Solutions, Dilutions, and Electrolytes in General Chemistry

Solutions and Solution Concentrations

Defining Solutions and Molarity

In chemistry, a solution is a homogeneous mixture composed of two or more substances. The solute is the substance dissolved, and the solvent is the substance doing the dissolving (often water in aqueous solutions). Molarity (M) is a common unit of concentration, defined as moles of solute per liter of solution.

• Molarity (M): $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Example: To make 500.0 mL of 0.100 M $(NH_4)_3PO_4$ solution, calculate the required grams using molarity and molar mass.

Visualizing Solutions

When drawing molecules in a beaker, represent the correct number and type of ions or molecules present in the solution. For example, two molecules of aqueous $(NH_4)_3PO_4$ would dissociate into their respective ions.

• Example: $(NH_4)_3PO_4$ dissociates into 3 $NH_4^+$ ions and 1 $PO_4^{3-}$ ion per formula unit.

Total Ion Concentration

The total ion concentration in a solution is the sum of the concentrations of all ions present after dissociation.

• Example: In a 2.0 M $(NH_4)_3PO_4$ solution, the total ion concentration is calculated by considering the stoichiometry of dissociation:

$\text{(NH}_4\text{)}_3\text{PO}_4 \rightarrow 3\ NH_4^+ + 1\ PO_4^{3-}$ Total ion concentration = $2.0\ \text{M} \times (3 + 1) = 8.0\ \text{M}$

Solution Dilutions

Concept and Calculations

Solutions are often supplied in concentrated form and must be diluted before use. Dilution involves adding solvent to decrease the concentration of solute.

• Key Principle: The number of moles of solute before and after dilution remains the same.

• Dilution Equation: $M_1V_1 = M_2V_2$

• Where: $M_1$ and $V_1$ are the initial molarity and volume; $M_2$ and $V_2$ are the final molarity and volume.

• Example: To prepare 250.0 mL of 0.10 M HCl from 6 M HCl, use the dilution equation to solve for the required volume of concentrated HCl.

Mixing Solutions and Ion Concentrations

When mixing solutions, calculate the new molarity for each ion by considering the total moles and the final combined volume.

• Example: Mixing 100.0 mL of 0.300 M $NH_4)_2CrO_4$ and 50.0 mL of 0.500 M $(NH_4)_3PO_4$—find the new molarity for each ion.

Multi-Component Reactions in Solution

For reactions involving multiple reactants and products in solution, determine the concentration of each ion at the end of the reaction by calculating the limiting reagent and the stoichiometry of the reaction.

• Example Reaction:

$2\ MnO_4^- + 5\ H_2O_2 + 6\ H^+ \rightarrow 2\ Mn^{2+} + 5\ O_2 + 8\ H_2O$

• Calculate initial moles, determine the limiting reactant, and use stoichiometry to find final concentrations.

Electrolytes and Ion Dissociation

Electrolytes: Strong, Weak, and Nonelectrolytes

When ionic compounds dissolve in water, they dissociate into ions, allowing the solution to conduct electricity. The ability to conduct electricity classifies solutions as strong electrolytes, weak electrolytes, or nonelectrolytes.

• Strong Electrolytes: Completely dissociate into ions (e.g., NaCl, strong acids).

• Weak Electrolytes: Partially dissociate; only some molecules form ions (e.g., weak acids).

• Nonelectrolytes: Do not form ions in solution (e.g., sugar).

Ion-Dipole Interactions and Dissociation

Water molecules surround and separate ions due to ion-dipole interactions. The partial positive and negative charges on water molecules allow them to compete with the ionic bonds in a crystal lattice, leading to dissociation.

• Example: NaCl dissolves in water as $Na^+$ and $Cl^-$ ions, each surrounded by water molecules.

Strong and Weak Acids

Strong acids (e.g., HCl, HNO3, H2SO4) completely ionize in water, producing a large number of ions. Weak acids (e.g., acetic acid) only partially ionize, resulting in fewer ions in solution.

• Strong Acid Equation: $HA \rightarrow H^+ + A^-$

• Weak Acid Equation: $HA \rightleftharpoons H^+ + A^-$

• Key Point: Strong acids are strong electrolytes; weak acids are weak electrolytes.

Table: Classification of Electrolytes

Type	Example	Dissociation in Water	Conductivity
Strong Electrolyte	NaCl, HCl	Complete	High
Weak Electrolyte	CH3COOH	Partial	Low
Nonelectrolyte	C6H12O6 (glucose)	None	None

Summary of Key Concepts

• Solution concentration is most commonly expressed as molarity (M).

• Dilution does not change the number of moles of solute, only the volume and concentration.

• Electrolytes are classified by their ability to dissociate and conduct electricity in solution.

• Strong acids and strong bases are strong electrolytes; weak acids and weak bases are weak electrolytes.

Additional info: The six strong acids to memorize are HCl, HBr, HI, HNO3, HClO4, and H2SO4.