If the pKa of HCHO₂ is 3.74 and the pH of an HCHO₂/NaCHO₂ solution is 3.11, then the statement which is correct is [HCHO₂] > [NaCHO₂].
Buffer solution is a solution which is a mixture of weak acid and its conjugate base or vice a versa. It shows a very little change in pH when strong acid or base is added in it.
Since NaCHO₂ is a basic salt of formic acid and it acts as a buffer solution. The pH of buffer solution is calculated by the formula
pH=pKa+log([base]/[acid])
We know that HCHO₂ is an acid, where NaCHO₂ is a base. Plug all values in the formula
3.11=3.74+log([NaCHO₂]/[HCHO₂])
3.11-3.74=log([NaCHO₂]/[HCHO₂])
log([NaCHO₂]/[HCHO₂])=-0.63
Take antilog on both side
[NaCHO₂]/[HCHO₂]=10^(-0.63)
[NaCHO₂]/[HCHO₂]=0.234423
Multiply both side by [HCHO₂]
([NaCHO₂]/[HCHO₂])×[HCHO₂]=0.234423×[HCHO₂]
[NaCHO₂]=0.234423×[HCHO₂]
From the above calculation, it is observed that concentration of NaCHO₂ is less than the concentration of HCHO₂. Hence, [HCHO₂] > [NaCHO₂].
Therefore, if the pKa of HCHO₂ is 3.74 and the pH of an HCHO₂/NaCHO₂ solution is 3.11, then the statement which is correct is [HCHO₂] > [NaCHO₂]. Hence, 'option d' is correct.
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