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Types of Solutions and Concentration Terms

A solution is a homogeneous mixture of two or more components, usually described using solvent, solute, and concentration terms such as mass percentage, mole fraction, molarity, molality, and parts per million.

In one line: Molarity uses litres of solution; molality uses kilograms of solvent.

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Understand it

Solutions may be gaseous, liquid, or solid depending on the physical state of the solvent and solute. In numerical questions, the concentration term must be chosen from the data given: mass percentage uses mass, mole fraction uses moles, molarity uses volume of solution in litres, molality uses mass of solvent in kilograms, and ppm is used for very dilute solutions.

Example

A solution containing 5 g glucose in 95 g water has mass percentage of glucose = 5/(5 + 95) × 100 = 5%.

Rule to remember

Mass % = mass of component/mass of solution x 100; mole fraction xi = ni/total moles; molarity M = moles of solute/litres of solution; molality m = moles of solute/kg of solvent; ppm = mass of solute/mass of solution x 106 for very dilute mass-based cases.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Find molarity when 0.5 mol NaCl is dissolved to make 250 mL solution.

  1. 1Convert volume: 250 mL = 0.250 L.
  2. 2M = 0.5/0.250 = 2.0 mol L−1.
  3. 3Interpretation: each litre of solution contains 2.0 mol NaCl.

Method you can reuse

  1. 1Identify given quantities first: mass, moles, solution volume, or solvent mass.
  2. 2Convert volume to litres and solvent mass to kilograms where needed.
  3. 3Select the concentration formula whose denominator matches the data.
  4. 4Substitute with units and state the final concentration term clearly.

Visual tip

A diagram is not essential. A compact comparison table of concentration terms with denominator and unit is more useful for revision.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Which concentration term is temperature independent: molarity or molality?

Think first, then show answer
Molality is temperature independent because it depends on mass of solvent, not volume of solution.

Common wrong answer

Writing molarity in mol kg−1 or molality in mol L−1 is a frequent unit error.

Don't mix this up

Students often use mass of solvent instead of mass of solution for mass percentage, or use volume of solvent instead of volume of solution for molarity.

Exam tip

Check whether the denominator is solution or solvent before applying the formula. Molarity changes with temperature because volume changes, while molality does not depend on solution volume.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask students to calculate mass percentage, mole fraction, molarity, molality, or ppm, or to compare why molality is preferred over molarity in temperature-variable situations.
  1. 1

    Enough for 1 mark

    A solution is a homogeneous mixture of two or more components, usually described using solvent, solute, and concentration terms such as mass percentage, mole fraction, molarity, molality, and parts per million.

  2. 2

    Add for 2 marks

    Example: A solution containing 5 g glucose in 95 g water has mass percentage of glucose = 5/(5 + 95) × 100 = 5%.

  3. 3

    Add for 3 marks

    Solutions may be gaseous, liquid, or solid depending on the physical state of the solvent and solute.

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