You are asked to prepare a buffer solution of pH 6.5 havingbuffer capacity of 0.

Question

You are asked to prepare a buffer solution of pH 6.5 havingbuffer capacity of 0.1 using Na2HPO4 (salt)and NaH2PO4 (acid). Which of followingcombination of buffer species can be used to make such a buffer?(pKa for NaH2PO4 (acid) ionizes toNa2HPO4+H+ is 7.21) (The answer is Salt=0.052 M, acid = 0.265, can you pleaseexpain what formula to use and how to work the proble, (simplicitywill be appreciated) Thanks) You are asked to prepare a buffer solution of pH 6.5 havingbuffer capacity of 0.1 using Na2HPO4 (salt)and NaH2PO4 (acid). Which of followingcombination of buffer species can be used to make such a buffer?(pKa for NaH2PO4 (acid) ionizes toNa2HPO4+H+ is 7.21) (The answer is Salt=0.052 M, acid = 0.265, can you pleaseexpain what formula to use and how to work the proble, (simplicitywill be appreciated) Thanks)

Explanation / Answer

We Know that :       According toHenderson's-Hasselbalch equation :      pH = pKa + log[salt] / [Acid]      6.5 = 7.21 + log [salt ] / [Acid]        [salt] /[Acid] = 10-0.71                                =  0.194         [ salt ] +[Acid] = 0.317 by using the buffer capacity         By solving the twoequations we get         The concentrationof salt = 0.052 M           concentration of Acid = 0.265 M                                =  0.194         [ salt ] +[Acid] = 0.317 by using the buffer capacity         By solving the twoequations we get         The concentrationof salt = 0.052 M           concentration of Acid = 0.265 M

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