Science, Tech, Math › Science Graham's Law Definition Chemistry Glossary Definition of Graham's Law Share Flipboard Email Print Thomas Graham. Wikimedia Commons/Public Domain Science Chemistry Chemical Laws Basics 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 July 03, 2019 Graham's Law is a relation which states that the rate of the effusion of a gas is inversely proportional to the square root of its density or molecular mass. Rate1 / Rate2 = (M2 / M1)1/2 Where:Rate1 is the rate of effusion of one gas, expressed as volume or as moles per unit time.Rate2 is the rate of effusion of the second gas.M1 is the molar mass of gas 1.M2 is the molar mass of gas 2.