What is the pH of a buffer that is 0.120 M in formic acid (HCO2H) and 0.080 M in potassium formate (HCO2K)? For formic acid, Ka = 1.8 10–4.

Question

What is the pH of a buffer that is 0.120 M in formic acid (HCO2H) and 0.080 M in potassium formate (HCO2K)? For formic acid, Ka = 1.8  10–4.

BlackTom AI · Accepted Answer

To determine the pH of a buffer solution containing a weak acid and its conjugate base, use the Henderson–Hasselbalch equation: pH = pKa + log([A-]/[HA]).

First, identify the acid/base pair: formic acid (HA = HCO2H) and its conjugate base (A- = HCO2K, i.e., formate). The given concentrations are [HA] = 0.120 M and [A-] = 0.080 M.

Next, calculate pKa from Ka. For formic acid, Ka = 1.8 × 10^-4. pKa = −log10(Ka) = −log10(1.8 × 10^-4).
− log10(1.8) ≈ −0.2553, and − log10(10^-4) = 4, so pKa ≈ 4 − 0.2553 = 3.7447. Round as needed; we’ll use pKa ≈ 3.745.

Now compute the ratio [A-]/[HA] = 0.080 / 0.120 = 2/3 ≈ 0.6667. Take the common logarithm (log10) of this ratio: log10(0.6667) ≈ −0.1761.

Plug these into the Henderson–Hasselbalch equation: pH ≈ 3.745 + (−0.1761) ≈ 3.5689. Rounding to two decimal places gives pH ≈ 3.57.

Common pitfalls to watch for: mixing up the ratio (A- vs HA) or using pH = pKa − log([HA]/[A-]) which numerically yields the same result, but can be confusing if you don’t keep track of the numerator and denominator. Also, ensure you’re using the pKa value derived from the given Ka; small changes in Ka yield small changes in pKa that typically don’t swing the result dramatically for this buffer.

If you encounter options that differ slightly, the key is applying pH = pKa + log([base]/[acid]) and plugging in the correct concentrations, which in this case leads to a pH around 3.57. If a choice is far from 3.57, it’s likely due to arithmetic error in the log calculation or in the ratio.

Chem 1C General Chemis... Self Assessment Quiz 17

What is the pH of a buffer that is 0.120 M in formic acid (HCO2H) and 0.080 M in potassium formate (HCO2K)? For formic acid, Ka = 1.8 10–4.

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