Molecular Mass Calculations

Molecular Mass Calculations The molecular mass of a molecule is the total mass of all the atoms making up the molecule. This example problem illustrates how to find the molecular mass of a compound or molecule. Molecular Mass Problem Find the molecular mass of table sugar (sucrose), which has a molecular formula C12H22O11. Solution To find the molecular mass, add the atomic masses of all of the atoms in the molecule. Find the atomic mass for each element by using the mass given in the Periodic Table.  Multiply the subscript (number of atoms) times the atomic mass of that element and add the masses of all of the elements in the molecule to get the molecular mass. For example, multiple the subscript 12 times the atomic mass of carbon (C). It helps to know the symbols for the elements  if you dont know them already. If you round off the atomic masses to four significant figures, you get: molecular mass C12H22O11 12(mass of C) 22(mass of H) 11(mass of O)molecular mass C12H22O11 12(12.01) 22(1.008) 11(16.00)molecular mass C12H22O11 342.30 Answer 342.30 Note that a sugar molecule is about 19 times heavier than a water molecule! When performing the calculation, watch your significant figures. Its common to work a problem correctly, yet get the wrong answer because its not reported using the correct number of digits. Close counts in real life, but its not helpful if youre working chemistry problems for a class. For more practice, download or print these worksheets: Formula or Molar Mass Worksheet (pdf)Formula or Molar  Mass Worksheet Answers (pdf) Note About Molecular Mass and Isotopes The molecular mass calculations made using the atomic masses on the periodic table apply for general calculations, but arent accurate when known isotopes of atoms are present in a compound. This is because the periodic table lists values that are a weighted average of the mass of all natural isotopes of each element. If you are performing calculations using a molecule that contains a specific isotope, use its mass value. This will be the sum of the masses of its protons and neutrons. For example, if all the hydrogen atoms in a molecule are replaced by deuterium, the mass for hydrogen would be 2.000, not 1.008. Problem Find the molecular mass of glucose, which has a molecular formula C6H12O6. Solution To find the molecular mass, add the atomic masses of all of the atoms in the molecule. Find the atomic mass for each element by using the mass given in the  Periodic Table. Multiply the subscript (number of atoms) times the  atomic mass  of that element and add the masses of all of the elements in the molecule to get the molecular mass. If we round off the atomic masses to four significant figures, we get: molecular mass C6H12O6   6(12.01) 12(1.008) 6(16.00) 180.16 Answer 180.16 For more practice, download or print these worksheets: Formula or Molar Mass Worksheet (pdf)Formula or Molas Mass Worksheet Answers (pdf)