Molar mass or molecular weight is a physical property that represents the mass of a substance divided by the amount of that substance. The common units are typically in grams per mole (g/mol), while the si units are in kilograms per mole (kg/mol).

**How to Calculate Molar Mass**

In order to determine the molar mass of a compound we must determine the number of atoms for the elements within a given compound.

For example, if we are asked to determine the molar mass of glucose, C_{6}H_{12}O_{6}, we can solve with steps shown below.

**STEP 1:** Count the **number of each element** within the compound to get 6 carbons, 12 hydrogens and 6 oxygens.

**STEP 2: **Find the **atomic mass** for each chemical element from the periodic table.

**STEP 3:** Multiply the number of each element by their atomic masses taken from the periodic table.

**STEP 4: **Add up all the masses you got from multiplication together to give the overall molar mass of the compound.

From simple compounds such as CO_{2}, H_{2}O, or ammonia to more complex compounds such as proteins, amino acids, acetone or acetic acid we now calculate their molar masses.

**PRACTICE 1:** Calculate the formula weight of aspirin, C_{9}H_{8}O_{4}, based on the given molecular formula.

Aspirin contains 9 carbon, 8 hydrogen and 4 oxygen atoms with a total mass of 180.154 g/mol.

**PRACTICE 2: **Calculate the molar mass of silver nitrate, AgNO_{3}.

Silver nitrate contains 1 silver, 1 nitrogen and 3 oxygen atoms with a total mass of 169.88 g/mol.

**PRACTICE 3: **Determine the molecular weight of ammonium phosphate, (NH_{4})_{3}PO_{4}.

Ammonium phosphate contains 3 nitrogens, 12 hydrogens, 1 phophate and 4 oxygen atoms with a total mass of 149.096 g/mol.

**Why is the molar mass so important? **

Molar mass serves as the foundation for many calculations and ideas found in chemistry. It serves as the bridge that connects together concepts in stoichiometry such as mass conversions, moles, avogadro’s number (6.022 x 10^{23}) or in dimensional analysis such as density, amu, and moles.

In addition we discuss the average molar mass or average molecular mass of compounds when we cover isotopes, the colligative properties such as boiling point elevation or freezing point depression, and the rates of gases through effusion and diffusion.

Calculate the formula weight for each of the following compounds:AgHSO3C3H5N3O3(NH4)3PO4

Given the following approximate values, calculate the molecular weight for NaCl. Atomic number for Na: 11, Atomic weight for Na: 23 g/mol, Atomic number for Cl: 17, Atomic weight for Cl: 35 g/mol, Boiling point for Cl: -34°C. A) 11 g/mol B) 28 g/mol C) 34 g/mol D) 40 g/mol E) 58 g/mol

Calculate the molar mass for each of the following: Part BC2H5OH (ethanol)Express your answer to two decimals places and include the appropriate units. Part C C6H12O6 (glucose) Express your answer to two decimals places and include the appropriate units. Part D Al2(SO4)3 (antiperspirant) Express your answer to two decimals places and include the appropriate units