① When two hydrochloric acid solutions with pH = 3 and pH=5 are mixed in equal volume, then C (h+) = (0.001v+0.0001v)/2v ≈ 0.0005 mol/L Note: when the concentration of one is less than or equal to one percent of the other, the small concentration can be ignored. So pH =-LGC (H+) =-LG0.0005 = 4-LG5 = 4-0.7 = 3.3 ② and ④ are the same as ①, 3.3 respectively; 2.3. When two hydrochloric acid solutions with pH = 3 and pH=4 are mixed in equal volume, C (h+) = (0.001v+0.0001v)/2v = 0.00055 (at this time, it is more than 1% and cannot be ignored), so pH =-lgc (.