Brass is an alloy of copper and zinc. Typical copper concentrations vary from 50% to 85%, depending upon the properties needed in the alloy. There may be small amounts of other metals present. A student found a method to determine the percentage of copper in a sample of brass. A known mass of brass powder is reacted with excess concentrated nitric acid. Both the copper and the zinc and any other metals present are oxidised into aqueous ions by the nitric acid. The amount of Cu²⁺(aq) ions present can be determined by a titration technique. step 1 Use a weighing boat to accurately weigh by difference approximately 2 g of brass powder and place the brass into a small glass beaker. step 2 In a fume cupboard add approximately 20 cm³ of concentrated nitric acid to the brass in the beaker. Allow the brass to completely react to form solution A. The equation for the reaction is shown. Cu(s) + 4HNO₃(aq) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l) step 3 Dilute all of solution A to form exactly 250.0 cm³ of solution B. step 4 Place 25.00 cm³ of solution B into a conical flask. step 5 Use a dropping pipette to add aqueous sodium carbonate, Na₂CO₃(aq), to solution B in the conical flask until there is no more acid present. step 6 Add approximately 20cm³ of aqueous potassium iodide, KI(aq), to the conical flask. A white precipitate forms as well as a brown solution of aqueous iodine, I₂(aq). step 7 Fill a burette with 0.100 moldm⁻³ sodium thiosulfate solution, Na₂S₂O₃(aq), so it is ready for the titration in step 8. step 8 Carry out a titration of the aqueous iodine produced in the conical flask against the 0.100 mol dm⁻³ Na₂S₂O₃(aq).