How to Calculate the Molarity of a Buffer Solution

What is the process to determine the moles of HCl needed to change the pH of a buffer solution by 0.18 units?


  • 0.063 mol
  • 1.0 mol
  • 0.025 mol
  • 0.082 mol
  • 0.50 mol

Final answer:

To change the pH of the given buffer solution by 0.18 units, we would need 0.20 moles of HCl, which is not given among the provided options.

The subject of this question is about buffer solutions and how they regulate pH changes when a strong acid or base is added. Buffer solutions are crucial in maintaining a stable pH level despite the addition of acids or bases.

Understanding the Calculation:

When HCl (a strong acid) is added to the buffer solution, the H2PO4- ion in the buffer solution neutralizes it by absorbing the H+ ions from HCl. To determine the moles of HCl needed to change the pH by 0.18 units, we can utilize the Henderson-Hasselbalch equation.

Henderson-Hasselbalch Equation:

pH = pKa + log ([A-]/[HA])

From the problem, we have [H2PO4-] = 1 M and [OH-] = [NaOH] = 1 M. The pKa2 value is 7.20. When aiming to increase the pH by 0.18 units, we can calculate the new pH.

Calculation Steps:

  1. Determine the new pH: 7.20 + 0.18 = 7.38
  2. Use the Henderson-Hasselbalch equation to find [OH-] concentration
  3. Solve for [OH-]: 10^(7.38 - 7.20) M = 1.5
  4. To achieve the desired pH change, decrease [OH-] by 0.5 M
  5. Since one molecule of HCl neutralizes one molecule of OH-, the required moles of HCl would be 0.20 moles

Therefore, when 0.20 moles of HCl are added to the buffer solution, the pH can change by 0.18 units. This calculation highlights the precise nature of buffer solutions in maintaining pH stability.

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