Science, Tech, Math › Science How to Calculate Limiting Reactant of a Chemical Reaction Determining the Limiting Reactant Share Flipboard Email Print Maskot / Getty Images Science Chemistry Biochemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Todd Helmenstine Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. our editorial process Todd Helmenstine Updated November 26, 2019 Chemical reactions rarely occur when exactly the right amount of reactants will react together to form products. One reactant will be used up before another runs out. This reactant is known as the limiting reactant. Strategy This is a strategy to follow when determining which reactant is the limiting reactant.Consider the reaction:2 H2(g) + O2(g) → 2 H2O(l)If 20 grams of H2 gas is reacted with 96 grams of O2 gas, Which reactant is the limiting reactant?How much of the excess reactant remains?How much H2O is produced? To determine which reactant is the limiting reactant, first determine how much product would be formed by each reactant if all the reactant was consumed. The reactant that forms the least amount of product will be the limiting reactant. Calculate the yield of each reactant. The mole ratios between each reactant and the product are needed to complete the calculation:The mole ratio between H2 and H2O is 1 mol H2/1 mol H2OThe mole ratio between O2 and H2O is 1 mol O2/2 mol H2OThe molar masses of each reactant and product are also needed:molar mass of H2 = 2 gramsmolar mass of O2 = 32 gramsmolar mass of H2O = 18 gramsHow much H2O is formed from 20 grams H2?grams H2O = 20 grams H2 x (1 mol H2/2 g H2) x (1 mol H2O/1 mol H2) x (18 g H2O/1 mol H2O)All the units except grams H2O cancel out, leavinggrams H2O = (20 x 1/2 x 1 x 18) grams H2Ograms H2O = 180 grams H2OHow much H2O is formed from 96 grams O2?grams H2O = 20 grams H2 x (1 mol O2/32 g O2) x (2 mol H2O/1 mol O2) x (18 g H2O/1 mol H2O)grams H2O = (96 x 1/32 x 2 x 18) grams H2Ograms H2O = 108 grams O2O Much more water is formed from 20 grams of H2 than 96 grams of O2. Oxygen is the limiting reactant. After 108 grams of H2O forms, the reaction stops. To determine the amount of excess H2 remaining, calculate how much H2 is needed to produce 108 grams of H2O.grams H2 = 108 grams H2O x (1 mol H2O/18 grams H2O) x (1 mol H2/1 mol H2O) x (2 grams H2/1 mol H2)All the units except grams H2 cancel out, leavinggrams H2 = (108 x 1/18 x 1 x 2) grams H2grams H2 = (108 x 1/18 x 1 x 2) grams H2grams H2 = 12 grams H2It takes 12 grams of H2 to complete the reaction. The amount remaining isgrams remaining = total grams - grams usedgrams remaining = 20 grams - 12 gramsgrams remaining = 8 gramsThere will be 8 grams of excess H2 gas at the end of the reaction.There is enough information to answer the question.The limiting reactant was O2.There will be 8 grams H2 remaining.There will be 108 grams H2O formed by the reaction. Finding the limiting reactant is a relatively simple exercise. Calculate the yield of each reactant as if it were completely consumed. The reactant that produces the least amount of product limit the reaction. More For more examples, check out Limiting Reactant Example Problem and Aqueous Solution Chemical Reaction Problem. Test your new skills by answering Theoretical Yield and Limiting Reaction Test Questions. Sources Vogel, A. I.; Tatchell, A. R.; Furnis, B. S.; Hannaford, A. J.; Smith, P. W. G. Vogel's Textbook of Practical Organic Chemistry, 5th Edition. Pearson, 1996, Essex, U.K.Whitten, K.W., Gailey, K.D. and Davis, R.E. General Chemistry, 4th Edition. Saunders College Publishing, 1992, Philadelphia.Zumdahl, Steven S. Chemical Principles, 4th Edition. Houghton Mifflin Company, 2005, New York.