   Chapter 17, Problem 120SCQ

Chapter
Section
Textbook Problem

Aluminum hydroxide reacts with phosphoric acid to give AlPO4. The substance is used industrially in adhesives, binders, and cements. (a) Write the balanced equation for the preparation of AlPO4 from aluminum hydroxide and phosphoric acid. (b) If you begin with 152 g of aluminum hydroxide and 3.00 L of 0.750 M phosphoric acid, what is the theoretical yield of AlPO4? (c) If you place 25.0 g of AlPO4 in 1.00 L of water, what are the concentrations of Al3+ and PO43− at equilibrium? (Neglect hydrolysis of aqueous Al3+ and PO43− ions.) Ksp for AlPO4 is 1.3 × l0−20. (d) Does the solubility of AlPO4 increase or decrease on adding HCl? Explain.

(a)

Interpretation Introduction

Interpretation:

A balanced chemical equation for the preparation of AlPO4 has to be written.

Concept introduction: The metals of group 3A form +3 ions because the highest oxidation number is always equal to the group number of that element. The non-metal gains these electrons to form anions with 1 and 2 charge.

The stoichiometric coefficients are multiplied with the compounds in the chemical equation to have an equal number of atoms on both sides of the equation. The equation is known as a balanced chemical equation if there are an equal number of atoms of each element in both the product and reactant side.

Explanation

The reaction of aluminum hydroxide with phosphoric acid is an example of double displacement reaction. Each reactant dissociates to form corresponding ions in the solution and then the exchange of ions between two reactant occurs on the basis of their charges either cation or anion to form a product.  Therefore the chemical reaction is given as,

Al(

(b)

Interpretation Introduction

Interpretation:

The theoretical yield of AlPO4 has to be calculated.

Concept introduction: Theoretical yield is the maximum amount of product that can be obtained from a chemical reaction such that one of the reactants is completely consumed called limiting reagent. The maximum amount of product formed can be predicted from the stoichiometric coefficient of the limiting reagent in the balanced chemical reaction.

(c)

Interpretation Introduction

Interpretation:

The concentration of Al3+ and PO43 at equilibrium has to be calculated.

Concept introduction:

Solubility product constant Ksp is an equilibrium constant and is defined as the product of the equilibrium concentration of the ions of the salt raised to the power of their coefficients in the balanced chemical equation.

For example, general salt AxBy when dissolved in water dissociates as,

AxBy(s)xAy+(aq)+yBx(aq)

The expression for Ksp of a salt is,

Ksp=[Ay+]x[Bx]y

(d)

Interpretation Introduction

Interpretation:

The solubility of AlPO4 increase or decrease upon addition of HCl to the solution has to be calculated.

Concept introduction: The solubility of insoluble salts containing anions (conjugate base of a weak acid) such as hydroxide, acetate, carbonate, phosphate and sulfide increases in the presence of strong acid than in pure water. Whereas the solubility of insoluble salts containing anions (conjugate base of a strong acid) such as chlorides decreases in the presence of strong acid than in the pure water.

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