Question

1. Methyl orange, HMO, is a common acid-base indicator. In solution it ionizes according to the equation: HMO(aq) = H(aq) + MO"(aq) red yellow If methyl orange is added to distilled water, the solution turns yellow. If a drop or two of 6 M HCl is added to the yellow solution, it turns red. If to that solution one adds a few drops of 6 M NaOH the color reverts to yellow a. Why does adding 6 M HCl to the yellow solution of methyl orange tend to cause the color to change to red? (Note that in solution HCl exists as H and Clions.) b. Why does adding 6 M NaOH to the red solution tend to make it turn back to yellow? (Note that in solution NaOH exists as Nat and OH ions. How does increasing (OH) shift Reaction 3 in the dis- cussion section? How would the resulting change in (H) affect the dissociation reaction of HMO?) 2. Magnesium hydroxide is only very slightly soluble in water. The reaction by which it goes into solution is: Mg(OH),(s) = Mg²+ (aq) + 2 OH(aq) a. Formulate the expression for the equilibrium constant, Kp for the above reaction. b. It is possible to dissolve significant amounts of Mg(OH), in solutions in which the concentration of either Mg2+ or OH" is very, very small. Explain, using K why this is the case. c. Explain why Mg(OH), might have very appreciable solubility in 1 M HCI. (Consider the effect of Reaction 3 on the Mg(OH), solution reaction.)

Answer 1

Page 2 / molt int 2 01"Can) — - 3 * Com). 6) = @ ksp = [gr] [on]² {Mg Cohe] since solubility of glong very less, [mg 6H2] = 1. ksp= [mgzt] [on]2 ② s from reaction. If concentration of [might] and [on ] very less, equilibrium will shift in right (forward) direction. This lead to increase in solubility of my (OH)2. ii) from equation ③, we know ksp is constant. As [mgar] and con very small. Numeration of equation ® will be small. To compensate such change [mg cong] has to be small. This is only possible when significant amount of my cond) dissolves in solution. My Corp is weak base which can reaction with acid like Hel (1M) stronger Mg (oh) 8) & 2 Hel (im) Cag) - Mg cl tt 24₂0 (salt) !") - Neutralisation reaction is favoured where Mach salt is precipitated. This reduces free Mght ion concentration in solution and favoury further dissolution Mg Cond in solution Mg (OH) = Mg 2+ + 204 + 24h teel – macht amo