calculate the mass of the species required to prepare a phosphoric acid buffer with a voncentration of 0.34M ,volume of 500ML and a PH of 11.8.calculate the mass of the varies species needed to prepare the buffer.salt of interest is sodium. consider the following information
H3PO4=H2PO4+H
H2PO4=HPO4+H
HPO4=PO4+H
Look up the pKa values for H3PO4.
K1 = 7.1E-3 which makes pK1 = about 2
K2 = 6,3E-8 which makes pK2 = about 7
K3 = 4.5E-13which makes pK3 about 12. This one is within about 1 pH value of the desired pH of 11.8 and HPO4^- and PO4^3- are the acid/base pair respectively that you want to use.
First you solve two simultaneous equations.
Equation 1 is the Henderson-Hasselbalch equation which is
11.8 = pK3 + log (base/acid)
Solve for b/a.
Equation 2 is
b + a = 0.34M
So you now have concn of acid and concn of base.
Convert M concns to grams HPO4^- and grams PO4^3-.
POst your work if you get stuck.
To calculate the mass of the species required to prepare a phosphoric acid buffer, we need to determine the molar mass and stoichiometry of each species involved in the buffer solution.
Given the concentration (0.34 M), volume (500 mL), and pH (11.8) of the buffer, we can start by determining the ratio of the acidic and conjugate base species needed to achieve the desired pH.
The pH of a buffer solution is determined by the ratio of the concentrations of the acidic and conjugate base species. In this case, we need to find the ratio of H2PO4- (conjugate base) to H3PO4 (acidic species) to achieve a pH of 11.8.
We can use the Henderson-Hasselbalch equation to find this ratio:
pH = pKa + log([conjugate base]/[acidic species])
In this case, the pKa values of phosphoric acid are as follows:
pKa1 = 2.15
pKa2 = 7.21
pKa3 = 12.67
For the desired pH of 11.8, we are interested in the second pKa value (pKa2 = 7.21).
Rearranging the Henderson-Hasselbalch equation and solving for the ratio of [conjugate base]/[acidic species], we get:
[conjugate base]/[acidic species] = 10^(pH - pKa)
Substituting the values, we have:
[conjugate base]/[acidic species] = 10^(11.8 - 7.21)
Calculating this ratio gives us the relative amounts of H2PO4- and H3PO4 needed to achieve the desired pH of 11.8.
Now, let's calculate the mass of each species required:
1. Phosphoric Acid (H3PO4):
Given the volume (500 mL) and concentration (0.34 M) of the phosphoric acid buffer, we can calculate the number of moles of H3PO4:
moles of H3PO4 = concentration * volume
moles of H3PO4 = 0.34 mol/L * 0.5 L
moles of H3PO4 = 0.17 mol
To convert moles to grams, we need to know the molar mass of H3PO4. Using the periodic table, we find:
Molar mass of H3PO4 = (1*3) + (1*1) + (16*4)
Molar mass of H3PO4 = 98 g/mol
mass of H3PO4 = moles of H3PO4 * molar mass of H3PO4
mass of H3PO4 = 0.17 mol * 98 g/mol
mass of H3PO4 = 16.66 g (rounded to two decimal places)
Therefore, we need approximately 16.66 grams of H3PO4 to prepare the buffer solution.
2. Conjugate Base (H2PO4-):
To determine the mass of H2PO4- needed, we use the previously calculated ratio:
[conjugate base]/[acidic species] ≈ 10^(11.8 - 7.21)
Since the ratio of H2PO4- to H3PO4 is 10^(11.8 - 7.21), the mass of H2PO4- can be calculated as follows:
mass of H2PO4- = mass of H3PO4 * [conjugate base]/[acidic species]
mass of H2PO4- = 16.66 g * 10^(11.8-7.21)
Calculating this value will give you the mass of H2PO4- required.
Note: The same process can be followed to determine the mass of HPO4^2- and PO4^3- if required.
Please note that in practice, it is common to prepare buffers using commercial buffer solutions or pre-made salts to ensure accurate concentrations.