1.

A weak acid HA after teratment with 12 mL of `0.1M` strong base BOH has a pH of 5. At the end point, the volume of same base required is `26.6mL.K_(a)` of acid is:A. `1.8xx10^(-5)`B. `8xx10^(-6)`C. `1.8xx10^(-8)`D. `8xx10^(-5)`

Answer» m.moles of `HA` taken =`27xx0.1=2.7`
`{:(,HA,+,OH^(-),rarr,A^(-),+,H_(2)O),(t=0,2.7,,1.2,,,,),(t_(eq),1.5,,-,,1.2,,):}`
`pH=pK_(a)+log(([A^(-)])/([HA]))rArr 5=pK_(a)+log((1.2)/(1.5))=pK_(a)+log((4)/(5))`
`:.pK_(a)=5.1 rArr K_(a)=8x10^(-6).`


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