KEY2CHEM

Solutions

A solution is made of a solute (substance being dissolved) and a solvent (substance doing the dissolving). A solute will dissolve appreciably in a given solvent if the intermolecular forces between solute and solvent are relatively similar. This rule is often referred to as “like dissolves like”. For example, \(\require{mhchem}\ce{NaCl}\) will dissolve in \(\require{mhchem}\ce{H2O}\) because the cumulative ion-dipole interactions are stronger than the individual ionic forces and hydrogen bonds holding either \(\require{mhchem}\ce{NaCl}\) or \(\require{mhchem}\ce{H2O}\) together, respectively. 


Example 1.

Which solute is expected to be most soluble in \(\require{mhchem}\ce{H2O}\)?

A. \(\require{mhchem}\ce{CH3OH}\)

B. \(\require{mhchem}\ce{CO2}\)

C. \(\require{mhchem}\ce{CCl4}\)

 

Solution 

C. \(\require{mhchem}\ce{CH3OH}\)

\(\require{mhchem}\ce{CH3OH}\) and \(\require{mhchem}\ce{H2O}\) can hydrogen bond to one another, while the other solutes, which are nonpolar, can only display weaker dipole-induced dipole intermolecular forces as the strongest type of intermolecular force with \(\require{mhchem}\ce{H2O}\).


Example 2.

Which diagram best describes the interaction of solute with solvent with \(\require{mhchem}\ce{MgCl2}\) is dissolved in \(\require{mhchem}\ce{H2O}\)?

A. 

B. 

C. 

 

 

Solution

A. 

\(\require{mhchem}\ce{MgCl2}\) is an ionic compound and \(\require{mhchem}\ce{H2O}\) is a polar molecule (specifically, capable of hydrogen bonding). The strongest type of interparticle force will be ion-dipole interaction, with positively-charged \(\require{mhchem}\ce{Mg^2+}\) cation attracted to the partial negative (\(\delta^-\)) of the oxygen atom in water. The negatively-charged \(\require{mhchem}\ce{Cl-}\) anion is attracted to the partial positive (\(\delta^+\)) of the hydrogen atoms in water molecules.


Example 3.

Which solute is expected to be most soluble in carbon tetrachloride (\(\require{mhchem}\ce{CCl4}\))?

A. \(\require{mhchem}\ce{C6H6}\)

B. \(\require{mhchem}\ce{H2O}\)

C. \(\require{mhchem}\ce{NaCl}\)

 

 

Solution

A. \(\require{mhchem}\ce{C6H6}\)

\(\require{mhchem}\ce{CCl4}\) is a nonpolar solvent with dispersion forces as the strongest type of interparticle force. Other nonpolar substances are expected to be most soluble in \(\require{mhchem}\ce{CCl4}\).