Molar Mass of Compounds



 

Learning Objective

  • Calculate the molar mass of an element or compound

Key Points

    • The molar mass is the mass of a given chemical element or chemical compound (g) divided by the amount of substance (mol).
    • The molar mass of a compound can be calculated by adding the standard atomic masses (in g/mol) of the constituent atoms.
    • Molar mass serves as a bridge between the mass of a material and the number of moles since it is not possible to measure the number of moles directly.

Terms

  • moleThe amount of substance of a system that contains as many elementary entities as there are atoms in 12 g of carbon-12.
  • molar massThe mass of a given substance (chemical element or chemical compound in g) divided by its amount of substance (mol).

Measuring Mass in Chemistry

Chemists can measure a quantity of matter using mass, but in chemical reactions it is often important to consider the number of atoms of each element present in each sample. Even the smallest quantity of a substance will contain billions of atoms, so chemists generally use the mole as the unit for the amount of substance.

One mole (abbreviated mol) is equal to the number of atoms in 12 grams of carbon-12; this number is referred to as Avogadro’s number and has been measured as approximately 6.022 x 1023. In other words, a mole is the amount of substance that contains as many entities (atoms, or other particles) as there are atoms in 12 grams of pure carbon-12.

amu vs. g/mol

Each ion, or atom, has a particular mass; similarly, each mole of a given pure substance also has a definite mass. The mass of one mole of atoms of a pure element in grams is equivalent to the atomic mass of that element in atomic mass units (amu) or in grams per mole (g/mol). Although mass can be expressed as both amu and g/mol, g/mol is the most useful system of units for laboratory chemistry.

Calculating Molar Mass

Molar mass is the mass of a given substance divided by the amount of that substance, measured in g/mol. For example, the atomic mass of titanium is 47.88 amu or 47.88 g/mol. In 47.88 grams of titanium, there is one mole, or 6.022 x 1023 titanium atoms.

The characteristic molar mass of an element is simply the atomic mass in g/mol. However, molar mass can also be calculated by multiplying the atomic mass in amu by the molar mass constant (1 g/mol). To calculate the molar mass of a compound with multiple atoms, sum all the atomic mass of the constituent atoms.

For example, the molar mass of NaCl can be calculated for finding the atomic mass of sodium (22.99 g/mol) and the atomic mass of chlorine (35.45 g/mol) and combining them. The molar mass of NaCl is 58.44 g/mol.

Molar Mass Calculations – YouTubeThis video shows how to calculate the molar mass for several compounds using their chemical formulas.