Solution mass fraction and dilution
Why does a solution become less concentrated when only water is added?
Change the masses of solute, water, and added water, then use conserved solute mass to explain dilution.
Learning goals
- Calculate solute mass fraction as solute mass divided by solution mass.
- Explain what changes and what is conserved when water dilutes a solution.
- Compare concentrations without treating mass fraction as volume.
Before you begin
- Calculate one quantity as a percentage of a total.
- Know that a dissolved solute is distributed through a solvent.
Interactive science experiment
- 1Build a mass account
- 2Add only water
- 3Change solute
- 4Design a dilution
Uses mass and assumes complete dissolution. The fixed liquid level and dots show composition only, not volume or particle-count proportions.
Observe, then explain
Work through four steps. Each button moves the interactive model to the matching observation state.
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A 20 g solute plus 80 g water makes 100 g solution. Its solute mass fraction is 20÷100=20%; the denominator includes solute and all water.
Your task: Select the starting solution and verify all three masses and the percentage.
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Add 100 g water: solute remains 20 g while total solution becomes 200 g, so the mass fraction falls to 10%. No solute disappears.
Your task: Hold solute constant, move Added water, and watch the denominator and percentage.
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With total water fixed, more fully dissolved solute increases both numerator and denominator. The percentage rises, but does not simply double with solute mass.
Your task: Compare 20 g and 40 g solute with 80 g water.
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With 40 g solute and 80 g water the fraction is about 33.3%. Adding 120 g water makes 240 g total and about 16.7%, so the mass account checks the dilution plan.
Your task: Choose the target dilution, then adjust added water to approach 20%.
Build an explanation
The numerator is solute mass
Mass fraction is the share of the whole solution mass contributed by solute. Percent means parts per 100 by mass.
Dilution conserves solute
If only water is added and nothing spills, reacts, or evaporates, solute mass stays fixed while total mass grows, putting the same numerator over a larger denominator.
Mass fraction is not a volume concentration
Every input here is in grams. The model does not calculate molarity or volume fraction; real preparation also depends on solubility, temperature, density, and measurement uncertainty.
Check your understanding
Printable worksheet
Complete the worksheet first. Answers and explanations are on a separate page.
Chemistry
Solution mass fraction and dilution
Why does a solution become less concentrated when only water is added?
- What is the solute mass fraction of 20 g solute with 80 g water?
A. 20% B. 25% C. 80%
- A solution contains 20 g solute and 80 g water. After adding 100 g more water, what is the new mass fraction?
A. 10% B. 20% C. 40%
- Which quantity stays fixed when only water is added?
A. Solute mass B. Total solution mass C. Mass fraction
- Why can this model not directly give the mixed volume?
A. It uses mass, and real mixed volumes need not add simply B. Solutions have no volume C. Grams always equal millilitres
Answer page
Solution mass fraction and dilution
- 20%
Solution mass is 20+80=100 g, so 20÷100×100%=20%.
- 10%
Solute remains 20 g and total mass becomes 200 g; 20÷200×100%=10%.
- Solute mass
No solute is added or removed, so its mass is conserved; water and total mass increase while the fraction falls.
- It uses mass, and real mixed volumes need not add simply
Mass conservation does not make volumes additive; volume also depends on density, temperature, and mixing behaviour.