AP® Chemistry · Unit 3 · Topic 3.7
Solutions and Molarity
Molarity is moles of solute per liter of solution. It lets chemists count particles in a liquid, which is what actually matters for reactions, not just mass.
Chemists care about how many particles are reacting, not how many grams. Two solutions with the same grams per liter of different solutes can have wildly different numbers of molecules, because molar masses differ. Molarity (M) fixes this by reporting moles of solute per liter of solution, so you can compare and predict reactions directly: moles react in whole-number ratios, grams don't.
To make 500.0 mL of 0.250 M NaCl, first find the moles needed: 0.500 L x 0.250 mol/L = 0.125 mol. Convert to mass using molar mass (58.44 g/mol): 0.125 mol x 58.44 g/mol = 7.30 g. Weigh out that solid, dissolve it in less water than the final volume, then add more water until the total solution reaches exactly 500.0 mL in a volumetric flask. The order matters: you dilute to final volume, not just add 500.0 mL of water to the solid.
When a salt dissolves, it splits into ions, so the ion concentration is not the same as the salt concentration. CaCl2 dissolving gives one Ca2+ and two Cl- per formula unit, so a 0.10 M CaCl2 solution is 0.10 M in Ca2+ but 0.20 M in Cl-. This idea connects directly to dilution and to later topics like reaction stoichiometry in solution and light absorption methods, which also read concentration through moles per liter rather than mass.
Key terms
- Molarity
- Concentration expressed as moles of solute divided by liters of solution.
- Solute
- The substance being dissolved, present in a smaller amount than the solvent.
- Volumetric flask
- Glassware marked with a single precise line used to measure a solution's total final volume exactly.
- Dissociation
- The splitting of an ionic compound into its individual ions when it dissolves in water.
Watch it explained
- How it worksWalk me through preparing 500.0 mL of 0.250 M NaCl starting from solid salt.
- Worked exampleFind the molarity of a solution made by dissolving 5.85 g of NaCl in enough water to reach 250 mL.
- Worked exampleFind the concentration of chloride ions in a 0.10 M solution of CaCl2.
- Common mistakeWhy isn't the volume of the solution the same as the volume of solvent you started with?
Check yourself
End of topic.