Calculate the pH change of 10mL of HCl added to 1M phosphate buffer in 1L with initil pH of 7.2?
Use the Henderson-Hasselbalch equation. It would be nice to know the concn HCl added.
To calculate the pH change when adding 10 mL of HCl to a 1M phosphate buffer in a total volume of 1 liter, we need to take into account the change in concentration of the acidic and basic components in the buffer solution.
Here's how you can calculate the pH change step by step:
Step 1: Calculate the initial concentration of H3PO4 and Na2HPO4 in the buffer solution.
The phosphate buffer is made up of two components, H3PO4 (acidic) and Na2HPO4 (basic). Since the buffer solution is 1M, this means both components are present at a concentration of 1M.
Step 2: Calculate the moles of acid and base initially present in the buffer.
Since the volume of the buffer is 1L and the concentration is 1M, the initial moles of H3PO4 and Na2HPO4 can be calculated as follows:
Initial moles of H3PO4 = concentration × volume = 1M × 1L = 1 mol
Initial moles of Na2HPO4 = concentration × volume = 1M × 1L = 1 mol
Step 3: Calculate the number of moles of HCl added.
Since 10 mL of HCl is added, we need to convert it to liters:
10 mL = 10/1000 L = 0.01 L
Assuming the concentration of HCl is known, multiply it by the volume to calculate the moles of HCl added. Let's assume the concentration of HCl is 1M:
Moles of HCl added = concentration × volume = 1M × 0.01L = 0.01 mol
Step 4: Calculate the new moles of acid and base after the reaction.
In this case, HCl reacts with Na2HPO4 to form NaH2PO4, reducing the concentration of the basic component and increasing the concentration of the acidic component.
Since the reaction stoichiometry is a 1:1 ratio, the moles of Na2HPO4 consumed will be equal to the moles of HCl added. Therefore, the new moles of Na2HPO4 can be calculated as follows:
New moles of Na2HPO4 = Initial moles of Na2HPO4 - moles of HCl added = 1 mol - 0.01 mol = 0.99 mol
Since the reaction equation is: Na2HPO4 + HCl ➞ NaH2PO4 + NaCl
And the stoichiometry is 1:1
Step 5: Calculate the new concentration of the acid and base.
The new concentration of H3PO4 can be calculated by dividing its new moles by the final volume, which is 1L:
New concentration of H3PO4 = New moles of H3PO4 / final volume = 1 mol / 1 L = 1 M
The new concentration of Na2HPO4 can be calculated in the same way:
New concentration of Na2HPO4 = New moles of Na2HPO4 / final volume = 0.99 mol / 1 L = 0.99 M
Step 6: Calculate the new pH of the buffer solution.
Using the Henderson-Hasselbalch equation, which relates the pH of a buffer solution to the ratio of its acidic and basic components:
pH = pKa + log ([base] / [acid])
Let's assume the pKa of the phosphate buffer is 7.2 (since the initial pH is given as 7.2).
pH = 7.2 + log (0.99M / 1M)
pH = 7.2 + log (0.99)
By evaluating this expression, you can find the new pH of the buffer solution after adding 10 mL of HCl.
Note: If the exact pKa is known, it can be used for more accurate calculations.