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Lesson · Grade 11 · Chemistry

Dilution: More Solvent, Same Moles of Solute

  • Grade 11
  • Chemistry
  • 2 min 10 s
  • British Columbia
  • BC11-SC-39

A small beaker of 50.0 mL of 2.00 mol/L solution (0.100 mol of solute) is diluted with water to 250.0 mL. The same solute particles spread out through the bigger volume, and a working panel shows C₁V₁ = C₂V₂ giving 2.00 × 50.0 ÷ 250.0 = 0.400 mol/L. The same answer then comes from parts (1 part concentrate in 5 parts total is one fifth of the strength). It ends by showing the two ways a "1:4" label can be read (one fifth or one quarter of the strength) and a one-line recap that the moles stay the same.

Concept: Dilution: more solvent, same amount of solute

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In one sentence

The question this lesson answers

Diluting a solution means adding more solvent. What changes, and what stays the same?

A solution is a solute dissolved in a solvent, here water.

Transcript

What the lesson says

Diluting a solution means adding more solvent. What changes, and what stays the same?

A solution is a solute dissolved in a solvent, here water.

Concentration is moles of solute per litre of solution. Ours is 50.0 millilitres at 2.00 moles per litre.

Moles of solute are concentration times volume in litres: 2.00 times 0.0500 is 0.100 moles.

Now add water until the total volume is 250.0 millilitres.

The solute spreads out, so the concentration drops, but there are still 0.100 moles.

Moles before equal moles after, so C one V one equals C two V two.

With both volumes in millilitres, 2.00 times 50.0 equals C two times 250.0.

So C two is 2.00 times 50.0 divided by 250.0, which is 0.400 moles per litre.

At work, people often think in parts. Each part here is 50.0 millilitres: 1, 2, 3, 4, 5.

1 part concentrate in 5 parts total gives one fifth of the strength: 2.00 divided by 5 is 0.400 again.

A label saying 1 to 4 may mean 1 part to 4 parts: 5 parts total, so one fifth of the strength.

Or it may mean 1 part in 4 parts total, so one quarter of the strength. Check which one is meant.

More solvent, same moles of solute: C one V one equals C two V two.

Curriculum

Where this fits

British Columbia, Grade 11

Chemistry 11, Solutions: stoichiometric calculations in aqueous solutionsBC11-SC-39

Previous: Molarity: moles of solute per litre of solutionNext: Ion concentrations when two solutions are mixed

British Columbia, Grade 11

Science For Citizens 11, Science at work: practical applications of science in the workplaceBC11-SC-287

Next: Fractional Distillation: Separating Crude Oil by Boiling Point

The outcomes and strands follow the British Columbia curriculum.

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