Chemistry · Solutions. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The concentration of a solution tells you how much solute is dissolved in a given amount of solvent or solution. The same solution can be described in several units, so always read which one the question is using.
Percentage units compare the solute with 100 parts of solution. Molarity counts moles of solute in 1 dm3 of solution. Molality counts moles of solute in 1 kg of solvent. Mole fraction compares moles with total moles.
Volume changes with temperature but mass does not. So molarity changes slightly when a solution is warmed, while molality and mole fraction stay the same. When a solution is diluted, the moles of solute do not change.
Molarity uses the volume of the whole solution, but molality uses the mass of the solvent only. Do not divide by the mass of the solution when finding molality, and change cm3 to dm3 and g to kg first.
Answer: 25 cm3 of the concentrated acid, made up to 600 cm3 with water.
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