在25℃时，将0.80 g氢氧化钠固体溶于水，配成200 mL溶液。取该溶液25.0 mL，加水稀释至500 mL。稀释后溶液的pH值最接近多少？（已知NaOH的摩尔质量为40 g/mol，忽略体积变化，25℃时Kw=1.0×10^-14）
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Correct answer
B. 12
Principle or equation
先求原溶液浓度，再计算稀释后浓度，由Kw求c(H+)，再算pH。
Why this answer is correct
n(NaOH)=0.80g/40g/mol=0.020mol，原溶液c=0.020mol/0.200L=0.10mol/L。取25.0mL含n=0.10mol/L×0.025L=0.0025mol，稀释至0.500L后c=0.0025mol/0.500L=0.0050mol/L。pOH=-lg(0.0050)=2.3，pH=14-2.3=11.7，最接近12。
Example
若取10mL 0.10mol/L NaOH稀释至100mL，浓度变为0.010mol/L，pH=12。
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