To prepare a buffer solution, you need to determine the appropriate amounts of NaOH and HCOOH to add to achieve the desired pH and maintain a specific ratio of COOH- to HCOOH. Here are the steps to calculate the quantities:
Step 1: Calculate the required moles of NaOH and HCOOH.
The volume of the buffer solution is given as 150.0 mL. Since we want to have equal molar concentrations of NaOH and HCOOH, we can assume we need an initial volume of 75.0 mL for both solutions.
To calculate the moles of NaOH needed, use the formula:
moles of NaOH = volume (in L) × concentration (in mol/L)
moles of NaOH = (75.0 mL / 1000 mL/L) × 0.500 mol/L
Similarly, to calculate the moles of HCOOH needed, use the same formula with the concentration of HCOOH:
moles of HCOOH = (75.0 mL / 1000 mL/L) × 0.500 mol/L
Step 2: Calculate the moles of COOH- and HCOOH required to achieve the desired ratio.
The Henderson-Hasselbalch equation gives you the ratio of COOH- to HCOOH needed to achieve the desired pH. Let's assume the ratio is x:1, where x is the number we need to determine.
pH = pKa + log(COOH- / HCOOH)
From your calculations, you already have the ratio at this step.
Step 3: Calculate the moles of COOH- and HCOOH required based on the ratio.
Since you have the ratio of COOH- to HCOOH from the Henderson-Hasselbalch equation, multiply the moles of HCOOH and COOH- by their respective ratios to obtain the final moles:
moles of HCOOH_final = moles of HCOOH × 1
moles of COOH-_final = moles of HCOOH × x
Step 4: Calculate the volume required to achieve the desired concentration.
To determine the volume needed for each solution to reach the final moles calculated in step 3, use the formula:
volume (in L) = moles / concentration (in mol/L)
volume of HCOOH = moles of HCOOH_final / concentration of HCOOH
volume of COOH- = moles of COOH-_final / concentration of NaOH
Convert the volumes to mL by multiplying by 1000.
Step 5: Prepare the buffer solution.
Now that you have the required volumes, mix the determined volumes of HCOOH and NaOH together in a container to make the final buffer solution.
It is essential to rinse the glassware before preparing the solution to ensure accurate measurements. Always wear appropriate safety equipment and follow lab protocols while handling chemicals.
Hope this helps! Let me know if you have further questions.