How would you prepare 1L of a .050 M phosphate buffer at pH 7.5 using crystalline K2HPO4 and a solution of 1 M HCl? pka of H2PO4 = 7.2

We need to find the amount of crystalline powder we need to weigh

in order to have 0.05 M of buffer
and the
volume of acid (1M HCl) we need to add and then make up the
volume
to 1L with water
...
.then we have our
buffer at pH 7.5.
N
umber
of moles = m/M
n = m/M
0.05mol/l
= m/174
g/mol
m = 8.7g
So we have to weigh 8.7g of K
2
HPO
4
and dissolve it in a
small
amount of water
How much of 1 M HCL do we need?
HCL

H
+
+ Cl
-
and the H
+
will go bind to our buffe
r
K
2
HPO
4
ioni
z
es in water to give 2K
+
and HPO
4
2
-
Then the
H
+
from HCL will bind the HPO
4
2
-
to give H
2
PO4
-
HPO
4
2
-
+
H
+
-
>
H
2
PO
4
-
If we know how much of acid
(
H
2
PO
4
-
)
is required
then we will know how
much of HCL we need to add since
for every 1mol of
acid
we need 1 mol of HCL (
1 mol of
H
+
from HCL)
Without considering pH we can easily calculate the volume we need
to make 0.05M of buffer
:
C
1
= 1M
C
2
= 0.05M
V
1
=
?
V
2
= 1
L
C
1
V
1
=
C
2
V
2
1M x V
1
= 0.05
M
x 1L
V
1
= 0.05L
This is
how much we need to make 0.05M of buffer at any pH b
ut
we don

t know how much of this
we
need
to adjust the pH to 7.5. So we need to use the Henderson
-
Hasselbalch equation
to find out how
much of the acid we need
pH =
pKa + log [base]
/[acid]
7.5 = 7.2 + log [base]/
[
acid]
0.3 = log [base]/[acid]
10
0.3
=
[base]/[acid]
2 = [base]/[acid]
2/
1
= base/acid
S
o
1/3 of
volume of acid
is
re
quired
to make pH 7.5
Volume of HCL required =
1/3 x
V
1
= 1/3 x
0.05L
= 0.0167L
= 16.67ml
(multiply by 1000 since 1000ml = 1L)
So to prepare your buffer of 0.05M at pH 7.5 you weigh out
8.7 g of
K
2
HPO
4
and dissolve it in a small amount
of water and add 16.67ml of 1M HCL and then make t
he volume up to 1L with water
...
.t
hen you have your

You want the pH of the buffer to be 7.5; therefore, calculate the ratio of base to acid using the Henderson-Hasselbalch equation.

pH = 7.2 + log(base/acid).
Solve for B/A. I get something like 2.

You have two unknowns; you need another equation. That is acid + base = 0.05
Using these two equation, solve for acid and base, convert base to grams using the molar mass of K2HPO4 and acid to mL using the molarity of HCl.
Post your work if you get stuck.

I got the first part when I have to find the ratio for base to acid. I didn't know where to go from there. I tried using the C1V1=C2V2 equation but again I am lost.

1.Weigh out 8.7 g k2hpo4(0.05 mole ×174 g/mole=8.7g).

2.dissolve it in small amount of water.

3.since the base/ acid ratio =2/1 you must convert 1third of the base (k2hpo4 )to the acid form - so you must add 0.05/3 L of 1M hcl to the solution.

4.dilute the resulting mix to 1 L .

bon appé :-)

Well, let's start by rustling up our ingredients and getting ready to concoct this magical potion! To prepare a 1L phosphate buffer at a concentration of 0.050 M with a pH of 7.5, we'll need crystalline K2HPO4 and a solution of 1 M HCl.

Step 1: Calculate the ratio of the acid to its conjugate base. The pKa of H2PO4 is 7.2, which is pretty close to our desired pH of 7.5. This means we want roughly equal amounts of the acid and its conjugate base in our buffer.

Step 2: Since we're dealing with K2HPO4 and HCl, we need to convert them to their respective moles. Remember, moles are sneaky little creatures hiding in chemicals.

Step 3: Determine how many moles of K2HPO4 we need by multiplying the concentration (0.050 M) by our desired volume (1L). This will give us the moles of K2HPO4.

Step 4: Conveniently, each mole of K2HPO4 will release two moles of H2PO4-. So, double the calculated moles of K2HPO4 to find the moles of H2PO4- we need.

Step 5: To find the moles of K+ we need, just double the moles of H2PO4-, because there are two K+ ions for every H2PO4- ion.

Step 6: Determine the volume of 1 M HCl required to achieve the desired pH. Since we're dealing with pH 7.5 and the pKa is 7.2, we need to shift the system toward the acid side. Adding HCl will do just that. Calculate the moles of HCl required using the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]). In this case, [A-] is the moles of H2PO4- and [HA] is the moles of H3PO4 (which came from the HCl). Solve for [HA] and convert it to moles.

Step 7: Convert the moles of H3PO4 to the volume of 1 M HCl needed by dividing by the concentration of HCl. Voila! You have the volume of HCl required!

Step 8: Pour the calculated volume of HCl into your 1L container, then add the calculated volume of K2HPO4. Mix it gently and give it a loving pat (optional, but highly recommended).

Congratulations, you've just created a 1L phosphate buffer at a concentration of 0.050 M with a pH of 7.5! Now sit back, relax, and revel in the glory of your buffer-making skills.

To prepare a 1L phosphate buffer solution at a pH of 7.5 using crystalline K2HPO4 and a solution of 1M HCl, you will need to follow a specific procedure. Here is a step-by-step guide on how to do it:

1. Determine the required molar concentrations of the buffer components:
- The pH of the buffer is given as 7.5, and the pKa of H2PO4 is 7.2.
- Use the Henderson-Hasselbalch equation to calculate the ratio of H2PO4 to HPO4:
pH = pKa + log([HPO4-] / [H2PO4])
7.5 = 7.2 + log([HPO4-] / [H2PO4])
Taking the antilog (10 raised to the power) of both sides: [HPO4-] / [H2PO4] ≈ 3.16

2. Determine the mole ratio between K2HPO4 and HCl:
- The balanced chemical equation between K2HPO4 and HCl is: 2K2HPO4 + 2HCl → 2KH2PO4 + K2H2P2O7
- The mole ratio is 2:2, meaning equal moles of K2HPO4 and HCl are required for the reaction.

3. Calculate the moles of K2HPO4 needed:
- Multiply the desired molarity (0.050M) by the desired volume (1L) to get the moles of H2PO4-:
moles of H2PO4- = 0.050M × 1L = 0.050 moles

4. Use the mole ratio to calculate the amount of HCl required:
- Since the ratio is 2:2, the moles of HCl needed will be the same as the moles of K2HPO4:
moles of HCl = 0.050 moles

5. Calculate the volume of 1M HCl solution needed:
- Convert the moles of HCl to volume using the molarity:
volume of 1M HCl solution = moles of HCl / molarity of HCl
volume of 1M HCl solution = 0.050 moles / 1M = 0.050L (or 50mL)

6. Prepare the buffer solution:
- Weigh out the required amount of crystalline K2HPO4 to obtain the desired moles (0.050 moles).
- Dissolve the K2HPO4 in distilled water until the final volume of the solution is 1L.
- Gradually add the 1M HCl solution to the K2HPO4 solution while monitoring the pH.
- Adjust the pH using small amounts of HCl or NaOH solution if necessary until the desired pH of 7.5 is achieved.

By following these steps, you will be able to prepare a 1L phosphate buffer solution at a pH of 7.5 using crystalline K2HPO4 and a solution of 1M HCl.