# 1.5.9 Calculate Concentrations of Solutions

### Calculate Concentrations of Solutions

#### Expressing Concentration

• A solid substance that dissolves in a liquid is called a solute, the liquid is called a solvent and the two when mixed together form a solution
• Most chemical reactions occur between solutes which are dissolved in solvents, such as water or an organic solvent
• Concentration simply refers to the amount of solute there is in a specific volume of the solvent
• The greater the amount of solute in a given volume then the greater the concentration
• A general formula triangle for concentration is thus: The concentration-moles formula triangle

• It is useful to a chemist to express concentration in terms of moles per unit volume
• Concentration can therefore be expressed in moles per decimetre cubed
• 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

#### 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 cmof water.

(Na= 23, H= 1, O= 16) #### Exam Tip

To go from cm3 to dm3 : divide by 1000

To go from dm3 to cm3 : multiply by 1000 ### 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.
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