Answer:
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
