Chapter 13 · Solutions
13.6Specifying Solution Concentration: Molarity
12 min · two checks
Predict
What is the molarity of a solution with 0.50 mol of solute in 2.0 L of solution?
The idea
- Calculate molarity.
- Convert between moles and liters using M.
Molarity (M) is moles of solute per liter of solution. It is the concentration unit stoichiometry prefers, because it already speaks moles. A 0.100 M NaCl solution contains 0.100 mol of NaCl, and therefore 0.100 mol of Na⁺ and 0.100 mol of Cl⁻, in each liter. A 0.100 M CaCl₂ solution contains 0.200 mol of Cl⁻ per liter, because the formula has two chlorides. Read the question: moles of compound, or moles of a particular ion?
To prepare a solution, dissolve the needed moles and then add solvent until the final volume is the volume you used in the definition. Adding 1.00 L of water to a solid is not the same as making 1.00 L of solution, because the solid occupies space. In calculations, moles = M × V, with V in liters. Milliliters must be converted or you will be off by 1000.
Keep these
- M = mol solute / L solution.
- moles = M × V(L).
- Ion concentration may be a multiple of the compound’s molarity.
Worked path
What mass of NaOH (40.00 g/mol) is needed to prepare 250. mL of 0.200 M solution?
Observe
V = 0.250 L. Moles needed = 0.200 × 0.250.