A student must make a buffer solution with a pH of 4.0. Determine which of the a

Question

A student must make a buffer solution with a pH of 4.0. Determine which of the acids and conjugate base listed below are the best options to make a buffer at the specified pH. Weak acids: propionic acid, K_a = 1.34 times 10^-5, 3.00 M formic acid, K_a = 1.77 times 10^-4, 2.00 M phosphoric acid, K_a = 7.52 times 10^-3, 1.00 M acetic add, K_a = 1.75 times 10^-5, 5.00 M Conjugate bases: sodium propionate. CH_3CH_2COONa sodium acetate trihydrate, CH_3COONa* 3H_2O sodium dihydrogen phosphate monohydrate, NaH_2PO_4* H_2O sodium formate, HCOONa The final volume of buffer solution must be 100,00 mL and the final concentration of the weak acid must be 0.100 M. Based on this information, what mass of solid conjugate base should the student weigh out to make the buffer solution with a pH = 4.0? Based on this information, what volume of acid should the student measure to make the 0.100 M buffer solution?

Explanation / Answer

Part 1Given the final volume of buffer = 100mL and [HA] = 0.1

pH 0f buffer = 4.0

to have a good buffer the pH of the buffer should be as close as to the pKa of the acid.

of the given acids

pKa of formic acid = 3.75

pKa of acetic acid = 4.76

pKa of propionic acid = 4.87 and

pKa of phosphoric acid = 2.12

Thus the best option is formic acid which has very close value to the required pH of buffer.

the acid to be taken is formic acid and the conjugate base is sodium formate.

Part 2

We know pH of buffer is calculated using Hendersen equation

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

4.0 = 3.75 + log [A-]/0.1

thus [A-] = 0.1778 M

So the mass of socium acetate to be added to make 100mL of 0.1778 M solution is

= molarity x molarmass x100/1000

= 0.1778x 68g/mol /10

= 1.209 g

Part 3

to have 0.1M acid solution from stock of 2M formic acid

0.1 M x100mL = 2 M x VmL

Thus volume of acid to be measured = 5mL

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.