Molarity formula:
story, analogy and memory trick
The Molarity formula is M = moles of solute ÷ litres of solution. Molarity tells you how many moles of solute are dissolved in each litre of solution.
The Molarity formula
- Mmolarity, in mol/L
- moles of soluteamount of dissolved substance
- litres of solutiontotal volume of the solution
The story
Priya makes chai for the whole family. One spoon of sugar in a cup tastes mild, and three spoons in the same cup tastes sweet. The spoons are the moles of solute, the cup is the volume, and the sweetness is the molarity.
Real-life analogy
Think of cold drink syrup. More syrup in the same glass gives a stronger drink, and the same amount of syrup in a bigger glass gives a weaker one.
Memory trick
Moles on top, litres at the bottom. The unit itself, mol/L, gives you the formula.
Exam tip
Use the volume of the whole solution in litres, not the volume of the solvent. Convert mL to L by dividing by 1000. Moles = mass ÷ molar mass.
Questions about Molarity
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For an ideal gas, pressure times volume equals the number of moles times the gas constant times the absolute temperature.
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P₁V₁ = P₂V₂
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Read the storypH
pH = −log₁₀[H⁺]
pH is the negative base-10 logarithm of the hydrogen-ion concentration, so a lower pH means a more acidic solution.
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