Question

A student pours a 10.0mL sample of a solution containing HC2H3O2(pKa=4.8) and NaC2H3O2 into a test tube. The student adds a few drops of bromocresol green to the test tube and observes a yellow color, which indicates that the pH of the solution is less than 3.8. Based on this result, which of the following is true about the relative concentrations of HC2H3O2 and NaC2H3O2 in the original solution?

A. [HC2H3O2]>[NaC2H3O2]
B. [HC2H3O2]=[NaC2H3O2]
C. [HC2H3O2]<[NaC2H3O2]
D. The relative concentrations cannot be determined without knowing the value of pKb for C2H3O2−.

Answer 1

pH = pKa + log [(acetate)/(acetic acid)]
3.8 = 4.8 + log base/acid
-1.0 = log base/acied
base/acid = 10^-1 = 0.1 or
base = 0.1*acid
so base (NaC2H3O2) < acid (HC2HO2)