Two buffers, (X) and (Y) of pH 4.0 and 6.0 respectively are prepared from acid HA and the salt NaA. Both the buffers are 0.50M of HA. What would be the pH of the solution obtained by mixing equal volumes of the buffers ? KHA=1.0×10−5
A
8.4
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B
4.3
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C
5.7
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D
7.5
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Solution
The correct option is C 5.7 Henderson equation is given by pH=−log10Ka+log10[Salt][Acid] When pH=4 4=−log101.0×10−5+log10[Salt]0.5 4=5+log10[Salt]0.5 log10[Salt]0.5=−1 [Salt]=0.1×0.5=0.05 When pH=6, 6=−log10(1.0×10−5)+log10[Salt]0.5 6=5+log10[Salt]0.5 log10[Salt]0.5=1 [Salt]=10×0.5=5M Suppose V litre each of both buffers are mixed. The concentration of the salt in this solution will be: [Salt]=(0.05×V)+(5×V)2V=5.052M Concentration of [HA] in mixed buffer =(0.5×V)+(0.5×V)2V=0.5M pH=−log10(1.0×10−5)+log10[5.052]0.5 =5+0.7033=5.7033