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Problem: During an acid-base titration, 25.0 mL of a hydrochloric acid (HCl) solution of unknown concentration is completely neutralised by 15.0 mL of a 0.10 M sodium hydroxide (NaOH) solution. Calculate the concentration of the HCl solution. Step 1: Write down the balanced chemical equation for the neutralisation reaction to understand the mole ratio between the acid and the base. The equation is HCl + NaOH -> NaCl + H2O. From this, we see that 1 mole of HCl reacts with exactly 1 mole of NaOH. Step 2: List out all the given values from the problem statement. Volume of acid (Va) = 25.0 mL, Volume of base (Vb) = 15.0 mL, Concentration of base (Mb) = 0.10 M, and Concentration of acid (Ma) = unknown. Step 3: Use the titration formula derived from the mole ratio, which is (Ma * Va) / (Na) = (Mb * Vb) / (Nb), where Na and Nb are the stoichiometric coefficients from the balanced equation (both are 1 here). Thus, Ma * Va = Mb * Vb. Step 4: Rearrange the formula to solve for the unknown concentration of the acid: Ma = (Mb * Vb) / Va. Step 5: Substitute the known values into the equation: Ma = (0.10 M * 15.0 mL) / 25.0 mL. Doing the math, Ma = 1.5 / 25.0 = 0.06 M. Therefore, the concentration of the hydrochloric acid solution is 0.06 M.