【单选题】This pair of shoes is too small for me. Would you like to show me pair? [ ]
【简答题】听长对话,选择正确答案。对话读两遍。 1. What does the woman want to buy? A. A pair of jeans B. A pair of running shoes C. A pair of trousers 2. How much is the black pair? A. 25 dollars B. 45 dollars C. 50 dollars 3. W...
【单选题】My brother has five _____ in his room. [ ]
【单选题】欲配制 PH=3 的缓冲溶液,下列缓冲对最合适的是( )
A.
HCOOH — HCOOK ( Ka=1.77X10 -4
B.
H a c — NaAc ( Ka=1.75X10 -5
C.
NH3-- NH4Cl ( Kb=1.8X10 -5
D.
HCN — NaCN(Ka=6.2X10 -10
【单选题】欲配制pH=6.50的缓冲溶液,用下列何种酸最好
A.
(CH3)2AsO2H)(Ka=6.40×10-7)
B.
CICH2COOH (Ka=1.40X10-3)
【单选题】欲配制 pH=6.50 的缓冲溶液,最好选用 _____
A.
(CH 3 ) 2 AsO 2 H ( K a Θ =6.40×10 -7 )
B.
ClCH 2 COOH ( K a Θ =1.40×10 -3
C.
CH 3 COOH ( K a Θ =1.76×10 -5
D.
HCOOH ( K a Θ =1.77×10 -4
【单选题】欲配制 pH = 4~5 的缓冲溶液 , 最好选择:
A.
NH 3 + NH 4 Cl ( K b =1.8 × 10 -5 )
B.
KH 2 PO 4 +K 2 HPO 4 ( K a2 =6.30 × 10 -8 )
C.
NaAc+HAc ( K a =1.8 × 10 -5 )
D.
KHC 2 O 4 + H 2 C 2 O 4 ( K a =1.4 × 10 -3 )
【单选题】欲配制pH约为9的缓冲溶液,下列缓冲对选择哪一对最合适?()。
A.
NaH 2 PO 4 -Na 2 HPO 4 ; K a2 =6.23×10 -8
B.
HCN-NaCN ; K a =4.93×10 -10
C.
NH 3 -NH 4 Cl ; K b =1.76×10 -5
D.
HAc-NaAc ; K a =1.76×10 -5
【单选题】汽化器式燃油系统中 , 喷油嘴出口与浮子式内燃油液面高度的关系为
【单选题】欲配制pH=6.50的缓冲溶液,下列酸最好的是:( )
A.
(CH 3 ) 2 AsO 2 H (K a =6.40×10 -7 )
B.
ClCH 2 COOH (K a =1.40×10 -3 )
C.
CH 3 COOH (K a =1.76×10 -5 )
D.
HCOOH (K a =1.77×10 -4 )