Three salts Na_3X, Na_2Y, and NaZ are made into 0.30M solutions with pH values o

Question

Three salts Na_3X, Na_2Y, and NaZ are made into 0.30M solutions with pH values of 8.5, 7.0, and 10.5, not necessarily in order. (Na=sodium ion, X, Y, and Z = anions) Explain how anions can act as bases in aqueous solution. Write a chemical equation to explain your answer. Which pH most likely goes with which salt? Explain. What is the approximate pH of a 0.010M solution of HZ? Explain your reasoning. Calculate the acid dissociation constants, Ka, for each acid HX^2-, HY^-, and HZ, and rank them in order or increasing strength, (weakest first) Justify your reasoning.

Explanation / Answer

2. a. Reactions as bases in aqueous solution

For Na3X : X^3- + H2O <==> HX^2- + OH-

For Na2Y : Y^2- + H2O <==> HY- + OH-

For NaZ : Z- + H2O <==> HZ + OH-

b. The larger is the -ve charge on the ion, the more number of OH- it can form in solution. That is X^3- can react with 3 H2O o form H3X and 3OH-. Similarly Y^2- can react with 2 H2O to give H2Y and 2OH-, and finally Z- can react with 1 H2O to form HZ and 1OH-. So as can be seen the greater is the reactivity with water, more basic will be the solution and higher would be the pH of solution.

Thus,

Na3X pH 10.5

Na2Y pH 8.5

NaZ pH 7

c. HZ <==> H+ + Z-

NaZ has a pH of 7, that means it is a salt of strong acid and strong base. Conjuagte acid of it is HZ, which would be a strong acid which dissociates completely into H+ and Z- ions, thus

pH = -log[H+] = -log(0.010) = 2

d. For Ka we need more information like degree of dissociation of acids.

Order of increasing strength : HX^2- < HY- < HZ

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