Science, Tech, Math › Science Redox Reactions: Balanced Equation Example Problem Share Flipboard Email Print Rafe Swan, 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 January 20, 2020 This is a worked example redox reaction problem showing how to calculate volume and concentration of reactants and products using a balanced redox equation. Key Takeaways: Redox Reaction Chemistry Problem A redox reaction is a chemical reaction in which reduction and oxidation occur.The first step in solving any redox reaction is to balance the redox equation. This is a chemical equation that must be balanced for charge as well as mass.Once the redox equation is balanced, use the mole ratio to find the concentration or volume of any reactant or product, provided the volume and concentration of any other reactant or product is known. Quick Redox Review A redox reaction is a type of chemical reaction in which reduction and oxidation occur. Because electrons are transferred between chemical species, ions form. So, to balance a redox reaction requires not only balancing mass (number and type of atoms on each side of the equation) but also charge. In other words, the number of positive and negative electrical charges on both sides of the reaction arrow is the same in a balanced equation. Once the equation is balanced, the mole ratio may be used to determine the volume or concentration of any reactant or product as long as the volume and concentration of any species are known. Redox Reaction Problem Given the following balanced redox equation for the reaction between MnO4- and Fe2+ in an acidic solution: MnO4-(aq) + 5 Fe2+(aq) + 8 H+(aq) → Mn2+(aq) + 5 Fe3+(aq) + 4 H2O Calculate the volume of 0.100 M KMnO4 needed to react with 25.0 cm3 0.100 M Fe2+ and the concentration of Fe2+ in a solution if you know that 20.0 cm3 of solution reacts with 18.0 cm3 of 0.100 KMnO4. How to Solve Since the redox equation is balanced, 1 mol of MnO4- reacts with 5 mol of Fe2+. Using this, we can obtain the number of moles of Fe2+: moles Fe2+ = 0.100 mol/L x 0.0250 Lmoles Fe2+ = 2.50 x 10-3 molUsing this value:moles MnO4- = 2.50 x 10-3 mol Fe2+ x (1 mol MnO4-/ 5 mol Fe2+)moles MnO4- = 5.00 x 10-4 mol MnO4-volume of 0.100 M KMnO4 = (5.00 x 10-4 mol) / (1.00 x 10-1 mol/L)volume of 0.100 M KMnO4 = 5.00 x 10-3 L = 5.00 cm3 To obtain the concentration of Fe2+ asked in the second part of this question, the problem is worked the same way except solving for the unknown iron ion concentration: moles MnO4- = 0.100 mol/L x 0.180 Lmoles MnO4- = 1.80 x 10-3 molmoles Fe2+ = (1.80 x 10-3 mol MnO4-) x (5 mol Fe2+ / 1 mol MnO4)moles Fe2+ = 9.00 x 10-3 mol Fe2+concentration Fe2+ = (9.00 x 10-3 mol Fe2+) / (2.00 x 10-2 L)concentration Fe2+ = 0.450 M Tips for Success When solving this type of problem, it's important to check your work: Check to make certain the ionic equation is balanced. Make certain the number and type of atoms is the same on both sides of the equation. Make certain the net electrical charge is the same on both sides of the reaction.Be careful to work with the mole ratio between reactants and products and not the gram quantities. You may be asked to provide a final answer in grams. If so, work the problem using moles and then use the molecular mass of the species to convert between units. The molecular mass is the sum of the atomic weights of the elements in a compound. Multiply the atomic weights of atoms by any subscripts following their symbol. Don't multiply by the coefficient in front of the compound in the equation because you've already taken that into account by this point!Be careful to report moles, grams, concentration, etc., using the correct number of significant figures. Sources Schüring, J., Schulz, H. D., Fischer, W. R., Böttcher, J., Duijnisveld, W. H., eds (1999). Redox: Fundamentals, Processes and Applications. Springer-Verlag, Heidelberg ISBN 978-3-540-66528-1.Tratnyek, Paul G.; Grundl, Timothy J.; Haderlein, Stefan B., eds. (2011). Aquatic Redox Chemistry. ACS Symposium Series. 1071. ISBN 9780841226524.