Science, Tech, Math › Science Use Avogadro's Number to Convert Molecules to Grams Share Flipboard Email Print You can apply Avogadro's number to determine the mass of molecules. Lawrence Lawry, Getty Images Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated May 06, 2019 Avogadro's number is the number of items in one mole. The number is experimentally determined based on measuring the number of atoms in precisely 12 grams of the carbon-12 isotope, giving a value of approximately 6.022 x 1023. You can use Avogadro's number in conjunction with atomic mass to convert a number of atoms or molecules into the number of grams. For molecules, you add together the atomic masses of all the atoms in the compound to get the number of grams per mole. Then you use Avogadro's number to set up a relationship between the number of molecules and mass. Here's an example problem that shows the steps: Avogadro's Number Example Problem Question: Calculate the mass in grams of 2.5 x 109 H2O molecules. Solution: Step 1 - Determine the mass of 1 mole of H2O To obtain the mass of 1 mole of water, look up the atomic masses for hydrogen and oxygen from the Periodic Table. There are two hydrogen atoms and one oxygen for every H2O molecule, so the mass of H2O is: mass of H2O = 2 (mass of H) + mass of Omass of H2O = 2 ( 1.01 g ) + 16.00 gmass of H2O = 2.02 g + 16.00 gmass of H2O = 18.02 g Step 2 - Determine the mass of 2.5 x 109 H2O molecules One mole of H2O is 6.022 x 1023 molecules of H2O (Avogadro's number). This relation is then used to 'convert' a number of H2O molecules to grams by the ratio: mass of X molecules of H2O / X molecules = mass of a mole of H2O molecules / 6.022 x 1023 molecules Solve for the mass of X molecules of H2O mass of X molecules of H2O = ( mass of a mole H2O · X molecules of H2O ) / 6.022 x 1023 H2O molecules mass of 2.5 x 109 molecules of H2O = ( 18.02 g · 2.5 x 109) / 6.022 x 1023 H2O moleculesmass of 2.5 x 109 molecules of H2O = ( 4.5 x 1010) / 6.022 x 1023 H2O moleculesmass of 2.5 x 109 molecules of H2O = 7.5 x 10-14 g. Answer The mass of 2.5 x 109 molecules of H2O is 7.5 x 10-14 g. Helpful Tips for Converting Molecules to Grams The key to success for this type of problem is paying attention to the subscripts in a chemical formula. For example, in this problem, there were two atoms of hydrogen and one atom of oxygen. If you're getting the incorrect answer for this type of problem, the usual cause is having the number of atoms wrong. Another common problem is not watching your significant figures, which can throw off your answer in the last decimal place.