A) \[{{H}_{3}}P{{O}_{4}},{{H}_{2}}PO_{4}^{-}\]
B) \[{{H}_{2}}PO_{4}^{-},PO_{4}^{3-}\]
C) \[HPO_{4}^{2-},PO_{4}^{3-}\]
D) \[{{H}_{3}}P{{O}_{4}},\,PO_{4}^{3-}\]
Correct Answer: B
Solution :
\[{{H}_{2}}PO_{4}^{-}+{{H}_{2}}OHPO_{4}^{2-}+\underset{acid}{\mathop{{{H}_{3}}{{O}^{+}}}}\,\] \[{{H}_{2}}PO_{4}^{-}+{{H}_{2}}O{{H}_{3}}P{{O}_{4}}+\underset{base}{\mathop{O{{H}^{-}}}}\,\] Similarly, \[HPO_{4}^{2-}+{{H}_{2}}OPO_{4}^{3-}+\underset{acid}{\mathop{{{H}_{3}}{{O}^{+}}}}\,\] \[HPO_{4}^{2-}+{{H}_{2}}O{{H}_{2}}PO_{4}^{-}+\underset{base}{\mathop{O{{H}^{-}}}}\,\] \[\therefore \,\,{{H}_{2}}PO_{4}^{-}\] and \[HPO_{4}^{2-}\] are amphiprotic anions which can undergo ionisation to form \[{{H}^{+}}\] ions and can undergo hydrolysis to form \[O{{H}^{-}}\]ions. Thus, for preparing a buffer solution of \[pH=7.0\], the buffer system of weak acid \[{{H}_{2}}PO_{4}^{-}\] and its conjugated base \[HO_{4}^{2-}\]are used.
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