Methyl orange indicator in .1 M NH3(aq)

Question

I'm a new york state chemistry student preping for the regents and need help with this regents question. Why is it that when methyl orange indicator is put into a 0.10 M solution of NH3 (aq) it turns yellow? It is in my NYS chemistry ref table but I don't understand

Answer 1

Methyl orange (MeO) is yellow in basic/alkaline solutions, and is red in acidic solutions.  The equilibrium looks something like this:

H-MeO _(red) <==>H+ + MeO- _(yellow)

What is the pH of a 0.10 M solution of NH₃?

NH₃ + H₂O ==> NH4⁺ + OH^-

K_b NH₃ = 1.8x10^-5

1.8x10^-5 = [NH₄⁺][OH^-]/[NH₃] = (x)(x)/0.10

x² = 1.8x10^-6

x = 1.34x10^-3 M = [OH^-] 

pOH = -log [OH^-] = 2.87

pH = 14 - 2.87 = 11.1 which is certainly alkaline/basic.  Thus, methyl orange would exist in the anionic, yellow form.