C
CraftExam
medium difficulty7 min

Concentration of a Solution

Concentration tells how much solute is present in a given amount of solution or solvent.

In one line: Percent concentration means part out of whole solution.

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Understand it

A concentrated solution has comparatively more solute, while a dilute solution has comparatively less solute. In Class 9 numerical questions, concentration is commonly expressed as mass percentage or volume percentage.

Example

If 10 g salt is dissolved to make 100 g solution, the mass percent of salt is 10%.

Rule to remember

Mass percent = (mass of solute / mass of solution) × 100.

Volume percent = (volume of solute / volume of solution) × 100.

See it worked out

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

Worked example

A solution has 5 g sugar dissolved in 95 g water.

  1. 1Mass of solution = 5 g + 95 g = 100 g.
  2. 2Mass percent of sugar = 5/100 × 100 = 5%.

Method you can reuse

  1. 1Identify solute amount.
  2. 2Find total solution amount if not directly given.
  3. 3Choose mass or volume formula.
  4. 4Substitute carefully.
  5. 5Add percent sign.

Visual tip

A labelled calculation box can show solute amount over total solution amount multiplied by 100.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is the denominator mass of solution in mass percent concentration?

Think first, then show answer
The denominator is mass of solution because mass percent compares the mass of solute with the total mass of the final solution, not only with the solvent.

Common wrong answer

For 5 g sugar in 95 g water, writing 5/95 × 100.

Why it loses marks: 95 g is only solvent, not solution.

Don't mix this up

Students often divide by mass of solvent instead of mass of solution in mass percent questions.

Exam tip

Underline whether the question asks mass by mass percent or volume by volume percent before substituting values.

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 usually provide solute and solvent quantities and ask for mass percent or volume percent concentration.
  1. 1

    Enough for 1 mark

    Concentration tells how much solute is present in a given amount of solution or solvent.

  2. 2

    Add for 2 marks

    Example: If 10 g salt is dissolved to make 100 g solution, the mass percent of salt is 10%.

  3. 3

    Add for 3 marks

    A concentrated solution has comparatively more solute, while a dilute solution has comparatively less solute.

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