Worksheet: Controlling Precipitation with Common Ions

In this worksheet, we will practice calculating the concentrations of common ions required to trigger or prevent the precipitation of a salt.

Q1:

Silver chloride, AgCl, has a very low solubility in water, as shown. AgCl()Ag()+Cl()saqaq+–sp𝐾=1.6×10Adding ammonia significantly increases the solubility of AgCl because a complex ion is formed, as shown. Ag()+2NH()[Ag(NH)]()+332+faqaqaq𝐾=1.7×10

What mass of NH3 is required to prepare 1.00 L of a solution that will dissolve 2.00 g of AgCl by formation of [Ag(NH)]32+?

Q2:

Unexposed silver halides are removed from photographic film when they react with sodium thiosulfate (NaSO)223 to form the complex ion [Ag(SO)]2323–. AgBr()Ag()+Br()Ag()+2SO()[Ag(SO)]()saqaqaqaqaq+–sp+232–2323–f,𝐾=5.0×10,𝐾=4.7×10What mass of NaSO223 is required to prepare 1.00 L of a solution that will dissolve 1.00 g of AgBr by the formation of Ag(SO)2323–?

Q3:

The concentration of Zn2+ ions in a 0.10 M aqueous HS2 solution is 0.050 M. What is the minimum concentration of HO3+ ions required to prevent the precipitation of ZnS? ZnS()Zn()+S()2HO()+HS()2HO()+S()saqaqlaqaqaq+22–sp223+2–𝐾=1.0×10𝐾=1.0×10

Q4:

The concentration of Mg2+ ions in a 0.10 M aqueous ammonia solution is 0.10 M. What is the minimum concentration of ammonium ions required to prevent the precipitation of Mg(OH)2? Mg(OH)()Mg()+2OH()NH()+HO()NH()+OH()22+–sp324+–bsaqaqaqlaqaq,𝐾=8.9×10,𝐾=1.8×10

Q5:

Will KClO4 precipitate when 20 mL of a 0.050 M solution of K+ is added to 80 mL of a 0.50 M solution of ClO4–? 𝐾sp for KClO4 is 1.05×10.

  • ANo, the reaction quotient, 𝑄, is greater than 𝐾sp, shifting the equilibrium position in favor of the dissolved ions.
  • BYes, the reaction quotient, 𝑄, is less than 𝐾sp, shifting the equilibrium position in favor of solid KClO4.
  • CNo, the reaction quotient, 𝑄, is less than 𝐾sp, shifting the equilibrium position in favor of the dissolved ions.
  • DYes, the reaction quotient, 𝑄, is greater than 𝐾sp, shifting the equilibrium position in favor of solid KClO4.
  • ENo, the reaction quotient, 𝑄, is equal to 𝐾sp, so the position of equilibrium does not shift in either direction.

Q6:

Does silver chloride precipitate when equal volumes of a 2.0×10 M solution of AgNO3 and a 2.0×10 M solution of NaCl are mixed? The solution also contains Na+ and NO3– ions, but sodium nitrate is very soluble and does not form a precipitate. 𝐾sp for AgCl is 1.6×10.

  • ANo, the reaction quotient, 𝑄, is greater than 𝐾sp, shifting the equilibrium position in favor of the dissolved ions.
  • BYes, the reaction quotient, 𝑄, is less than 𝐾sp, shifting the equilibrium position in favor of solid silver chloride.
  • CNo, the reaction quotient, 𝑄, is equal to 𝐾sp, so the position of equilibrium does not shift in either direction.
  • DNo, the reaction quotient, 𝑄, is less than 𝐾sp, shifting the equilibrium position in favor of the dissolved ions.
  • EYes, the reaction quotient, 𝑄, is greater than 𝐾sp, shifting the equilibrium position in favor of solid silver chloride.

Q7:

CaHPO4 has a solubility product of 7×10. Will calcium hydrogen phosphate precipitate from a solution with [Ca]2+ = 0.0001 M and [HPO]42– = 0.001 M?

  • AYes, the reaction quotient, 𝑄, is greater than 𝐾sp, shifting the equilibrium position in favor of solid calcium hydrogen phosphate.
  • BNo, the reaction quotient, 𝑄, is equal to 𝐾sp, so the position of equilibrium does not shift in either direction.
  • CCalcium hydrogen phosphate is too insoluble for a solution to form.
  • DYes, the reaction quotient, 𝑄, is greater than 𝐾sp, shifting the equilibrium position in favor of the dissolved ions.
  • ENo, the reaction quotient, 𝑄, is less than 𝐾sp, shifting the equilibrium position in favor of the dissolved ions.

Q8:

The first step in the preparation of magnesium metal is the precipitation of Mg(OH)2 from sea water by the addition of lime, Ca(OH)2, a readily available inexpensive source of OH– ions. Mg(OH)()Mg()+2OH()22+–spsaqaq,𝐾=8.9×10 The concentration of Mg()2+aq in sea water is 0.0537 M. Will Mg(OH)2 precipitate when enough Ca(OH)2 is added to give a hydroxide ion concentration of 0.0010 M?

  • ANo, the reaction quotient, 𝑄, is less than 𝐾sp, shifting the equilibrium position in favor of the dissolved ions.
  • BYes, the reaction quotient, 𝑄, is greater than 𝐾sp, shifting the equilibrium position in favor of solid Mg(OH)2.
  • CNo, the reaction quotient, 𝑄, is greater than 𝐾sp, shifting the equilibrium position in favor of solid Mg(OH)2.
  • DYes, the reaction quotient, 𝑄, is less than 𝐾sp, shifting the equilibrium position in favor of solid Mg(OH).2
  • ENo, the reaction quotient, 𝑄, is greater than 𝐾sp, shifting the equilibrium position in favor of the dissolved ions.

Q9:

What concentration of [Ag]+ is required to reduce [CO]32– to 2.8×10 M by precipitation of AgCO23? The 𝐾sp of AgCO23 is 8.1×10.

  • A2.9×10 M
  • B4.0×10 M
  • C4.8×10 M
  • D6.6×10 M
  • E1.7×10 M

Q10:

What is the concentration of Ag+ required to begin precipitation of AgCO23 in a solution that is 2.50×10 M in CO32–? The 𝐾sp of AgCO23 is 8.1×10.

  • A1.3 M
  • B7.8×10 M
  • C4.6×10 M
  • D1.8×10 M
  • E3.2×10 M

Q11:

What is the concentration of F– required to begin precipitation of CaF2 in a solution with a Ca2+ concentration of 0.015 M? The 𝐾sp of CaF2 is 4.0×10.

  • A2.7×10 M
  • B5.2×10 M
  • C3.3×10 M
  • D7.1×10 M
  • E1.9×10 M

Q12:

What concentration of [F]– is required to reduce [Ca]2+ to 1.0×10 M by precipitation of CaF2? The 𝐾sp of CaF2 is 4.0×10.

  • A4.0×10 M
  • B6.3×10 M
  • C5.1×10 M
  • D4.0×10 M
  • E4.0×10 M

Q13:

A 0.800 L sample of a 2.8×10 M  Ba(NO)32 solution is added to 0.200 L of 4.8×10 M LiSO24. Does BaSO4 precipitate? The 𝐾sp of BaSO4 is 2.3×10.

  • ANo, the concentrations of SO42– and Ba2+ are not high enough: 𝑄<𝐾sp.
  • BYes, the concentrations of SO42– and Ba2+ are high enough: 𝑄>𝐾sp.
  • CYes, the concentrations of SO42– and Ba2+ are just high enough to enable precipitation: 𝑄=𝐾sp.

Q14:

A solution contains 0.20 M of both Pb2+ and Ag+ ions. If Cl– ions are added to the solution, what is the concentration of Ag+ ions when PbCl2 begins to precipitate? The 𝐾sp of PbCl2 is 1.6×10 and that of AgCl is 1.6×10.

  • A1.6×10 M
  • B5.1×10 M
  • C7.1×10 M
  • D1.0×10 M
  • E1.8×10 M

Q15:

About 50% of kidney stones consist of calcium phosphate, Ca(PO)342. The typical mass of Ca2+ excreted in urine is 0.10 g per day. The typical amount of urine passed is 1.4 L per day. What is the maximum concentration of phosphate ions that urine can contain before a kidney stone begins to form? The 𝐾sp of Ca(PO)342 is 1.3×10.

  • A7.3×10 M
  • B1.5×10 M
  • C9.1×10 M
  • D2.3×10 M
  • E1.7×10 M

Q16:

The base dissociation constant, 𝐾b, of NH3 is 1.8×10 and the solubility product, 𝐾sp, of Mg(OH)2 is 8.9×10. A 50 mL solution containing 0.95 g   MgCl2 is mixed with an equal volume of 1.80 M aqueous NH3. What mass of NHCl4 must be added to the solution to prevent the precipitation of Mg(OH)2?

Q17:

Calcium ions in serum, the liquid component of blood, are necessary for coagulation. Adding potassium oxalate (KCO224) to blood samples prevents coagulation because most of the calcium is removed as a precipitate of CaCO24, which has a solubility product (𝐾sp) of 1.96×10. It is necessary to remove all but 1.0% of the calcium ions in serum in order to prevent coagulation. A blood sample contains 5.50 mL of serum with a calcium concentration of 89 mg/L. What mass of KCO224 is required to prevent coagulation of this blood sample?

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