A nitrous acid buffer solution contains 0.85 M nitrous acid (HNO2) and 0.61 M of its conjugate base (NO2-). If a chemist adds 0.15 mol of hydrobromic acid (HBr), a strong acid, to 0.50 L of the buffer solution, what will be the final pH of the solution? (The pKa of HNO2 is 3.25. Assume the volume of the added HBr is negligible.)

Question

Chemistry

Harper Shelton

6 May, 17:24

A nitrous acid buffer solution contains 0.85 M nitrous acid (HNO2) and 0.61 M of its conjugate base (NO2-). If a chemist adds 0.15 mol of hydrobromic acid (HBr), a strong acid, to 0.50 L of the buffer solution, what will be the final pH of the solution? (The pKa of HNO2 is 3.25. Assume the volume of the added HBr is negligible.)

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Christina Green · Answer 1

2.68

Explanation:

At the solution, the number of moles of each substance (acid and conjugate base) is the volume multiplied the concentration

nHNO₂ = 0.50 L * 0.85 mol/L = 0.425 mol

nNO₂⁻ = 0.50 L * 0.61 mol/L = 0.305 mol

At the buffer, the substances are in equilibrium. When HBr is added, it dissociantes in H⁺ and Br⁻, and the H⁺ will react with NO₂⁻ to form more HNO₂. So, NO₂⁻ will be consumed and HNO₂ will be formed at a 1:1:1 reaction:

nH⁺ = nHBr = 0.15 mol

nNO₂⁻ = 0.305 - 0.15 = 0.155 mol

nHNO₂ = 0.425 + 0.15 = 0.575 mol

The pH of a buffer can be calculated by the Handerson-Halsebach equation:

pH = pKa + log[A⁻]/[HA]

Where [A⁻] is the concentration of the conjugate base, and [HA], the concentration of the acid. Because the volume is the same, it can be used the number of moles:

pH = 3.25 + log (0.155/0.575)

pH = 2.68