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Questions and Answers

What must the product of ion concentrations satisfy in a saturated solution?

  • The molar solubility
  • The equilibrium constant for all reactions
  • The Ksp expression (correct)
  • The ionization constant

In order to compare the solubilities of two salts using their Ksp values, what condition must be met?

  • The solubilities must be identical
  • The salts must produce the same number of ions (correct)
  • The salts must be of the same mass
  • The salts must have equal Ksp values

If two salts have Ksp values 7.5 x 10-10 and 3.0 x 10-8 respectively, what can be inferred about their solubilities?

  • Solubility cannot be compared directly (correct)
  • Both salts are equally soluble
  • The salt with the lower Ksp is more soluble
  • The salt with the higher Ksp is always more soluble

Which of the following salts produces two ions when dissolved?

<p>Ag2S (D)</p> Signup and view all the answers

When comparing salts that dissolve to form different numbers of ions, what conclusion can be drawn about their Ksp values?

<p>Solubility cannot be directly inferred from Ksp values (A)</p> Signup and view all the answers

Which of these salts has the lowest solubility based on its Ksp value of 1.1 x 10-73?

<p>Bi2S3 (A)</p> Signup and view all the answers

Why can't Ksp values be used directly to predict relative solubilities of different salts?

<p>Different salts can produce different amounts of ions (A)</p> Signup and view all the answers

What is the relationship between a solute’s molar solubility and Ksp?

<p>Molar solubility can be determined from Ksp (D)</p> Signup and view all the answers

How does the solubility of Mg(OH)2 change in acidic versus basic solutions?

<p>Mg(OH)2 is soluble only in acidic solutions. (A)</p> Signup and view all the answers

What effect does adding NH4+ to a solution containing NH3 have on the precipitation of Mg(OH)2?

<p>It decreases the precipitation of Mg(OH)2. (B)</p> Signup and view all the answers

Why does the solubility of silver phosphate (Ag3PO4) increase in acidic solutions?

<p>The reaction with H+ decreases PO43- concentration. (D)</p> Signup and view all the answers

How does the pH of a solution influence the solubility of salts containing basic anions?

<p>Decreased pH often increases the solubility of basic anions. (C)</p> Signup and view all the answers

What is the solubility behavior of silver chloride (AgCl) in acidic solutions?

<p>It has the same solubility in acid as in pure water. (B)</p> Signup and view all the answers

What is the primary method used to separate ions in a solution when they can precipitate with the same reagent?

<p>Fractional precipitation technique. (D)</p> Signup and view all the answers

Which of the following anions is NOT considered an effective base, potentially increasing salt solubility in acidic solutions?

<p>Cl- (B)</p> Signup and view all the answers

Which of the following salts is expected to be more soluble in acidic solutions?

<p>Mg(OH)2 (A), ZnCO3 (D)</p> Signup and view all the answers

Which factor directly influences the solubility Product Constant (Ksp) for a salt in solution?

<p>The temperature of the solution. (C)</p> Signup and view all the answers

The solubility product constant (Ksp) is best described as:

<p>The product of the concentrations of the ions in a saturated solution. (A)</p> Signup and view all the answers

How do weak acids influence the solubility of salts containing their corresponding anions?

<p>They increase the solubility of the salts by shifting the equilibrium. (A)</p> Signup and view all the answers

In the dissolution of Mg(OH)2, what occurs when hydronium ions are added to the solution?

<p>The solubility of Mg(OH)2 increases as OH- ions are consumed. (A)</p> Signup and view all the answers

What is a key characteristic of common anions that are effective bases?

<p>They generally increase solubility in acidic solutions. (B)</p> Signup and view all the answers

Which statement best conveys the limitations of using Ksp for solubility predictions?

<p>Ksp is useful for order-of-magnitude predictions but may lack high accuracy. (D)</p> Signup and view all the answers

What is the expected outcome when a solution of AgNO3 is added slowly to a mixture containing CrO42- and Br- ions?

<p>AgBr precipitates first, leaving CrO42- in the solution. (D)</p> Signup and view all the answers

What drives the dissolution of slightly soluble salts like Mg(OH)2 in acidic conditions?

<p>Le Châtelier’s principle indicates that equilibrium shifts toward dissolution. (A)</p> Signup and view all the answers

Flashcards

Solubility Product Constant (Ksp)

The equilibrium constant for the dissolution of a sparingly soluble salt. It represents the product of the ion concentrations raised to the power of their stoichiometric coefficients in the balanced dissolution equation.

Molar Solubility

The concentration of a dissolved substance in a saturated solution measured in mol/L. Indicates how much of a substance can dissolve in a given amount of solvent (water).

Relative Solubilities

Comparing the solubilities of different salts. The Ksp value alone doesn't give the complete picture, especially when the salts produce different numbers of ions in solution.

Same Number of Ions

Solubilities of salts that produce the same number of ions in solutions can be directly compared by their Ksp values.

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Different Number of Ions

If salts generate different numbers of ions when dissolved, direct comparison of Ksp values is NOT a valid way to predict relative solubilities.

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Ksp Comparison

A method of determining relative solubility of compounds with the same number of ions in solutions using Ksp values from a balanced dissolution equation. Higher Ksp = greater solubility.

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Saturated Solution

A solution in which no more solute can be dissolved at a given temperature and pressure.

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Solubility

The ability of a substance to dissolve in another substance

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Solubility and pH

Solubility of ionic compounds can be affected by pH. Acidic solutions can increase the solubility of salts with anions that are effective bases.

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Mg(OH)2 solubility

Magnesium hydroxide is more soluble in acidic solutions than in basic solutions due to the reaction of H+ with OH-.

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Ag3PO4 solubility in acid

Silver phosphate solubility increases in acidic solutions due to H+ reacting with PO43- thereby lowering its concentration. This shift the equilibrium right on the overall solubility.

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AgCl solubility in acid

Silver chloride solubility is the same in acidic and neutral solutions. Since chloride is a weak base, adding H+ does not affect [Cl-], thus solubility remains constant.

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Effective base anion

An anion is an effective base if its conjugate acid is weak (HX is a weak acid).

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Solubility equilibrium

The equilibrium between a solid ionic compound and its dissolved ions in solution.

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Buffer solution

A solution that resists changes in pH when small amounts of acid or base are added.

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Ksp

Solubility product constant; equilibrium constant for the dissolution of a slightly soluble ionic compound.

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Limitations of Ksp

While Ksp values give an estimate of solubility, they may not be highly accurate in all cases.

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Fractional Precipitation

Separating ions that can be precipitated by the same reagent by controlling the addition of the reagent.

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Solubility and pH

pH affects the solubility of salts, especially those with basic anions.

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Mg(OH)2 and pH

Mg(OH)2, a basic salt, is more soluble in acidic solutions because of OH- ion reaction with H3O+.

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Solubility and Weak Acids

Weak acids increase the solubility of salts containing conjugate bases of weak acids.

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Fractional vs. Full Precipitation

Fractional precipitation separates ions (using slow addition), full precipitation precipitates everything.

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Mixed Precipitate

Adding a reagent to solutions with multiple ions capable of precipitation can form mixed precipitate of both types.

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Le Chatelier's Principle

Changes in conditions (like pH) shift equilibrium to counteract the change.

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Study Notes

Solubility Equilibria

  • Solubility equilibria describes the dynamic equilibrium between a solid solute and its dissolved ions in a solution.
  • A saturated solution exists when the rates of dissolution and crystallization are equal.
  • The concentration of a saturated solution is its solubility.
  • Solubility varies with temperature.
  • A solubility-temperature graph is a solubility curve.
  • Most ionic substances' solubility increases with temperature.
  • Exceptions include anions such as SO32-, SO42-, AsO43-, and PO43-.
  • An unsaturated solution contains less solute than a saturated solution.
  • A supersaturated solution contains more solute than a saturated solution. Supersaturated solutions are unstable and will precipitate.

Saturated Solutions

  • The concentration of a saturated solution is called its solubility.
  • Solubility varies with temperature.
  • A solubility-temperature graph is called a solubility curve.
  • Solubility for various salts is shown on graphs, each line representing a different substance.

Aqueous Solubility

  • Aqueous solubility is the amount of solute that can dissolve in 100 g of water at a given temperature.
  • Solubility can be expressed in various ways such as molarities, mass percentages, or grams of solute/100 g of water.
  • Points on the solubility curve represent saturated solutions. Areas above the curve represent supersaturated solutions and below unsaturated solutions.

Solubility

  • Generally, solubilities of ionic substances increase with increasing temperature.
  • Compounds containing anions (SO32-, SO42-, AsO43-, and PO43-) are exceptions.
  • Adding solute to a solvent, with less than the amount needed for saturation, can result in an unsaturated solution, which the additional solute will dissolve.
  • A saturated solution can be prepared by dissolving the solute at a high temperature and then allowing it to cool slowly.

Supersaturated Solutions

  • Usually, excess solute crystallizes from a solution, but occasionally all the excess solute may stay in solution.
  • Supersaturated solutions are unstable.
  • If crystals of solute are added, the excess solute crystallizes.

Le Châtelier's Principle

  • A system at equilibrium subjected to a change will spontaneously shift to counteract the change.
  • Adding a product causes the equilibrium to shift to the left (reactants).
  • Adding a reactant causes the equilibrium to shift to the right (products).

The Solubility Product Constant, Ksp

  • The equilibrium constant for the reaction in which a solid salt dissolves to yield its constituent ions in solution.
  • The concentration of the solid is omitted from the equilibrium constant expression because it is in its standard state.
  • Example: CaSO4 (s) ⇌ Ca2+(aq) + SO42-(aq).
  • Ksp = [Ca2+] [SO42-]
  • Ksp = 9.1 × 10−6 (at 25°C)

Solubility vs. Solubility Product

  • Solubility is an equilibrium position for a solution; a specific solubility value for a salt at particular temperatures.
  • The solubility product, Ksp, is a constant value for a specific salt, at any specific temperature. The solubility product depends on ion activities.

The Relationship Between Solubility and Ksp

  • There's a relationship between the solubility product constant (Ksp) and the molar solubility of a solute.
  • Molar solubility is a solute's molarity in a saturated aqueous solution.

Relative Solubilities

  • Compare Ksp values only for salts that have the same number of ions (e.g., AgCl, Cul).
  • The larger the Ksp value, the more soluble the salt.

Fractional Precipitation

  • Fractional precipitation is a technique used to separate two or more ions that can be precipitated by the same reagent.
  • The rate of reagent addition is controlled to selectively precipitate different ions in a solution.
  • The slower the reagent addition, the greater the chance the less soluble of the two products will precipitate first.

Solubility and pH

  • The pH of a solution can greatly affect the solubility of a salt.
  • This is especially true for salts whose anions are the conjugate bases of weak acids or the base OH⁻ itself.
  • An example is Mg(OH)2, which is highly insoluble in water and is a common antacid commonly known as milk of magnesia.
  • Hydroxide ions from magnesium hydroxide react with hydronium ions from stomach acid to form water.
  • Increasing acidity increases the solubility of salts containing basic anions.

Incomplete Dissociation of Solute into Ions

  • In solubility calculations, assume complete dissociation into ions, but this is often not a valid assumption.
  • Solute might not be 100% ionic and some may dissolve as molecules or as ion pairs.

Simultaneous Equilibria

  • The reversible reaction between a solid solute and its ions in solution, is never the only process.
  • Other simultaneous processes occurring include water autoionization and reactions occurring between solute ions and other solution species. These may include acid-base reactions and complex ion formation.

Limitations of Ksp

  • Ksp values are often limited to slightly soluble or essentially insoluble solutes.
  • When calculating Ksp it is important to consider if other equilibria are occurring simultaneously
  • Experimental solubility values can differ from calculated Ksp values, as the calculated ones are based on assuming complete dissociation.
  • Molarities are often used in place of activities to simplify calculations.

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