Question

In the buffer lab, students were asked to prepare a phosphate buffer given a target pH. Malic acid (H₂C₄H₄O₅, M_r = 134.088 g/mol) is a diprotic acid in which the pK_a1 for dissociation of the first proton is 3.40 and the pK_a2 for dissociation of the second proton is 5.11. In a similar scenario, given 1.650 grams of NaHC₄H₄O₅ (M_r = 156.070 g/mol), prepare 100.0 mL of a pH = 3.75 buffer. Determine the identity and mass of material needed to complete the buffer. (Assume K_w = 1.0 ✕ 10⁻¹⁴.)

(a) How many moles of NaHC₄H₄O₅ are in 1.650 grams of NaHC₄H₄O₅?

(b) Select the proper compound needed to prepare a pH 3.75 buffer.

H₂C₄H₄O₅ (M_r = 134.088 g/mol) OR Na₂C₄H₄O₅ (M_r = 178.052 g/mol)   

(c) How many grams of the compound chosen in part (b) are needed to prepare 100.0 mL of a pH 3.75 buffer?

Answer 1

a)

Equation to find out the number of moles is given below:

Number of moles = Given mass / Molar mass

Given mass = 1.650 grams

Molar mass = 156.070 g/mol

Moles of NaHC4H4O5 are in 1.650 grams = 1.650 g/ 156.070 g

Moles of NaHC4H4O5 are in 1.650 grams = 0.0106 moles

b)

There are two types of buffer solutions into which buffer solutions : acidic and alkaline buffers.

Acidic buffer solution

These solutions are used to maintain acidic environments. Acid buffer has acidic pH and is prepared by mixing a weak acid and its salt with a strong base. pH of these solutions is below seven. These solutions consist of a weak acid and a salt of a weak acid.

Alkaline Buffer solution

These buffer solutions are used to maintain basic conditions. Basic buffer has a basic pH and is prepared by mixing a weak base and its salt with strong acid.

Now, to prepare a buffer solution of pH=3.75 (Acidic buffer, since pH less than 7), we need an acid and its salt with strong base.

i.e.,, The acid H2C4H4O5 and its salt Na2C4H4O5