Experiment 22 Advance Study Assignment: Properties of Systems in Chemical Equili

Question

Experiment 22
Advance Study Assignment: Properties of Systems in Chemical Equilibrium

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 Cl ions.)

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 Na+ 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?)

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) 2

Formulate the expression for the equilibrium constant, Ksp, for the above reaction.

It is possible to dissolve significant amounts of Mg(OH)2 in solutions in which the concentration of

either Mg2+ or OH is very, very small. Explain, using Ksp, why this is the case.

Explain why Mg(OH)2 might have very appreciable solubility in 1 M HCl. (Consider the effect of

Reaction 3 on the Mg(OH)2 solution reaction.)

Explanation / Answer

According to LeChatelier principle if you add H+'s , the reaction will shift to the left in order to eliminate the effect of increasing the right side.
when it shifts to the left it produces the reddish HMO. this shift restores the constant ratio equilibrium constant.

By adding the NaOH, you didn't add anything to the equilibrium, but it did remove some of the H+'s.
LeChatelier principle says that if you remove H+'s , the reaction will shift to the right in order to replace the the H+'s , when it shifts to the right it produces the yellowish MR-. this shift restores the constant ratio equilibrium constant.

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Ksp = [Mg+2] * [ OH-]^2

It is possible to dissolve Mg(OH)2 till it reaches its ksp value. So, you need at least one to be small

if you add H+ it reduces OH- concentration by reacting with it. So the equilibrium will shift to the right and solubility will increase.

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