Chemistry · Year 10 · Quantitative Chemistry · Summer Term

Concentration of Solutions

Students will calculate the concentration of solutions in g/dm³ and mol/dm³.

• National Curriculum Attainment Targets
• GCSE: Chemistry - Quantitative Chemistry
• GCSE: Chemistry - Chemical Measurements

About This Topic

Concentration of solutions measures the amount of solute dissolved in a given volume of solvent, using units of grams per cubic decimeter (g/dm³) or moles per cubic decimeter (mol/dm³). Year 10 students calculate these with simple formulae: for g/dm³, divide mass of solute in grams by volume in dm³; for mol/dm³, divide moles of solute by volume in dm³. They convert mass to moles using molar mass and apply this to real scenarios, aligning with GCSE Quantitative Chemistry requirements on chemical measurements.

Students differentiate the two units, recognising that g/dm³ suits any soluble substance while mol/dm³ standardises comparisons across chemicals via moles. They also learn to prepare solutions of specific concentrations by accurately weighing solute, dissolving it, and making up to a precise volume in a volumetric flask. These steps build precision in lab techniques and underpin topics like stoichiometry and rates of reaction.

Active learning excels for this topic because calculations link directly to tangible measurements. When students weigh salts, measure volumes, and dilute in small groups, they see how changes affect concentration firsthand. Peer discussions of results versus predictions correct errors in real time, making abstract maths concrete and boosting retention.

Key Questions

1. Calculate the concentration of a solution given mass/moles and volume.
2. Differentiate between concentration in g/dm³ and mol/dm³.
3. Explain how to prepare a solution of a specific concentration.

Learning Objectives

• Calculate the concentration of a solution in g/dm³ given the mass of solute and volume of solution.
• Calculate the concentration of a solution in mol/dm³ given the moles of solute and volume of solution.
• Convert between concentration in g/dm³ and mol/dm³ using molar mass.
• Design a step-by-step procedure to prepare a solution of a specified concentration using a volumetric flask.
• Compare the meaning and application of concentration expressed in g/dm³ versus mol/dm³.

Before You Start

Moles and Molar Mass

Why: Students must be able to calculate the number of moles of a substance from its mass and molar mass before they can calculate concentration in mol/dm³.

Units of Volume (cm³ and dm³)

Why: Students need to be comfortable converting between cubic centimeters and cubic decimeters to use the correct volume units in concentration calculations.

Key Vocabulary

Concentration	A measure of the amount of solute dissolved in a specific amount of solvent or solution.
Solute	The substance that is dissolved in a solvent to form a solution.
Solvent	The substance that dissolves a solute to form a solution; water is a common solvent.
dm³ (cubic decimeter)	A unit of volume equal to one liter. It is commonly used in chemistry for concentration calculations.
Molar mass	The mass of one mole of a substance, expressed in grams per mole (g/mol).

Watch Out for These Misconceptions

Common Misconception

Concentration is always expressed as a percentage.

What to Teach Instead

Concentration uses g/dm³ for mass-based or mol/dm³ for amount-based measures, not just percentages. Hands-on preparation shows percentages depend on relative densities, while active weighing and diluting clarifies fixed units. Peer comparisons during stations reveal why standard units aid precise communication.

Common Misconception

Moles and grams are directly interchangeable without molar mass.

What to Teach Instead

Convert grams to moles by dividing by molar mass first. Calculation challenges in pairs highlight errors when skipping this, as students test predictions with actual solution colours. Group discussions reinforce the step, building accurate mental models.

Common Misconception

Volumes of solutions are always additive.

What to Teach Instead

Mixing equal volumes dilutes, not maintains concentration. Dilution relays demonstrate this empirically, as measured results differ from naive additions. Collaborative analysis helps students adjust preconceptions through evidence.

Active Learning Ideas

Pairs: Dilution Relay

Pairs prepare a concentrated solution, then pass it along a chain for serial dilutions using pipettes and water. Each pair calculates expected concentrations before and after dilution, records absorbance with a colorimeter if available, or compares colours. Discuss discrepancies as a class.

35 min·Pairs

Small Groups: Solution Preparation Stations

Set up stations with balances, spatulas, and volumetric flasks. Groups weigh solute for target concentrations in g/dm³ and mol/dm³, dissolve, and make up to volume. Rotate stations to try different solutes, then label and store for later use.

45 min·Small Groups

Whole Class: Concentration Calculation Challenge

Project problems on screen with varied data (mass, moles, volume). Students work individually first, then share pair solutions before class vote on correct methods. Use mini-whiteboards for quick reveals and corrections.

25 min·Whole Class

Individual: Mystery Solution Solver

Provide data cards for unknown solutions. Students calculate concentrations in both units, predict dilution effects, and match to labelled samples by observation or simple tests. Submit worksheets for feedback.

20 min·Individual

Real-World Connections

• Pharmaceutical companies use precise concentration calculations to formulate medications, ensuring correct dosages for patients. For example, determining the exact amount of active ingredient in a liquid antibiotic requires careful measurement of solute and solvent.
• Food scientists and manufacturers control the concentration of ingredients like sugar, salt, and flavorings in products such as soft drinks and sauces. This ensures consistent taste and quality, and adherence to nutritional guidelines.

Assessment Ideas

Quick Check

Present students with a scenario: 'A student dissolves 5.0 g of sodium chloride (NaCl, molar mass 58.5 g/mol) in 250 cm³ of water. Calculate the concentration of the solution in g/dm³ and mol/dm³.' Ask students to show their working and final answers.

Discussion Prompt

Pose the question: 'When would it be more useful to express concentration in g/dm³ compared to mol/dm³? Give a specific example.' Facilitate a class discussion where students justify their choices, referencing different types of solutes or experimental contexts.

Exit Ticket

Ask students to write down the steps required to prepare 100 cm³ of a 0.1 mol/dm³ solution of sulfuric acid (H₂SO₄). They should include the calculation to find the mass of solute needed and the laboratory equipment they would use.

Frequently Asked Questions

How do you calculate concentration in g/dm³?

Measure the mass of solute in grams and the volume of solution in dm³, then divide mass by volume. For example, 5 g of salt in 0.5 dm³ gives 10 g/dm³. Practise with balances and measuring cylinders to ensure accuracy, as small errors amplify in calculations. This method applies to any soluble solid.

What is the difference between g/dm³ and mol/dm³?

g/dm³ uses mass of solute directly, suitable for quick lab checks without knowing formula. mol/dm³ requires converting mass to moles using molar mass (moles = mass / molar mass), allowing comparisons between substances. Use mol/dm³ for reactions involving particle numbers, as in stoichiometry.

How to prepare a solution of a specific concentration?

Calculate required mass or moles for the target volume, weigh accurately on a balance, dissolve in less than full volume of solvent, then transfer to a volumetric flask and make up to the mark. Stir thoroughly. Volumetric apparatus ensures precision; practise with sodium chloride for familiarity.

How can active learning help students understand concentration of solutions?

Activities like dilution relays and preparation stations let students manipulate variables directly, linking formulae to observations such as colour intensity changes. Pair work encourages explaining steps aloud, correcting peers' errors instantly. Whole-class challenges with mini-whiteboards build confidence in calculations through low-stakes practice and immediate feedback, deepening conceptual grasp over rote memorisation.