M = (% × d × 10) / MW
Stock chemicals are often labeled with percent concentration and density. To find molarity: M = (% × density × 10) / molar mass. The factor of 10 converts from g/100mL to g/L. Examples: concentrated HCl (37%, d=1.19) → M = 37×1.19×10/36.46 = 12.1 M. Concentrated H₂SO₄ (96%, d=1.84) → M = 96×1.84×10/98.08 = 18.0 M. This conversion is essential when preparing dilute solutions from concentrated stocks using C₁V₁ = C₂V₂. Always remember: concentrated acids should be added to water, never the reverse (to safely dissipate the heat of dilution). Label all prepared solutions with the final concentration, date, and preparer's initials.