Solutions & Mixtures
- Calculate molarity and use it to relate moles, mass, and solution volume
- Predict solubility with the "like dissolves like" rule and identify ion–dipole solvation
- Explain how chromatography and distillation separate the components of a mixture
Concentration and molarity
A solution is a homogeneous mixture of a solute dissolved in a solvent. Chemists express how concentrated a solution is with molarity (M) — the number of moles of solute per liter of solution. Molarity is the workhorse of solution chemistry because it directly connects a volume you can measure in a flask to the moles a reaction actually consumes.
What is the molarity of a solution made by dissolving 0.50 mol of NaCl in enough water to make 250 mL of solution?
- 1.Convert the volume to liters: 250 mL × (1 L ÷ 1000 mL) = 0.250 L.
- 2.Apply the definition: M = mol ÷ L.
- 3.Substitute: M = 0.50 mol ÷ 0.250 L.
What mass of NaOH (molar mass 40.0 g·mol⁻¹) is needed to prepare 500. mL of 0.100 M NaOH solution?
- 1.Convert volume to liters: 500. mL = 0.500 L.
- 2.Find moles of solute: mol = M × L = 0.100 mol·L⁻¹ × 0.500 L = 0.0500 mol.
- 3.Convert moles to mass: mass = mol × molar mass = 0.0500 mol × 40.0 g·mol⁻¹.
"Like dissolves like"
A solute dissolves when solute–solvent attractions can replace the solute–solute and solvent–solvent attractions. Polar and ionic solutes dissolve in polar solvents (water dissolves salt and sugar); nonpolar solutes dissolve in nonpolar solvents (oil dissolves in hexane). When an ionic solid dissolves in water, the strong ion–dipole forces between each ion and the polar water molecules pull the lattice apart and surround each ion — a process called hydration.
Separating mixtures
Because the components of a mixture keep their own properties, physical differences let us separate them. Distillation exploits differences in boiling point: heat a mixture, and the more volatile component vaporizes first, then recondenses in a cooler tube. Chromatography exploits differences in how strongly components adhere to a stationary phase versus travel with a moving (mobile) phase: a component that clings weakly and dissolves well in the mobile phase moves farther and separates from the rest.
In chromatography, a spot that travels far up the paper is more attracted to the mobile phase (and less to the stationary phase) than a spot that stays near the start line. Relative distance traveled tells you relative affinity.
What is the molarity of a solution containing 0.25 mol of glucose dissolved in enough water to make 500 mL of solution?
10.0 mL of 6.0 M HCl is diluted with water to a final volume of 60.0 mL. What is the concentration of the diluted solution?
Which solute is most likely to dissolve well in water, a polar solvent?
When a problem gives grams but asks about a reaction or concentration, convert to moles first — moles are the currency of both molarity (mol ÷ L) and stoichiometry. Watch that volume is in liters and that "solution" volume, not solvent volume, goes in the denominator.
Answer the 3 checkpoints as you read.
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