The mole (abbreviated mol) is the SI unit for the amount of substance. It is defined as the amount of a substance that contains the same number of entities as the number of atoms in 12 g of carbon-12.

This number called Avogadro’s number (in honor of the 19th-century Italian physicist Amedeo Avogadro) is enormous:

• 1 mole (1 mol) contains 6.022×1023 entities (to four significant figures)

A counting unit, such as a dozen, gives you the number of items but not their mass; a mass unit, such as a kilogram, tells you the mass but not the number of objects.

The mole tells you both—the number of things in a given quantity of substance and the number of objects in a given mass of the substance.

• 1 mol of carbon-12 contains 6.022×1023 carbon-12 atoms and has a mass of 12 g

When we mix multiple chemicals to perform a reaction, knowing the amount (in moles), mass (in grams), and a number of entities becomes critical. The fundamental connection between weights on the atomic and macroscopic scales.

The same holds true for elements and compounds: Elements are a type of element. The mass of one atom of an element in atomic mass units (AMU) equals.

The same numerically as the mass in grams (g) of one mole of the element's atoms. The periodic table gives the atomic mass.

The mole lets us relate the number of entities to the mass of a sample of those entities.

The mole maintains the same numerical relationship between mass on the atomic scale (atomic mass units, AMU) and mass on the macroscopic scale (grams, g).

A grocer does not know that there are 1 dozen eggs based on their weight or that there is 1 kilogram of coffee beans based on their count in daily words since eggs and coffee beans do not have fixed masses.

However, a chemist can determine the number of copper atoms by weighing 63.55 g (1 mol) of copper, because all copper has 6.0221023 atoms.

The atomic mass of an atom is 63.55 amu.

1 One mole (6.022×1023 entities) of some familiar substances. From left to right: 1 mol of copper (63.55 g), of liquid H2O (18.02 g), sodium chloride (table salt, 58.44 g), sucrose (table sugar, 342.3 g), and of aluminum (26.98 g).

Components. Look up the atomic mass and note if the element is monatomic or molecular to get the molar mass.

Elements with a single atom. The periodic table is the molar mass of a monatomic element. The value expressed in grams per mole* The molar mass of neon, for example, is 20.18 g/mol. Gold has a molar mass of 197.0 g/mol.

Elements of a molecular nature. To calculate the molar mass of a molecular element, you must first know its formula (see Figure 2.16).

For example, oxygen is the most abundant element in the air. O2 is most often found as a diatomic molecule, therefore its molar mass is twice that of oxygen.

• Molar mass (ℳ) of O2 = 2 × ℳ of O = 2 × 16.00 g/mol = 32.00 g/mol

The most common form of sulfur exists as octatomic molecules, S8:

• ℳ of S8 = 8 × ℳ of S = 8 × 32.06 g/mol = 256.5 g/mol

A mole of material is the number of chemical entities that contain Avogadro's number (6.0221023). (atoms, ions, molecules, or formula units).

The mass (in grams) of a mole of a certain entity (atom, ion, molecule, or formula unit) has been calculated. The same numerical number as the entity's mass (in AMU) As a result, the mole enables us to count Entities by weighing them.

Using an element's (or compound's) molar mass (M, g/mol) and Avogadro's number as a starting point we can convert between quantity (mol), mass (g), and a number of entities using conversion factors.

The mass fraction of element X in a compound is used to calculate the mass of X in a particular compound.

Mass analysis is used to derive the empirical formula. It displays the element's lowest entire number of moles, and hence the relative number of atoms.

For example, one part by mass of hydrogen is present in hydrogen peroxide for every 16 parts oxygen by mass.

Because hydrogen's atomic mass equals the atomic mass of hydrogen is 1.008 amu, while the atomic mass of oxygen is 16.00 amu; there is one H atom for every O atom. (a H/O atom ratio of one to one). As a result, the empirical formula is HO.

• The term molecular formula shows the actual number of atoms of each element in a molecule: the molecular formula of hydrogen peroxide is H2O2, twice the empirical formula. Notice that the molecular formula exhibits the same 1/1 H/O atom ratio as in the empirical formula.

• The term structural formula also shows the relative placement and connections of atoms in the molecule: the structural formula of hydrogen peroxide is H⏤O⏤O⏤H.

If we know the molar mass of a compound, we may apply the empirical formula to get the molecular formula, which uses the actual numbers of moles of each element in 1 mol of the compound as subscripts. For some substances, such as water (H2O),

The empirical and molecular formula for ammonia (NH3) and methane (CH4) are identical, but for many others, the molecular formula is a whole-number multiple of the empirical formula.

The empirical formula for hydrogen peroxide is HO. Using the empirical formula, divide the molar mass of hydrogen peroxide (34.02 g/mol) by The mass of HO (17.01 g/mol) produces a whole-number multiple:

• Whole-number multiple = compound molar mass (g/mol)/empirical formula mass (g/mol) = 34.02 g/mol/ 17.01 g/mol = 2.000 = 2

Since the molar mass of hydrogen peroxide is twice as large as the empirical formula mass, the molecular formula has twice the number of atoms as the empirical formula.