欲配制 pH=5.0 缓冲溶液用的一对物质是( )
A.
HAc( K a =1.8×10 -5 ) ~ NaAc
B.
NH 3 ·H 2 O ( K b =1.8×10 -5 ) ~ NH 4 Cl
C.
H 2 C 2 O 4 -Na H C 2 O 4 ( K a1 =5.6×10 -2 , K a2 =1.5×10 -4 )
D.
KH 2 PO 4 -Na 2 HPO 4 ( K a1 =7.5×10 -3 , K a2 =6.2×10 -8 , K a3 =5.0×10 -13 )