Given stocks of 1M lactic Acid (pka 3.86) at pH 3.5, 2 M NaOH, 2M HCl and water; how would you prepare 4 Liters of 0.05M lactic acid at pH 3.25.

I know you use the Hendersen-Hasselbeck eqn, just not sure how to start. Please help.

I have posted this twice and it doesn't want to post correctly. Here is another try. On the second HH equation it takes the I (initial) line and what follows onto the equation line of

base + H^+ ==> acid and it may do that again.

Use the HH equation with 3.5 for pH and solve for (base)/(acid) (which I will call b/a
b/a = ? and this is equation 1.
a + b = 0.05*4 is equation 2.
Solve these two equations simultaneously; a and b will be in mols.

Since your solution is 3.5 and you want the final solution to be 3.25, it must be more acidic; therefore, the equation you now need is
..........base + H^+ = acid
I.......above...0.....above
add..............x.............
C.........-x....-x.......+x
E.......above-x..0.......above+x

Substitute the E line into the HH equation and solve for x which is the mols H^+ that must be added.
Then M = mols/L. YOu know mols needed and M of the HCl, solve for L HCl needed.
I suggest you take the answer you get, prepare the solution on paper and work it to see that the pH really is 3.25.

It appears to have posted correctly this time.

To prepare 4 liters of 0.05M lactic acid at pH 3.25, you can start by using the Henderson-Hasselbalch equation, which relates the pH of a solution to its pKa and the ratio of the concentrations of the conjugate acid-base pair.

The Henderson-Hasselbalch equation is given by:

pH = pKa + log([A-]/[HA])

Where:
- pH is the desired pH value (3.25 in this case).
- pKa is the dissociation constant of the acid (3.86 in this case).
- [A-] is the concentration of the conjugate base (lactate ion) - this is what you want to find.
- [HA] is the concentration of the acid (lactic acid) - this is what you want to calculate.

Now let's go step by step:

Step 1: Calculate the ratio [A-]/[HA]:
Using the given pH and pKa, rearrange the Henderson-Hasselbalch equation to solve for [A-]/[HA]:

3.25 = 3.86 + log([A-]/[HA])

Take the antilog of both sides:

10^(3.25 - 3.86) = [A-]/[HA]

The ratio [A-]/[HA] is equal to 10^(3.25 - 3.86).

Step 2: Determine the final concentration of lactic acid ([HA]):
To calculate the concentration of lactic acid, you can use the following equation:

M1V1 = M2V2

Where:
- M1 is the initial concentration of lactic acid.
- V1 is the initial volume of lactic acid in liters.
- M2 is the final concentration of lactic acid (0.05M in this case).
- V2 is the final volume of the solution in liters (4 liters in this case).

Since you have 1M lactic acid and want to find V1, rearrange the equation:

V1 = (M2V2) / M1

Substitute the known values to find the initial volume of lactic acid.

Step 3: Calculate the volume of the stock solution of lactic acid to use:
Now that you have the initial volume (V1) of lactic acid, you can calculate the volume of the stock solution of lactic acid to use.

However, keep in mind that it's not possible to directly measure volumes larger than the available stock solutions. In this case, if the stock solution is 1M lactic acid, you need 0.05M lactic acid. So you'll need to dilute the stock solution.

To dilute the stock solution, you can use the dilution equation:

C1V1 = C2V2

Where:
- C1 is the initial concentration of the lactic acid stock solution (1M).
- V1 is the volume of the stock solution to be used (unknown).
- C2 is the final concentration of the diluted lactic acid solution (0.05M).
- V2 is the final volume of the diluted lactic acid solution (4 liters).

Substitute the values into the equation and solve for V1 to determine the volume of stock solution to use.

Step 4: Calculate the volumes of NaOH, HCl, and water:
Now that you know the volume of the stock solution of lactic acid to use, you can calculate the volume of NaOH, HCl, and water needed.

First, determine the number of moles of lactic acid needed. This can be calculated using the equation:

moles = concentration (M) x volume (liters)

Substitute the known concentration (0.05M) and volume (4 liters) to calculate the moles of lactic acid needed.

Since you have 2M NaOH and 2M HCl, you need to determine the volume of each solution required to neutralize the calculated moles of lactic acid. To do this, use the equation:

moles = concentration (M) x volume (liters)

Substitute the known concentration (2M) and moles to find the volume of NaOH and HCl needed.

Finally, to make up the remaining volume to 4 liters, subtract the volumes of lactic acid, NaOH, and HCl from 4 liters. The resulting volume will be the volume of water needed.

By following these steps, you should be able to prepare 4 liters of 0.05M lactic acid at pH 3.25 using the given stock solutions.