Textbook in PDF format The fundamentals of mass balances, relevant for chemical engineers summarized in an easy comprehensible manner. Plenty of examples, schemes and flow diagrams facilitate understanding. The book proposes an original methodology based on the intensive application of the spreadsheet. This approach introduces the concept of "simulation" which allows to determine the effect of operating parameters on individual units but also on entire processes. Case studies from relevant topics such as sustainable chemistry illustrate the theory behind current applications. New in the Second Edition: Extending the application of mass balances to the circular economy - a new topic is introduced with the calculation of the Material Circularity Indicator (MCI), one of the most widespread in industry, which is also included in the standard ISO 59020 published in 2024. Aspects related to the potential use of Artificial Intelligence in data search and critical analysis of the results obtained in mass balances have been introduced. Its use in the design of Excel functions and applications is also presented in an appendix. Finally, it should be noted that both the number of solved examples and proposed practical cases have increased by 25%. Mass balance fundamentals in systems with and without chemical reactions. Easy to understand with plenty of example calculations, schemes and flow diagrams. Current practice examples from the field of sustainable chemistry. Conservation principles Balances in systems without chemical reaction Balances in systems with chemical reaction Balances in multiple systems Balances in circular economy processes