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Chemistry?? Preparing a buffer??

Preparing a Buffer

Suppose you are asked to prepare a buffer solu9on to maintain a pH = 4.30 and you have the following acids: HSO4-, CH3COOH and HCN.

a. What combination of acid and conjugate base would you choose?

asked Mar 18, 2015 in CHEMISTRY by heather Apprentice

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1 Answer

Step 1 :

Buffer pH value : $pH=pK_{a}+log\frac{\left [A^{^{-} \right ]}}\left \left [{HA} \right \right ]$ .

Where $K_{a}$ is the strength of an acid.

Step 2 :

Here the acids are $HSO_{4}^{-}$ , $CH_{3}COOH$ and $HCN$ .

Buffer pH value = 4.30 .

For Acetic acid,

$4.30=4.75+log\frac{\left [CH_{3}COO^{^{-} \right ]}}\left \left [{CH_{3}COOH} \right \right ]$

$4.30-4.75=log\frac{\left [CH_{3}COO^{^{-} \right ]}}\left \left [{CH_{3}COOH} \right \right ]$

$-0.45=log\frac{\left [CH_{3}COO^{^{-} \right ]}}\left \left [{CH_{3}COOH} \right \right ]$ .

For Sulfuric acid,

$4.30=1.92+log\frac{\left [SO_{4}^{^{2-} \right ]}}\left \left [{HSO_{4}^_{-}} \right \right ]$

$4.30-1.92=log\frac{\left [SO_{4}^{^{2-} \right ]}}\left \left [{HSO_{4}^_{-}} \right \right ]$

$3.1=log\frac{\left [SO_{4}^{^{2-} \right ]}}\left \left [{HSO_{4}^_{-}} \right \right ]$ .

For Hydrocyanic acid,

$4.30=9.21+log\frac{\left [CN^{^{-} \right ]}}\left \left [{HCN} \right \right ]$

$4.30-9.21=log\frac{\left [CN^{^{-} \right ]}}\left \left [{HCN} \right \right ]$

$-4.91=log\frac{\left [CN^{^{-} \right ]}}\left \left [{HCN} \right \right ]$ .

Conjugate acids(cations) of strong bases are ineffective bases.

If acid strength increases, then conjugate base strength decreases.

Observe the above calculations.

I will choose the combination of Sulfuric acid, because the ratio is greater than 1 comparing with other two combinations.

Solution :

The combination of acid and conjugate base of sulfuric acid would be better.

answered Mar 18, 2015 by lilly Expert
edited Mar 18, 2015 by lilly

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