Q19.78P - Chemistry: Molecular Nature Of Matter And Change

Question

Which compound in each pair is more soluble in water? 

(a) Magnesium hydroxide or nickel (II) hydroxide 

(b) Lead (II) sulphide or copper (II) sulphide 

(c) Silver sulphate or magnesium fluoride

Step-by-Step Solution

Answer

(a) Magnesium hydroxide is more soluble in water than Nickel (II)   hydroxide.

(b)Lead (II) sulphide is more soluble in water than Copper (II)   sulphide.

(c)Silver sulphate is more soluble in water than magnesium fluoride.

Concept Introduction

Q_sp - ion-product expression; Q_sp value is obtained when the concentrations of ions formed by dissolution of some compound are multiplied. When the solution is saturated Q_sp  value is called K_sp  value (solubility-product constant).

${\mathbf{\text{MX}}}_{\mathbf{\text{2}}}\mathbf{\to }{\mathbf{\text{M}}}^{\mathbf{\text{2 + }}}{\mathbf{\text{ + 2X}}}^{\mathbf{\text{ - }}}$

The solid and liquid state is not included in the K_sp equations.

${\mathbf{\text{K}}}_{\mathbf{\text{sp}}}{\mathbf{\text{ = [M}}}^{\mathbf{\text{2 + }}}{\mathbf{\text{][X}}}^{\mathbf{\text{ - }}}{\mathbf{\text{]}}}^{\mathbf{\text{2}}}$

S (Molar solubility) is equal to the concentration of one mol of the ion formed by the dissolution of some compound.

Compound More Soluble between Mg(OH) 2 and role="math" localid="1663353650637" Ni(OH) 2

(a)

To compare the solubility of each compound, just compare their K_sp values.

The K_sp  value of Magnesium Hydroxide $\mathrm{MgOH}{)}_2$is –

${\mathrm{K}}_{\mathrm{sp}}=6.3\times {10}^{-10}$

The K_sp   value of Nickel (II) Hydroxide ${\text{Ni(OH)}}_{\text{2}}$ is –

${\mathrm{K}}_{\mathrm{sp}}=6\times {10}^{-16}$

It can be observed that the K_sp of ${\text{Mg(OH)}}_{\text{2}}\text{ > }$K_sp of ${\text{Ni(OH)}}_{\text{2}}$. 

 

Therefore, ${\text{Mg(OH)}}_{\text{2}}$is more soluble in water.

Compound More Soluble between PbS and CuS

(b)

To compare the solubility of each compound, just compare their K_sp values.

The K_sp value of Lead (II) Sulphide $\mathrm{PbS}$ is –

${\mathrm{K}}_{\mathrm{sp}}=3\times {10}^{-25}$

The K_sp value of Copper (II) Sulphide $\mathrm{CuS}$ is –

${\mathrm{K}}_{\mathrm{sp}}=8\times {10}^{-34}$

It can be observed that the K_sp of $\mathrm{PbS}$ > K_sp of $\mathrm{CuS}$. 

 

Therefore, is $\mathrm{PbS}$ more soluble in water.

Compound More Soluble between Ag 2 SO 4 and MgF 2

(c)

To compare the solubility of each compound, just compare their K_sp values.

The K_sp  value of Silver Sulphate ${\text{Ag}}_{\text{2}}{\text{SO}}_{\text{4}}$ is –

${\mathrm{K}}_{\mathrm{sp}}=1.5\times {10}^{-5}$

The K_sp  value of Magnesium Fluoride ${\text{MgF}}_{\text{2}}$is –

${\mathrm{K}}_{\mathrm{sp}}=7.4\times {10}^{-9}$

It can be observed that the K_sp  of ${\text{Ag}}_{\text{2}}{\text{SO}}_{\text{4}}$> K_sp  of ${\text{MgF}}_{\text{2}}$. 

 

Therefore, ${\text{Ag}}_{\text{2}}{\text{SO}}_{\text{4}}$ is more soluble in water.