Describe the preparation of 100 ml of phosphate buffer pH 7.2 using a 0.5 M H2PO4 and O .5 M NaOH
I don't know what molarity you want the buffer to be. I have assume 1 M but that may not be suitable. I work in millimoles and not concentrations; however, since M = millimols/mL then the volume cancels (since it's the same solution) and that works out OK.
M buffer = 1 M or millimoles = mL x M = 100 x 1 = 100 mmols.
pH = pKa2 + log base/acid
7.2 = 7.2 + log base/acid
base/acid = 1 or b = a and that's equation 1.
b + a = 100 mmols = equation 2.
Solve these two equations simultaneously to get
b = 50 mmols and a = 50 mmols.
So you want to take some amount of 0.5 M H2PO4^- (say x mL), add some quantity of 0.5 M NaOH(say y mL) to convert it to HPO4^- and have the final volume = 100 mL(or x mL + y mL = 100 mL)
Start with x volume of H2PO4^- initially and add y of NaOH
...........H2PO4^- + OH^- ==> HPO4^2- + H2O
I.........0.5x..............0..................0....................
add.......................0.5y............................................
C......-0.5y...........-0.5y..............+0.5y
E........0.5x-0.5y........0...............0.5y
pH = pKa2 + log (b/a)
7.2 = 7.2 + log (0.5y/0.5x-0.5y)
1 = 0.5y/(0.5x-0.5y) or
y = x/2 but you want x mL + y mL = 100 so
Solve the two equations to get x = 66.67 mL H2PO4^- then you add 66.67/2 = ? mL NaOH. Check it.
...........H2PO4^- + OH^- ==> HPO4^2- + H2O
I.........0.5(66.67).......0..................0....................
............33.34 mmols
add.......................0.5(33.34).= 16.67..........................................
C......-16.67.............-16.67..............16.67
E.........16.67..............0.....................16.67
So base = acid which we calculated and total volume is 66.67 + 33.34 = 100 mL
To prepare a 100 ml phosphate buffer pH 7.2 using a 0.5 M H2PO4 and 0.5 M NaOH, you will need to follow a specific procedure. Here are the steps:
1. Calculate the amount of H2PO4 and NaOH needed:
Since both the H2PO4 and NaOH solutions are 0.5 M, you need to determine the volumes required to achieve the desired final pH.
The pH of a phosphate buffer can be calculated using the Henderson-Hasselbalch equation:
pH = pKa + log ([salt] / [acid])
The pKa of H2PO4 is approximately 7.2. To prepare a pH 7.2 buffer, the ratio of [salt] (Na2HPO4) to [acid] (H2PO4) should be 1:1. Hence, you will need equal amounts of 0.5 M H2PO4 and NaOH.
To calculate the volume required, you can use the following equation:
Moles = Molarity × Volume (in liters)
Let's assume the final desired concentration is 0.1 M. So, for both H2PO4 and NaOH, the moles required would be:
Moles = 0.1 × 0.1 (since we're preparing a 100 ml solution)
= 0.01 moles
Now, divide the moles by their respective molarities to get the required volumes in liters:
Volume H2PO4 = 0.01 moles / 0.5 M
= 0.02 L = 20 ml
Volume NaOH = 0.01 moles / 0.5 M
= 0.02 L = 20 ml
2. Prepare the solutions:
Using a 0.5 M H2PO4 solution, measure 20 ml of it using a graduated cylinder or pipette and transfer it to a clean container (such as a beaker or glass bottle).
Similarly, measure 20 ml of 0.5 M NaOH using a graduated cylinder or pipette and add it to the same container containing the H2PO4 solution.
3. Add distilled water and adjust to 100 ml:
After adding both the solutions, the total volume will be 40 ml. To increase it to 100 ml, add distilled water to the container and mix well.
Alternatively, you can measure 40 ml of distilled water separately and add it to the container before adding the H2PO4 and NaOH solutions.
4. Check the pH:
Finally, use a pH meter or pH indicator strips to measure the pH of the buffer solution. Ideally, it should be around pH 7.2. If the pH deviates from the desired value, adjust it accordingly by adding small amounts of either H2PO4 or NaOH, and then mix and recheck the pH until it reaches 7.2.
And that's it! You have successfully prepared a 100 ml phosphate buffer pH 7.2 using a 0.5 M H2PO4 and 0.5 M NaOH.