H3PO4的pka1~pka3分别为2.12,7.20,12.40.当H3PO4溶液的PH=9.30时,溶液中的主要存在

用分布系数来计算
H3PO4的分布系数=[H+]^3 / {[H+]^3 +Ka1×[H+]^2+Ka1×Ka2×[H+] +Ka1×Ka2×Ka3}
=(10^-9.30)^3/{(10^-9.30)^3 +(10^-2.12)×(10^-9.30)^2+(10^-2.12)×(10^-7.20)×(10^-9.30)
+(10^-2.12)×(10^-7.20)×(10^-12.40)}
=(1.26×10^-28) / {(1.26×10^-28) +(1.90×10^-21 )+(2.40×10^-19)+ (1.90×10^-22 )}
=(1.26×10^-28) /{ 2.42×10^-19 }
=5.20×10^-10
H2PO4-的分布系数=Ka1×[H+]^2 / {[H+]^3 +Ka1×[H+]^2+Ka1×Ka2×[H+] +Ka1×Ka2×Ka3}
=(1.90×10^-21 )/{ 2.42×10^-19 }
=7.85×10^-3
HPO42-的分布系数=Ka1×Ka2×[H+] / {[H+]^3 +Ka1×[H+]^2+Ka1×Ka2×[H+] +Ka1×Ka2×Ka3}
=(2.40×10^-19)/{ 2.42×10^-19 }
=7.85×10^-3
=0.99
PO43-的分布系数=Ka1×Ka2×Ka3/ {[H+]^3 +Ka1×[H+]^2+Ka1×Ka2×[H+] +Ka1×Ka2×Ka3}
=(1.90×10^-22)/{ 2.42×10^-19 }
=7.85×10^-4
可见是答案是C

PH=9.3的时候二三级都有,9.3离7.2近,选C

PH是12.4以后才是（HPO4）2ˉ变成PO43ˉ为主所以是c