³)
- n
- ā amount of solute, in moles (mol)
- V
- ā volume of solution, in cubic decimetres (dmĀ³)
When to use it: Whenever you need to find how concentrated a solution is, or how many moles/what volume are needed to make one of a given concentration.
Worked Example
Calculate a solution's concentration
What is the concentration of a solution containing 0.5 mol of solute dissolved in 2 dmĀ³ of solution?
Why Does This Work?
Dividing the total amount of dissolved substance by the total volume it is spread through gives exactly how much of it is present per unit of volume ā the definition of concentration.
Real-Life Example
Preparing a solution in the lab
A chemist needs to prepare 1 dmĀ³ of a 0.1 mol/dmĀ³ solution for a titration.
Rearranging c = n/V to n = c Ć V tells them exactly how many moles (and therefore grams) of solute to dissolve.
Practice
What is the concentration of a solution with 1 mol of solute in 4 dmĀ³ of solution?
EasyHow many moles of solute are in 0.5 dmĀ³ of a 2 mol/dmĀ³ solution?
MediumCommon mistake
Using volume in cmĀ³ instead of dmĀ³ without converting ā remember 1 dmĀ³ = 1000 cmĀ³, so forgetting to divide by 1000 gives a concentration 1000 times too large.
Quick Review
- c = n/V, with V in dmĀ³.
- Units are mol/dmĀ³ (molarity).
- 1 dmĀ³ = 1000 cmĀ³ ā always check your units.