Q1: You wish to prepare a buffer solution with pH = 11.10. What volume of 6.0 M HCl would you add to 500 mL of 0.10 M (C2H5)2NH to prepare the buffer? You may assume that the solution’s volume remains constant.
Q2: What is the resulting pH when 20 mL of 0.20 M NaOH is added to the buffer prepared in Q1?
Q3: Imagine diluting 250 mL of the buffer prepared in Q1 with distilled water to a final volume of 500 mL. What is the resulting pH when 20 mL of 0.20 M NaOH is added to the diluted buffer?
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Q1: To prepare a buffer solution with a specific pH, we need to use an acidic solution and its conjugate base (or a basic solution and its conjugate acid) in appropriate proportions. In this case, we have (C2H5)2NH, which is a weak base, and we want to prepare a buffer with pH 11.10.
The Henderson-Hasselbalch equation is often used to calculate the quantities needed for buffer solutions:
pH = pKa + log ([A-]/[HA])
In this case, (C2H5)2NH acts as the base (A-) and its conjugate acid is (C2H5)2NH2+ (HA). To calculate the pKa, we can use a pKa table or look up the dissociation constant (Ka) for (C2H5)2NH.
Once we have the pKa, we can rearrange the Henderson-Hasselbalch equation to solve for the ratio of [A-]/[HA] that gives us the desired pH. We can then calculate the amount of (C2H5)2NH and (C2H5)2NH2+ needed to prepare the buffer.
To find the volume of 6.0 M HCl needed, we need to calculate the moles of HCl required using the volume of the buffer solution, the desired pH, and the known concentrations of (C2H5)2NH and HCl.
Here are the steps to calculate the volume of 6.0 M HCl needed to prepare the buffer:
1. Determine the pKa of (C2H5)2NH.
2. Use the Henderson-Hasselbalch equation to calculate the ratio of [A-]/[HA] that gives pH 11.10.
3. Calculate the moles of (C2H5)2NH and (C2H5)2NH2+ required to prepare the buffer using the ratio from step 2 and the volume of the buffer solution.
4. Calculate the moles of HCl required to neutralize the (C2H5)2NH and (C2H5)2NH2+ and adjust the pH using the volume of the buffer solution and the concentration of HCl.
5. Convert the moles of HCl to the volume of 6.0 M HCl using the molarity of HCl.
Once you have the volume of 6.0 M HCl needed, you can proceed to Q2 for the next part.
Q2: Now that we have prepared the buffer solution using the calculated volume of 6.0 M HCl, we need to determine the resulting pH when 20 mL of 0.20 M NaOH is added.
To calculate the resulting pH, we need to consider the reaction between the added NaOH, the (C2H5)2NH, and (C2H5)2NH2+ in the buffer solution. Since NaOH is a strong base and (C2H5)2NH is a weak base, the NaOH will react with (C2H5)2NH to form the conjugate acid (C2H5)2NH2+.
To calculate the resulting pH, we can use stoichiometry and the Henderson-Hasselbalch equation. We know the initial concentrations of (C2H5)2NH and (C2H5)2NH2+ from Q1 and the volume of the buffer solution. We also know the concentration of NaOH and the volume added.
Here are the steps to calculate the resulting pH:
1. Calculate the moles of (C2H5)2NH and (C2H5)2NH2+ in the buffer solution before the addition of NaOH.
2. Use stoichiometry to determine the amount of (C2H5)2NH that reacts with the added NaOH.
3. Calculate the new concentrations of (C2H5)2NH and (C2H5)2NH2+ after the reaction using the volume of the buffer solution and the volume of NaOH added.
4. Use the Henderson-Hasselbalch equation to calculate the resulting pH using the new concentrations of (C2H5)2NH and (C2H5)2NH2+.
Now that we have the resulting pH when 20 mL of 0.20 M NaOH is added, we can proceed to Q3 for the next part.
Q3: In this question, we dilute the buffer prepared in Q1 with distilled water to a final volume of 500 mL. After dilution, we need to determine the resulting pH when 20 mL of 0.20 M NaOH is added.
To calculate the resulting pH, we need to take into account the dilution factor and the reaction between NaOH and the weak base (C2H5)2NH in the buffer solution. Similar to Q2, we can use stoichiometry and the Henderson-Hasselbalch equation to calculate the resulting pH.
Here are the steps to calculate the resulting pH:
1. Calculate the moles of (C2H5)2NH and (C2H5)2NH2+ in the diluted buffer solution after adding distilled water.
2. Use the dilution factor to adjust the concentrations of (C2H5)2NH and (C2H5)2NH2+ to account for the volume increase.
3. Calculate the moles of (C2H5)2NH that react with the added NaOH using stoichiometry.
4. Calculate the new concentrations of (C2H5)2NH and (C2H5)2NH2+ after the reaction using the adjusted concentrations and the volume of NaOH added.
5. Use the Henderson-Hasselbalch equation to calculate the resulting pH using the new concentrations of (C2H5)2NH and (C2H5)2NH2+.
By following these steps, you can calculate the resulting pH when 20 mL of 0.20 M NaOH is added to the diluted buffer solution.