# Composition of Substances and Solutions

## Overview

### Description

Avogadro's number, $6.022\times{10}^{23}$, is the number of particles of a substance per mole of that substance. A mole is the amount of a substance that contains as many particles as 12 grams of pure carbon-12. Molar mass, then, is the mass of one mole of a substance. It can be used to calculate the number of moles or molecules of a given substance when the mass of the substance is known. A compound's molecular formula shows the number of atoms of each element present in the compound, whereas its empirical formula shows only the ratio of elements in the compound. Percent composition is the percentage by mass of each element in a compound. Percent composition and molar mass can be used to determine a compound's molecular and empirical formulas. Molarity describes the concentration of an aqueous solution in which materials are dissolved in water. Other measurements, such as mass percent or parts per million, can also be used to describe solution concentrations.

### At A Glance

• Molar mass is the mass of a substance in a mole of that substance. It can be used to calculate the number of moles and molecules in a given amount of that substance.
• The elements that make up the molecules of a compound can be described by the number of atoms of each element in a molecule or by the ratio of atoms of the elements in a molecule.
• An aqueous solution is composed of a material dissolved in water. This type of solution can be described in terms of molarity, the number of moles of a solute dissolved in 1 liter of water. However, other measurements can be especially useful for solutions that contain complex solutes or multiple solutes, or when the solution is made of two liquids.
• Different ways are needed to describe solutions with different or multiple compositions. Solution concentrations can be described by mass, volume, mass-volume percent, parts per million, or parts per billion.