Question

Bromothymol blue is a chemical indicator that turns blue at a pH value greater than 7. In which of these solutions would a colour be observed if 2 drops of cromothyml blue were added? The concentration of each solution is 0.10 M. Ka for HC2H3O2(aq) = 1.8 * 10^-5; Kb for NH3 (aq) = 1.8 * 10^-5

A - NH4Cl B - NaNO3 C - NH4C2H3O2 D - NaC2H3O2

Answer 1

Concentration of each solution = 0.10 M

For HC2H3O2(aq): ka = 1.8 * 10^-5

HC2H3O2      H+ + C2H3O2-
Initial 0.1 0 0
Final 0.1 - x x x

Ka = [H+][C2H3O2-] / [HC2H3O2]

1.8 * 10^-5 = x*x / 0.1 - x

x = 0.00133

[H+] = 0.00133 M

pH = - log (0.00133)

= 2.87

For NH3(aq): Kb = 1.8 * 10^-5

NH3(aq) + H2O      NH4+ + OH-
Initial 0.1 0 0
Final 0.1 - x x x

Kb = x*x / (0.1 - x)

1.8 * 10^-5 = x*x / (0.1 - x)

x = 0.00133

[OH-] = 0.00133 M

pOH = - log (0.00133)

pOH = 2.87

pH = 14 - 2.87

= 11.13

Since the pH of NH3 is greater than 7.

So, in NH3 solution there would be a color change observed when 2 drops of Bromothymol blue were added.