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According to the Arrhenius concept, if HNO3 were dissolved in water, it would act as
17. Acid and Base Equilibrium
Acids Introduction
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A 230.0 mL buffer solution initially contains 3.0×10⁻² M of HCHO₂ and 3.0×10⁻² M of NaCHO₂. In order to adjust the buffer pH to 4.20, what mass of NaOH should you add?
A
0.023 g
B
0.184 g
C
0.046 g
D
0.092 g
Verified step by step guidance1
Identify the components of the buffer solution: HCHO₂ (formic acid) and NaCHO₂ (sodium formate). The buffer solution resists changes in pH upon the addition of small amounts of acid or base.
Use the Henderson-Hasselbalch equation to relate the pH of the buffer to the concentrations of the acid and its conjugate base: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \). Here, \( \text{pK}_a \) is the negative logarithm of the acid dissociation constant \( K_a \) for formic acid.
Calculate the \( \text{pK}_a \) of formic acid using its \( K_a \) value. For formic acid, \( K_a \approx 1.8 \times 10^{-4} \), so \( \text{pK}_a = -\log(1.8 \times 10^{-4}) \).
Set up the Henderson-Hasselbalch equation with the desired pH of 4.20 and solve for the ratio \( \frac{[\text{A}^-]}{[\text{HA}]} \). Substitute \( \text{pH} = 4.20 \) and the calculated \( \text{pK}_a \) into the equation.
Determine the change in concentration of \( \text{A}^- \) (sodium formate) needed to achieve the desired pH. Calculate the moles of NaOH required to convert the necessary amount of HCHO₂ to NaCHO₂, and then convert this amount to mass using the molar mass of NaOH (approximately 40.00 g/mol).
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