5 Biological and environmental importance (preview)

• Dissolved nutrients and gases (O₂, CO₂) can be carried in the blood, sap and sea water.
• Chemical reactions inside cells take place in solution (Module 5).
• The same solvent power makes water easily polluted: soluble pollutants (fertiliser nitrates, heavy-metal ions) spread through the aquatic ecosystem (Module 8).

Problem. 11.7 g of sodium chloride (NaCl = 58.5 g/mol) dissolves in water to make 500 mL of solution. Find (a) moles of NaCl, (b) its molar concentration, (c) the concentration of each ion, (d) the number of positive ions (Avogadro's number = 6.02 × 10²³).

Solution. (a) n = 11.7 ÷ 58.5 = 0.2 mol. (b) C = n/V = 0.2 ÷ 0.5 = 0.4 mol/L. (c) NaCl → Na⁺ + Cl⁻ (1 : 1), so [Na⁺] = [Cl⁻] = 0.4 mol/L. (d) 0.2 × 6.02 × 10²³ = 1.2 × 10²³ Na⁺ ions.