氢氧根离子：[OH-] = (0.025 * 0.02 *2 + 0.15 * 0.02) ÷ 0.04
= 0.004 ÷ 0.04 = 0.1 (mol/L)
则：[H+] = Kw ÷ [OH-] = 10^-14 ÷ 0.1 = 10^-13 (mol/L)
所以：PH = 13