Counting Atoms

• Each element in a formula begins with a capital letter.
• CO2 is made of two elements, carbon and oxygen.
• A subscript tells you the number of atoms for an element in 1 molecule and it belongs to the element it comes after.
• In CO2, there are two oxygen atoms.
• If there are parentheses, multiply only the elements inside the parenthesis by the subscript outside the parenthesis, it’s just like distributing in math.
• Cu(OH)2 requires you to multiply the O by 2 and the H by 2.
• The coefficient is the number of molecules.
• If there are 4 H2O molecules, you multiply H2 by 4 and O by 4.
• To find the total number of atoms for each element, remember if an element is in the molecule in 2 separate places you have to add them together.
• CH3COOH has 2 carbons, 2 oxygens, and 4 hydrogens, 2 + 2 + 4 = 8 atoms.
• Molecular (formula) mass is the total mass of a molecule and to find the molecular mass, you must use your periodic table.
• To find the molecular mass, you must use your periodic table and find out the mass of each element in the molecule, and multiply it by the number of atoms of each element.
• ZnSO4 has a total of 65 amu for Zn and 32 amu for S, and 4 amu for O, molecular mass is 161 amu.
• If there are 5 molecules of ZnSO4, multiply 161 amu by 5 to get the mass of all of the molecules.