The purpose of this descriptive study was to create and test questions on stoichiometry with number ratios for quick mental calculations and to identify students' problem-solving strategies. The present study was a component of a more comprehensive investigation in which 7,441 German senior high school students were asked to work on 154 test items from 10 different topics. A random sample of 4,181 out of the 7,441 students completed the paper and pencil tests on stoichiometry. Additionally, discussions with students about how they solved the test items were videotaped. The study uncovered five strategies used by students to solve the test items on stoichiometry. With some test items the possibility that students came to the correct result though they had used incorrect solving strategies could not be excluded. However, it was possible to improve these test items. This indicates that students' successful solving strategies have to be known in order to decide whether a test item is valid or not. It was noted that most students successfully applied problem-solving strategies that were not presented in chemistry textbooks. The test items developed in the study and the problem solving strategies observed may be used to introduce students to stoichiometric calculations. (Author)