Solutions and concentration
Make it up to the mark, then dilute it and watch the moles stay put.
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- Mass to weigh out
- 1.461 g
- Amount
- 0.0250 mol
- Concentration
- 0.100 mol/L
Per litre of solution, not of solventDissolve the solute first, then make the volume up to the mark. Adding a litre of water to the solute gives a different concentration, because the solute takes up room of its own.
Dilution
c₁V₁ = c₂V₂Give any three and the fourth follows.
Starting concentration
Starting volume
Final concentration
Final volumeSolve for
500 mL
Unchanged — dilution conserves moles, not volume.
Description of this simulation
1.461 g of NaCl dissolved and made up to 0.25 L of solution is 0.1 mol/L. That is 0.025 moles in 0.25 litres. The volume is the volume of the finished solution, not the volume of solvent added — the solute takes up room of its own, so the two are not the same thing. Starting with 0.05 L of 1 mol/L NaCl and making it up to 0.5 L gives 0.1 mol/L — 10 times more dilute. Adding 0.45 L of solvent changed the volume and changed nothing about the amount of solute: there were 0.05 moles before and there are 0.05 moles after. Dilution conserves moles, not volume.
Molar mass
58.440
g/mol
How that molar mass is built
- Na1 × 22.99022.99
- Cl1 × 35.45035.45
The same solution, other ways
- Mass per litre
- 5.84 g/L
- Percent mass/volume
- 0.584 %
- Parts per million
- 5,844 ppm
Milligrams per litre equals parts per million only because a litre of dilute aqueous solution weighs very nearly a kilogram.
What this model shows — and what it simplifies
Educational ModelMolar masses are computed from the standard atomic weights in the element table, so they agree with the periodic table by construction rather than by a second copy being kept in step.
Where do we see this in real life?
A saline drip is 0.9 per cent sodium chloride because that matches the concentration inside a cell. A little either way and the cells swell or shrivel — which is why the arithmetic on this page is done carefully in every hospital pharmacy.