pKa = 3.74
Use the Henderson-Hasselbalch equation.
pH = pKa + log base/acid
4.00 = 3.74 + log (HCOONa)/0.1
Solve for (HCOONa).
calculate the initial concentration of HCOONa you would add to 0.10M HCOOH to make a buffer with a pH of 4.00 [Ka (HCOOH) = 1.8 x 10^-4]
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