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# Acid/Base Chemistry Problems

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1. Solve the following problem related to the solubility of equilibria of some metal hydroxides in aqueous solution.

a) The solubility of Cu(OH)2 (solid)is 1.72x10^-6 gram per 100 ml of solution at 25 degrees C.
i. Write the balanced chemical equation for the dissociation of Cu(OH)2 (solid) in aqueous solution.
ii. Calculate the solubility (in moles/liter) if Cu(OH) 2 at 25 degrees C.
iii. Calculate the value of the solubility-product constant Ksp for Cu(OH)2 at 25 degrees C.
b) The value of the solubility product constant, Ksp, for Zn(OH) 2 is 7.7x10^-17 at 25 degrees C.
i. Calculate the solubility of Zn(OH)2 at 25 degrees C in a solution with a pH of 9.35
ii. At 25 degrees C, 50 mL of .1 molar Zn(NO3) 2 is mixed with 50mL of a .3 molar NaOH. Calculate the molar concentration of Zn2+ (aq) in the resulting solution once equilibrium has been established. Assume the volume are additive

2. A 30mL sample of a weak monoprotic acid was titrated with a standardized solution of NaOH. A pH meter was used to measure the pH after each increment of NaOH was added, and the curve attached was constructed

a) Explain how this curve could be useful determining the molarity of the acid
b) Explain how this curve could be used to determine the dissociation constant, K of the weak monoprotic acid.
c) If you were to repeat the titration using an indicator in the acid to signal the endpoint, which of the following indicators should you select? Why?
Methyl Red Ka= 1x10^-5
Cresol Red Ka= 1x10^-8
Alizarin yellow Ka= 1x10^-11

d) Sketch the titration curve that would result if the monoprotic acid were replaced by a strong monoprotic acid, such as HCL of the same molarity. Identify differences between this titration curve and the curve shown attached.

3. A buffer solution contains .4 mole of formic acid HCOOH, and .6 mol of sodium formate, HCOONa, in 1L of solution. The ionization constant, Ka, of formic acid is 1.8x10^-4

a) Calculate the pH of the solution
b) If 100mL of this buffer solution is diluted to a volume of 1L with pure water, the pH does not change. Discuss why the pH remains constant on dilution.
c) A 5.00mL sample of 1.00 molar HCL is added to 100mL of the original buffer solution. Calculate the [H3O-] of the resulting solution
d) An 800mL sample of 2 molar formic acid is mixed with 200mL of 4.8 molar NaOH. Calculate the [H3O-] of the resulting solution.

4. H3PO2, H3PO3, and H3PO4 are monoprotic, diprotic, and triprotic acids, respectively, and they are about equal weak acids.

HClO2, HClO3, and HClO4, are all monoprotic acids, but HClO2 is a weaker acid than HClO3, which a weaker acid than HClO4. Account for:

A) The fact that the molecules of the three phosphorus acids can provide different numbers of protons.

B) The fact that the three chlorine acids differ in strength.

##### Solution Summary

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1. a)
i. Cu(OH)2 (s) >< Cu+2 (aq) + 2 OH- (aq)

ii. The molecular weight of Cu(OH)2 is 97.5 so 1.72

x10^-6 g equals 1.76x10^-8 moles. To calculate its

solubility in moles/liter we must divide it to total

volume that is 100 ml= 0.1 lt. Then the solubility is

1,76x10^-7

iii. Ksp= [Cu] x [OH]^2

[Cu]= 1,76 x 10^-7
[OH]= 3,52 x 10^-7 Thus when put in the equation

Ksp= 2.26 x 10^-20

b) i. When pH is 9.65 pOH is 4.65 then [OH]= 2.23 x 10^-5

Zn(OH)2 (s) >< Zn+2 (aq) + 2 OH- (aq)

-S +S 2.23 x 10^-5 + 2S

( 2.23 x 10^-5 + 2S)^2 x S = 7.7 x 10^-17

S=5.01 x 10^-10

ii. [Zn]= 0.05 moles
[OH]= 0.15 moles

Zn+2 (aq) + 2 OH- (aq) >< Zn(OH)2 (s)

0.5 1.5
-0.5 -1.0 ...

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