25 ℃时，a mol·L一元酸HA与b mol·L-1NaOH等体积混合后，pH为7，则下列关系一定正确的是
A．a=b
B．a＞b
C．c(A-)=c(Na+)
D．c(A-)＜c(Na+)
答案：C
假如酸是强酸，则酸和碱恰好反应显中性，其物质的量应该相等，,即体积相等时，a=b；假如酸为弱酸，pH=7，酸应该过量，体积相等时，a＞b；故，A、B错；要显中性，c（H+）=c（OH-），根据电荷守恒，则c（Na+）=c（A-），D错；答案选C。
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