What is Solution Concentration?
Solution concentration describes how much solute is dissolved in a solvent or total solution. Different concentration units are used for different applications: molarity for lab work, molality for colligative properties, and percent for everyday applications.
Solution concentration is the amount of solute per unit volume (molarity: mol/L) or mass (molality: mol/kg, percent: g solute per 100 g solution). Choice of unit depends on the application.
- •mol solute / L solution
- •Used in lab, easy to measure volume
- •M = n/V
- •Example: 0.5 M NaCl
- •mol solute / kg solvent
- •Used for colligative properties
- •m = n/mass(solvent)
- •Example: 0.5 m NaCl
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Step-by-step worked examples
What is the molarity of 2 mol of NaCl dissolved in 500 mL of solution?
M = n/V = 2 mol / 0.5 L = 4 M
A 0.2 M HCl solution contains how many grams of HCl per liter? (M = 36.5 g/mol)
n = M × V = 0.2 × 1 L = 0.2 mol mass = 0.2 mol × 36.5 g/mol = 7.3 g
Calculate molality: 50 g glucose (M = 180 g/mol) in 500 g water.
n = 50 g / 180 g/mol = 0.278 mol m = n / mass(solvent in kg) = 0.278 / 0.5 = 0.556 m
Flashcards
Quick quiz
Q1.What is molarity?
Q2.If you dilute 1 L of 1 M HCl to 2 L, the new molarity is…
Q3.Why use molality for colligative properties instead of molarity?
Q4.Percent by mass formula?
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Common mistakes
Confusing molarity and molality. — Correct: Molarity uses solution volume; molality uses solvent mass.
Using mL instead of L in molarity calculation. — Correct: Convert mL to L first: 500 mL = 0.5 L.
Forgetting to multiply percent by 100. — Correct: Always express percent as 0–100, not 0–1.
Assuming percent by mass and percent by volume are the same. — Correct: They differ unless solute density = 1 g/mL.
FAQ
What is solution concentration?
Concentration describes the amount of solute per unit of solution (molarity, mol/L), solvent (molality, mol/kg), or total mass (percent).
Which concentration unit is most common in the lab?
Molarity (mol/L), because volumes are easy to measure with graduated cylinders and flasks.
Why does molarity change with temperature but molality doesn't?
Molarity depends on solution volume, which expands with heating. Molality depends on mass, which is constant.
How do you convert from molarity to molality?
Need the solution density: m = M × d / (1000 + M × M_solute), where d is density in g/mL and M_solute is molar mass.