Science, Tech, Math › Science Mole Relations in Balanced Equations Chemistry Problems with Balanced Equations Share Flipboard Email Print Chemical equations state moles of reactants and products. Comstock/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 These are worked chemistry problems showing how to calculate the number of moles of reactants or products in a balanced chemical equation. Mole Relations Problem #1 Determine the number of moles of N2O4 needed to react completely with 3.62 mol of N2H4 for the reaction 2 N2H4(l) + N2O4(l) → 3 N2(g) + 4 H2O(l). How to Solve the Problem The first step is to check to see that the chemical equation is balanced. Make sure the number of atoms of each element are the same on both sides of the equation. Remember to multiply the coefficient by all atoms following it. The coefficient is the number in front of a chemical formula. Multiply each subscript only by the atom right before it. The subscripts are the lower numbers found immediately following an atom. Once you verify the equation is balanced, you can establish the relationship between the number of moles of reactants and products. Find the relation between moles of N2H4 and N2O4 by using the coefficients of the balanced equation: 2 mol N2H4 is proportional to 1 mol N2O4 Therefore, the conversion factor is 1 mol N2O4/2 mol N2H4: moles N2O4 = 3.62 mol N2H4 x 1 mol N2O4/2 mol N2H4 moles N2O4 = 1.81 mol N2O4 Answer 1.81 mol N2O4 Mole Relations Problem #2 Determine the number of moles of N2 produced for the reaction 2 N2H4(l) + N2O4(l) → 3 N2(g) + 4 H2O(l) when the reaction begins with 1.24 moles of N2H4. Solution This chemical equation is balanced, so the molar ratio of reactants and products may be used. Find the relation between moles of N2H4 and N2 by using the coefficients of the balanced equation: 2 mol N2H4 is proportional to 3 mol N2 In this case, we want to go from moles of N2H4 to moles of N2, so the conversion factor is 3 mol N2/2 mol N2H4: moles N2 = 1.24 mol N2H4 x 3 mol N2/2 mol N2H4 moles N2 = 1.86 mol N2O4 Answer 1.86 mol N2 Tips for Success The keys to getting the correct answer are: Make sure the chemical equation is balanced.Use the coefficients in front of compounds to get molar ratios.Check to make sure you use the appropriate number of significant figures for atomic masses and report mass using the correct number of figures.