Structure 1.4.5 — Molar Concentration, Amount of Solute, and Volume of Solution

 Molar Concentration

Molar concentration, also known as molarity, refers to the number of moles of solute dissolved in one cubic decimetre (1 dm³) of solution.

It provides a quantitative measure of the amount of substance present in a given volume of solution.

Represented as: [X], where X is the chemical formula or symbol of the solute.

Example: [NaCl] = 0.25 mol dm⁻³ means 0.25 mol of NaCl is dissolved per dm³ of solution.

 Formula (from the data booklet):

\( n = C \times V \)

• \( n \) = amount of solute in moles (mol)
• \( C \) = molar concentration in mol dm⁻³
• \( V \) = volume of solution in dm³

Rearranged Forms:

• \( C = \frac{n}{V} \)
• \( V = \frac{n}{C} \)

Unit Conversions:

• Volume: cm³ → dm³ → divide by 1000
• Mass concentration: g dm⁻³
• To convert molar concentration to mass concentration:
  \( C_{\text{g dm}^{-3}} = C_{\text{mol dm}^{-3}} \times M \) (M = molar mass in g mol⁻¹)
• To convert mass concentration to molar concentration:
  \( C_{\text{mol dm}^{-3}} = \frac{C_{\text{g dm}^{-3}}}{M} \)

Additional Notes:

• Ensure all volumes are expressed in dm³ before substituting into the formula.
• Concentration is a temperature-dependent property – use standard conditions unless stated otherwise.
• Use a calibrated volumetric flask to prepare solutions of known concentration accurately.
• Label all concentrations clearly using square brackets and correct units (e.g., [HCl] = 2.0 mol dm⁻³).