Question

In: Chemistry

(a) Which two of the following compounds would you mix to make a buffer of pH...

(a) Which two of the following compounds would you mix to make a buffer of pH 7.45? H3PO4 (FM 98.00), NaH2PO4 (FM 119.98), Na2HPO4 (FM 141.96), and Na3PO4 (FM 163.94)?

NaH2PO4 and Na2HPO4 This is the correct answer for this part.


(b) If you wanted to prepare 1.00 L of buffer with a total phosphate concentration of 0.0513 M, how many grams of the two selected compounds would you mix together?

acidic component _____g
basic component ______g


(c) If you did what you calculated in part (b), you would not get a pH of exactly 7.45. Explain how you would really prepare this buffer in the lab.

Solutions

Expert Solution

solution:

The pKa of given acids are

H3PO4 = 2.14

H2PO4- = 7.10

HPO4-2 = 12.32

so we will take a combination of NaH2PO4 and Na2HPO4, as the pKa of NaH2PO4 is nearest to the desired pKa value.

b) The total volume required = 1L

The total concentration required = 0.0513

it means

[NaH2PO4] + [Na2HPO4] = 0.0513 .......(1)

From Hendersen Equations

pH = pKa + log [salt] / [acid] = 7.10 + log [Na2HPO4] / [NaH2PO4] = 7.45

log [Na2HPO4] / [NaH2PO4] = 0.35

[Na2HPO4] / [NaH2PO4] = 2.24 ......(2)

Putting (2) in (1)

2.24 NaH2PO4]+[NaH2PO4] = 0.0513

[NaH2PO4] = 0.0154 M

[Na2HPO4] = 0.050 - 0.0154 = 0.0346 M

as the solution is 1L

so the moles of Na2HPO4 required = 0.0346 moles

Mass of Na2HPO4 = moles X molar mass = 0.0346 moles X 141.96 = 4.91 grams

Moles of NaH2PO4 = 0.0154

Mass of NaH2PO4 = 0.0154 X 119.98 = 1.85 g

c) we will take the salt initially i.e. Na2HPO4, 0.0513moles

It means we will weight out = 0.0513 X 141.96 = 7.28 grams of salt

And will dissolve it in 900mL of water. then we will start adding HCl to it so that HCl will react with the salt to give the weak acid. and will continue adding HCl till we get desired pH. Then we will make up water to 1L.

queen_honey_blossom answered 2 years ago
Next > < Previous

Related Solutions

You wish to make a buffer with a pH of 11. A) Which of the following...
You wish to make a buffer with a pH of 11. A) Which of the following amino acids would you use to make your buffer: glutamine, aspartate, histidine, or tyrosine? B) Why would your chosen amino acid be better than the others? C) Start with 0.02M of the base form of your amino acid and calculate the concentration of the acidic form needed to have your buffer at the desire pH of 11. Show your work. This is the third...
You wish to make a buffer with a pH of 11. A) Which of the following...
You wish to make a buffer with a pH of 11. A) Which of the following amino acids would you use to make your buffer: glutamine, aspartate, histidine, or tyrosine? B) Why would your chosen amino acid be better than the others? C) Start with 0.02M of the base form of your amino acid and calculate the concentration of the acidic form needed to have your buffer at the desire pH of 11. Show your work.
You wish to make a buffer solution with a pH of 3.5. Which of the following...
You wish to make a buffer solution with a pH of 3.5. Which of the following acids (and a salt of its conjugate base) would be the most ideal choice? Acid A (Ka = 3.2 x 10^-3) Acid B (Ka = 7.1 x 10^-4) Acid C (Ka= 1.4 x 10^-5) Acid D (Ka = 8.5 x 10^-6)
Which of the following combinations would make a buffer closest to pH 9.0? HClO2 Ka =...
Which of the following combinations would make a buffer closest to pH 9.0? HClO2 Ka = 1.1 x 10-2                             NH3 Kb = 1.8 x 10-5 HNO2 Ka = 7.2 x 10-4                              C5H5N Kb = 1.5 x 10-9 A. Equal volumes of 0.10 M HNO2 and 0.10 M NaNO2 B. Equal volumes of 0.10 M C5H5NH+Cl- and 0.10 M C5H5N C. Equal volumes of 0.10 M NH4Cl and 0.10 M NH3 D. Equal volumes of 0.10 M HClO2 and 0.10 M NaClO2
if you want to prepare a buffer at pH = 5.2 which buffer would you choose?...
if you want to prepare a buffer at pH = 5.2 which buffer would you choose? a) MES --> Ka = 7.9 x 10^-7 b) CHES --> Ka = 5.0 x 10^-7 c) Tris --> Ka = 6.3 x 10^-9 d) acetic aid --> Ka = 1.8 x 10^-5
You would like to make 100.00 mL of a 4.76 pH acetic acid/acetate buffer, in which...
You would like to make 100.00 mL of a 4.76 pH acetic acid/acetate buffer, in which the acid concentration is 0.070M. Write the reaction of the buffer with HCl solution. How many mL of 0.100 M HCl can react with the buffer before it is exhausted? And Write the reaction of the buffer with NaOH solution. How many mL of 0.100 M NaOH can react with the buffer before it is exhausted?
You would like to make a buffer that has a pH of 7.00. Starting with 100.0...
You would like to make a buffer that has a pH of 7.00. Starting with 100.0 mL solution of 0.10 M NaH2PO4 and 0.10 M Na2HPO4, how much 0.1 M NaOH would you need to add to bring the pH of the buffer up to 7.00?
Which of the following acids would be most suitable for preparing a buffer of pH 3.10...
Which of the following acids would be most suitable for preparing a buffer of pH 3.10 and why: hydrogen peroxide (pKa= 11.65), propanoic acid (pKa=4.874), cyanoacetic acid (pKa= 2.472), and 4-aminobenzenesulfonic acid (pKa=3.232)?
Describe how you would prepare each of the following: 100.0 mL of a buffer with pH...
Describe how you would prepare each of the following: 100.0 mL of a buffer with pH 7.4 using species from the ionizations of phosphoric acid. 100.0 mL of a buffer with pH 7.4 using species from the ionizations of carbonic acid. Will these buffers be more effective against added acid or added base?
For each of the following compounds, indicate the pH at which 50% of the compound will...
For each of the following compounds, indicate the pH at which 50% of the compound will be in a form that possesses a charge and at which pH more than 99% of the compound will be in a form that possesses a charge. CH3COOH (pKa = 4.76) CH3NH+3 (pKa = 10.7) Determine a pH at which pH more than 99% of CH3COOH will be in a form that possesses a charge. Express your answer using two decimal places.
ADVERTISEMENT
Subjects
ADVERTISEMENT
Latest Questions
ADVERTISEMENT