Laura_2707 wrote:Hi

Im just wondering about question 16.

Would you be able to talk me through it in a bit more detail? I don't understand the solution given!

How do you arrive at [A-]/[HA] = 10-3 ?

Thank you!

Masters Series GAMSAT Question 16

The question stem mentions pKa and pH so we need to find a way that they relate. The passage provides that relationship by describing the Henderson-Hasselbach equation:

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

The question stem describes this relationship: [A-]/[HA] = 1/10^3 = 10^-3. We are also told that the pKa is 5.

Substituting:

pH = pKa + log([A-]/ [AH]) = 5 + log (10^-3) = 5 - 3 = 2