1.

(a) How can you prepare a buffer solution of pH 9. you are provided with 0.1 M NH_4 OH solutionand ammonium chloride crystals. (Given : pK_(b) for NH_4 OH is 4.7 at 25^@ C.

Answer»

SOLUTION :(a) `POH =pK_(b)+log.(["salt"])/(["base"])`
We KNOW that
`pH+pOH=14 `
`therefore 9+pOH=14`
`RARR pOH=14-9=5`
`5=4.7+log.([NH_4 CI])/([NH_4 OH])`
`0.3 =log.([NH_4 CI])/(0.1)`
`([NH_4 CI])/([0.1])` = antilog of `(0.3)`
`[NH_4 CI]=0.1 M xx 1.995`
`0.1995 M`
`0.2 M`
Amount of `NH_4 CI` required to prepare 1 litre 02 M solution
= Strength of `NH_4 CI xx` molar mass of `NH_4 CI`
`=0.2 xx 53.5`
`10.70g`
10.70 g ammonium chloride is dissolved in water and the solution is made up to one litre of the given `NH_4 CI` solution and the prepared `NH_4 CI` solution will give a buffer solution with required pH VALUE `(pH=9)`.


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