Question

# How many grams of solid NaOH must be added to 100 ml of a buffer solution which is 0.1 M each with respect to Acid HA and salt Na+A− to make the pH of the solution 5.5? ( Given pka(HA)=5)

A
2.08 ×101
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B
2.10 ×101
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C
2.01 ×102
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D
None of these
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Solution

## The correct option is D 2.08 ×10−1pH=pka+log[A−][HA]5.5=5+log[A−][HA]log[A−][HA]=0.5[A−][HA]=3.16 ....(1)Let W moles of NaOH are added[NaOH]=wmol100ml×1000ml1L=10wMNaOH+HA=NaOH+H2O10w molar NaOH will combine with10w molar HA to form 10wMA[A−]=(0.1+10w)M[HA]=(0.1−10w)M[A−][HA]=0.1+10w0.1−10w ...(2)(1)=(2)0.1+10w0.1−10w=3.160.1+10w=3.16(0.1−10w)0.1+10w=0.316−316w71.6w=0.216w=0.00519 moleMolecular weight of NaOH=40g/mol, mass of NaOH=0.00519×40=2.08×10−1gAns. is option (A).

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