moles HCOOH = M x L = ?
moles HCOONa = M x L = ?
(HCOOH) = moles/total volume.
(HCOONa) = moles/total volume.
Ka = (H^+)(HCOO^-)/(HCOOH)
Solve for (H^+).
(HCOO^-) from above.
(HCOOH) from above.
Convert (H^+) to pH.
This is a buffered solution and you can use the Henderson-Hasselbalch equation if you wish. It is a little simpler because you may use moles/V and not actually determine the concns of each (since the volume cancels).
Calculate the pH of a solution that results from the mixture of 50.0 mL of 0.15 M HCOOH with 75.0 mL of 0.13 M HCOONa. Now, don't forget these solutions are being mixed and you haven't accounted for that yet in the concentrations! Ka for HCOOH is 1.8 x 10-4.
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