Solutions and Mixtures

Learning Objective 3.7.A Calculate the number of solute particles, volume, or molarity of solutions.

Quick Notes

A solution is a homogeneous mixture with uniform properties throughout.
A heterogeneous mixture has regions with different properties.
Solutions can be solid, liquid, or gas.
The most common way to describe a solution’s composition is molarity (M):
- M = moles of solute / liters of solution
Key conversions:
- moles = M × L
- volume (L) = moles / M
- M = moles / L

Full Notes

What Is a Solution?

A solution is a type of homogeneous mixture – a physical combination of substances with the same properties throughout the sample.

Example:Ethanol and water (CH₃CH₂OH in H₂O)

AP Chemistry diagram of a homogeneous mixture showing water molecules mixed uniformly with ethanol molecules at the particle level.

In contrast, a heterogeneous mixture (like sand in water or salad dressing) has parts with visibly different properties or compositions.

Example:Grease and water

AP Chemistry diagram of a heterogeneous mixture showing grease layer separate from water with different regions at the particle level.

Solute and Solvent

In any solution:

The solute is the substance being dissolved.
The solvent is the substance doing the dissolving (present in greater amount).

AP Chemistry solvation diagram showing solvent water surrounding and dispersing solute particles to form a solution.

Molarity (M)

Molarity is a way to express the concentration of a solution.

Formula:
AP Chemistry formula showing how to calcualte Molarity using volume and moles. M = moles of solute / liters of solution

Where:
M = molarity (mol/L)
moles = amount of solute
L = total volume of solution

Key relationships:

moles = M × L
L = moles / M

Note: Volume must always be in liters, not milliliters.

Worked Example

You dissolve 0.500 mol of NaCl in enough water to make 2.00 L of solution. What is the molarity?

Use the definition: M = moles / L
Substitute: M = 0.500 mol / 2.00 L
Calculate: M = 0.250 M

Answer: 0.250 M NaCl

Matt’s exam tip

Always check whether you're being asked for molarity, volume, or moles. It’s easy to mix them up. Also watch your units – make sure volume is in liters, not milliliters.

Units and Conversions

1000 mL = 1.00 L
Always convert volume to liters before using it in molarity calculations

Particle View of a Solution

The solute is broken into individual particles (ions or molecules)
These particles are evenly dispersed throughout the solvent
The concentration is the same in every part of the solution
This uniform distribution gives solutions their homogeneous character.

Summary

Solutions are homogeneous mixtures with consistent properties throughout.
The most common way to describe their composition is molarity, calculated using: M = moles of solute / liters of solution.
By manipulating this equation, you can calculate the number of moles, volume, or concentration of a solution.
This is essential for quantitative lab work and for preparing solutions accurately.