Q3. In NCERT’s gold process, gold dissolves in cyanide solution with oxygen and water, and zinc gives back the metal. Write balanced ionic equations for the two steps and say what is oxidised in each.
4Au + 8CN− + O2 + 2H2O → 4[Au(CN)2]− + 4OH−: oxygen oxidises gold from 0 to +1. 2[Au(CN)2]− + Zn → [Zn(CN)4]2− + 2Au: zinc is oxidised to +2 and reduces gold(I) to the metal (process from NCERT page 136; equations reasoned).
Q4. 25.0 mL of tap water needs 6.00 mL of 0.0125 M Na2EDTA to complex all its Ca2+ and Mg2+. Find their total concentration.
EDTA used = 0.0125 × 0.00600 = 7.50 × 10−5 mol, which equals the moles of Ca2+ + Mg2+ (1 : 1). Concentration = 7.50 × 10−5 ÷ 0.0250 = 3.00 × 10−3 mol L−1 (reasoned).