A buffer is created by combining 160 mL of 0.20 M HCHO2 with 75 mL of 0.25 M NaOH. Determine
the pH of the buffer.
Someone help me to figure out this question..

Question

A buffer is created by combining 160 mL of 0.20 M HCHO2 with 75 mL of 0.25 M NaOH. Determine 
the pH of the buffer.
Someone help me to figure out this question..

rogue · Answer

$$n = MV$$$$n_{HCHO_2} = .160 L * 0.20 M = 0.032 mol $$$$n_{NaOH} = 0.075 mL * 0.25 M = 0.01875mol$$$$OH^- + HCHO_2 \rightarrow  H_2O + CHO _{2}^{-}$$ 0.01875 moles of formic acid will be converted to formate ions.
0.01875 mol HCHO2 ---> 0.01875 mol CHO2-
0.032 mol - 0.01875 mol = 0.01325 mol HCHO2

$$pH = pK_a - \log_{10} \frac {[HA]}{[A^{-}]}$$You can use moles of HA/A- instead of their concentrations since their molarity and mole ratio are the same. The pKa of formic acid is 3.77.$$pH = 3.77 - \log_{10} \frac {0.01325 mol}{0.01875 mol}$$$$pH = 3.92$$

rogue · Answer

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