How many grams of solid KOH must be added to 100ml of a buffer solution which is 0.1M each with
respect to the weak acid HA and its potassium salt KA in order to make the pH of the solution 6.0?
You have 100 mL x 0.1M = 10 millimols HA and 10 mmols KA.
....HA + KOH = KA + H2O
I...10....0....10........
add.......x.............
C..-x....-x....+x
E...10-x..0....10+x
Substitute the E line into the Henderson-Hasselalch equation and solve for x = millimols KOH that must be added. Convert to grams KOH.
Post your work if you get stuck.
Yes... Got the answer 0.458 g.. Thank you...
To solve this problem, we need to calculate the moles of HA and KA in the buffer solution and then determine the amount of KOH needed to adjust the pH.
1. Calculate the moles of HA and KA in the initial buffer solution:
- We know that the initial buffer solution is 0.1M each for HA and KA.
- The volume of the solution is given as 100 mL, which is equivalent to 0.1 L.
- The number of moles can be calculated using the formula: moles = concentration (M) x volume (L).
- Moles of HA = 0.1 M x 0.1 L = 0.01 moles
- Moles of KA = 0.1 M x 0.1 L = 0.01 moles
2. Determine the number of moles of OH- ions needed to adjust the pH:
- To reach a pH of 6.0, we need to neutralize the excess H+ ions in the buffer solution.
- The pH scale is logarithmic, so a pH of 6.0 means 10^-6 moles of H+ ions per liter.
- Since the volume of the solution is 0.1 L, the number of moles of H+ ions is 10^-6 x 0.1 = 10^-7 moles.
3. Calculate the number of moles of KOH needed to neutralize the H+ ions:
- KOH is a strong base that completely dissociates in water, releasing OH- ions.
- For every mole of KOH, one mole of OH- ions is produced.
- Therefore, the number of moles of KOH needed is equivalent to the number of moles of OH- ions needed to neutralize the H+ ions.
- Moles of KOH = 10^-7 moles
4. Calculate the mass of KOH using its molar mass:
- The molar mass of KOH is 56.11 g/mol (39.10 g/mol for K, 16.00 g/mol for O, and 1.01 g/mol for H).
- Mass of KOH = moles of KOH x molar mass of KOH
- Mass of KOH = 10^-7 moles x 56.11 g/mol
Therefore, the number of grams of solid KOH that must be added to the buffer solution is 5.61 x 10^-6 grams (or 0.00561 mg) to make the pH of the solution 6.0.