A student mixes of a solution with of pure water

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Problem

A student mixes

10.0\,\text{g}

of a

2.0\,\text{mol}\,L^{-1}

\mathrm{CuSO}_4

solution with

90.0\,\text{g}

of pure water. Which of the following best explains why the mass fraction of

\mathrm{CuSO}_4

in the final mixture is much less than

2.0\,\%

? (a) Because the density of the solution drops on dilution, the larger volume makes the concentration expressed as mass percent smaller even though the masses remain constant. (b) Dilution causes some

\mathrm{CuSO}_4

to precipitate out of solution, so less solute remains in the mixture and the mass fraction falls. (c) Adding a large mass of solvent increases the total mass while the mass of

\mathrm{CuSO}_4

stays the same, so the solute’s mass fraction decreases. (d) A

2.0\,\text{mol}\,L^{-1}

solution already contains about

2.0\,\%

\mathrm{CuSO}_4

by mass, so any further dilution must lower the value below

2.0\,\%

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