Summary
Highlights
Molar mass is a fundamental chemistry skill used in various calculations when converting from grams to moles, in empirical or molecular formulas, or in limiting reactant problems. It's a simple but very important skill. The molar mass for any element can be found on the periodic table; for example, aluminum is 26.98 grams per mole and carbon is 12.01 grams per mole. The unit for molar mass is grams per mole (g/mol).
When calculating molar mass for a molecule like carbon dioxide (CO2), you need to consider all elements present. Carbon dioxide is a molecule with one carbon atom and two oxygen atoms. First, perform an atom inventory: Carbon (C) has 1 atom, and Oxygen (O) has 2 atoms. Look up the molar mass for each element on the periodic table: Carbon is 12.01 g/mol and Oxygen is 16.0 g/mol. Multiply the molar mass of each element by its count in the molecule: 1 * 12.01 g/mol = 12.01 g/mol for Carbon, and 2 * 16.0 g/mol = 32.0 g/mol for Oxygen. Add these values together to get the total molar mass for the molecule: 12.01 + 32.0 = 44.01 g/mol for CO2.
For molecules with parentheses, such as aluminum sulfate (Al2(SO4)3), the numbers outside the parentheses multiply everything inside. First, do an atom inventory: Aluminum (Al), Sulfur (S), and Oxygen (O). Count the atoms: there are 2 Al atoms. For Sulfur, multiply the subscript outside the parentheses by the understood 1 inside: 3 * 1 = 3 S atoms. For Oxygen, multiply the subscript outside by the subscript inside: 3 * 4 = 12 O atoms. Next, find the molar mass for each element: Al = 26.98 g/mol, S = 32.07 g/mol, O = 16.0 g/mol. Calculate the total mass for each element: Al: 2 * 26.98 = 53.96 g/mol; S: 3 * 32.07 = 96.21 g/mol; O: 12 * 16.0 = 192.0 g/mol. Finally, add these totals to find the molar mass of aluminum sulfate: 53.96 + 96.21 + 192.0 = 342.17 g/mol.