To adjust the pH of a 250.0-mL buffer solution initially containing 0.025 mol of HCHO2 and 0.025 mol of NaCHO2 to 4.10, should you add NaOH or HCl, and what mass of the correct reagent should you add?

Verified step by step guidance

Identify the components of the buffer solution: HCHO2 (formic acid) and NaCHO2 (sodium formate).

Use the Henderson-Hasselbalch equation: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where \( \text{A}^- \) is the conjugate base (formate ion) and \( \text{HA} \) is the weak acid (formic acid).

Calculate the initial pH of the buffer using the given concentrations and the \( \text{pK}_a \) of formic acid (3.74).

Determine whether the pH needs to be increased or decreased to reach 4.10. If the pH needs to be increased, add NaOH; if it needs to be decreased, add HCl.

Calculate the amount of NaOH or HCl needed to adjust the pH to 4.10 using the buffer capacity and the change in moles of \( \text{A}^- \) or \( \text{HA} \) required. Convert this amount to mass using the molar mass of the reagent.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Buffer Solutions

Buffer solutions are mixtures that resist changes in pH when small amounts of acid or base are added. They typically consist of a weak acid and its conjugate base, or a weak base and its conjugate acid. In this case, the buffer is composed of formic acid (HCHO2) and sodium formate (NaCHO2), which help maintain the pH around a desired value.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a buffer solution to the concentration of its acid and conjugate base. It is expressed as pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the base and [HA] is the concentration of the acid. This equation is essential for determining how to adjust the pH by adding either an acid or a base.

Stoichiometry of Acid-Base Reactions

Stoichiometry involves the calculation of reactants and products in chemical reactions. In the context of adjusting pH, it is important to determine the amount of acid (HCl) or base (NaOH) needed to achieve the desired pH. This requires understanding the molar relationships and the neutralization reactions that occur when these substances are added to the buffer solution.

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Acid-Base Reaction

Related Practice

Textbook Question

A 1.0-L buffer solution initially contains 0.25 mol of NH₃ and 0.25 mol of NH₄Cl. In order to adjust the buffer pH to 8.75, should you add NaOH or HCl to the buffer mixture? What mass of the correct reagent should you add?

Textbook Question

In analytical chemistry, bases used for titrations must often be standardized; that is, their concentration must be precisely determined. Standardization of sodium hydroxide solutions can be accomplished by titrating potassium hydrogen phthalate (KHC₈H₄O₄), also known as KHP, with the NaOH solution to be standardized. b. The titration of 0.5527 g of KHP required 25.87 mL of an NaOH solution to reach the equivalence point. What is the concentration of the NaOH solution?

Textbook Question

A 5.55-g sample of a weak acid with Ka = 1.3⨉10^-4 was combined with 5.00 mL of 6.00 M NaOH, and the resulting solution was diluted to 750.0 mL. The measured pH of the solution was 4.25. What is the molar mass of the weak acid?