A buffer is formed by mixing 100 mL of 0.2 M HClO2 and 200 mL of 0.7 M KClO2. What volume of 0.2 M KOH can be added before the buffer capacity is reached?
To determine the volume of 0.2 M KOH that can be added before the buffer capacity is reached, we need to calculate the initial pH of the buffer solution and identify the components that are responsible for maintaining the pH.
1. Calculate moles of HClO2:
moles of HClO2 = concentration (M) x volume (L)
= 0.2 M x 0.1 L (100 mL converted to L)
= 0.02 moles
2. Calculate moles of KClO2:
moles of KClO2 = concentration (M) x volume (L)
= 0.7 M x 0.2 L (200 mL converted to L)
= 0.14 moles
3. Determine the limiting reagent:
The limiting reagent is the component that is completely consumed in the reaction and determines the maximum amount of product that can be formed. In this case, HClO2 and KClO2 react in a 1:1 ratio, so the limiting reagent is the one with fewer moles, which is HClO2. Therefore, HClO2 will fully react with KOH.
4. Calculate the moles of HClO2 that react with KOH:
moles of HClO2 reacted = moles of HClO2 initially = 0.02 moles
5. Calculate the new concentrations of HClO2 and KClO2 after the reaction:
moles of HClO2 remaining = moles of HClO2 initially - moles of HClO2 reacted
= 0.02 moles - 0.02 moles
= 0 moles (HClO2 is completely reacted)
moles of KClO2 remaining = moles of KClO2 initially - moles of HClO2 reacted
= 0.14 moles - 0.02 moles
= 0.12 moles
volume of solution after reaction = volume of HClO2 + volume of KClO2
= 0.1 L + 0.2 L
= 0.3 L
concentration of HClO2 after reaction = 0 moles / 0.3 L
= 0 M
concentration of KClO2 after reaction = 0.12 moles / 0.3 L
= 0.4 M
6. Calculate the pOH of the solution:
pOH = -log10[OH-]
Since KOH is a strong base and fully dissociates, the concentration of OH- is equal to the concentration of KOH, which is 0.2 M.
pOH = -log10(0.2)
= 0.70
7. Calculate the pH of the solution:
pH + pOH = 14
pH = 14 - pOH
= 14 - 0.70
= 13.30
The initial pH of the buffer solution is 13.30. The buffer capacity is typically considered to be the pH range within one unit of the initial pH. Therefore, we want to find the volume of 0.2 M KOH that can be added before the pH changes by one unit.
To find this volume, we can calculate the moles of KOH needed to react with the remaining concentration of KClO2 and then divide by the concentration of KOH.
8. Calculate the moles of KOH:
moles of KOH = concentration (M) x volume (L)
= 0.2 M x volume (L)
Since the reaction between KOH and KClO2 occurs in a 1:1 ratio, the moles of KOH needed is equal to the moles of KClO2 remaining, which is 0.12 moles.
0.2 M x volume (L) = 0.12 moles
Solve for volume:
volume (L) = 0.12 moles / 0.2 M
Therefore, the volume of 0.2 M KOH that can be added before the buffer capacity is reached is 0.6 L or 600 mL.