# 3.4.1 Using Concentrations in mol/dm3

Higher Tier Only

### Concentration of Solutions in Moles

• It is more useful to a chemist to express concentration in terms of moles per unit volume rather than mass per unit volume
• Concentration can therfore be expressed in moles per decimetre cubed
• We can modify the concentration formula to include moles
• The units in the answer can be written as mol dm-3 or mol / dm3: • You may have to convert from g dm-3 into mol dm-3 and vice versa depending on the question
• To go from g dm-3 to mol dm-3:
• Divide by the molar mass in grams
• To go from mol dm-3 to g dm-3:
• Multiply by the molar mass in grams

#### Exam Tip

To go from cm3 to dm3 : divide by 1000

To go from dm3 to cm3 : multiply by 1000

Higher Tier Only

### Calculating Concentration from Reacting Solutions

• Solving problems on concentrations involves carefully working out moles and volumes in the correct units and applying the concentration formula
• Some students find formula triangles help them to understand the relaitonship: • The following examples show how to do this step-by-step

#### Worked Example

Example 1

Calculate the amount of solute, in moles, present in 2.5 dm3 of a solution whose concentration is 0.2 mol dm-3. #### Worked Example

Example 2

Calculate the concentration of a solution of sodium hydroxide, NaOH, in mol dm-3, when 80 g is dissolved in 400 cm3 of water.

(Na= 23, H= 1, O= 16) #### Titration Calculations

• If the concentration of one of the reactants is known (either the acid or the base), then the exact volumes from a titration along with the balanced chemical equation for the reaction can be used to calculate the concentration of the other reactant

#### Worked Example

Example 3

25.0 cm3 of a solution of 0.05 mol dm-3  sodium carbonate was completely neutralised by 20.00 cm3 of dilute hydrochloric acid. Calculate the concentration of the hydrochlroric acid in mol cm-3 #### Exam Tip

You are not given the concentration-moles formula triangle in exams so you have to learn it. It is a good idea to write it down before you start a problem, so you get all the parts in the correct place. ### Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
Close Close