A Level Chemistry OCR Practice Exam

Which of the following variables is not considered in stoichiometry?

1. The masses of reactants

2. The pressure of gases

3. The concentration of solutions

4. The net charge of ions

The correct answer is: The net charge of ions

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between the amounts of reactants and products in a chemical reaction. It relies on the balanced equation of the reaction to determine how much of each substance is involved and produced. The masses of reactants, pressure of gases, and concentration of solutions are all significant variables in stoichiometric calculations. The masses of reactants are directly used to find the moles of materials present, which is essential for determining how much product is formed based on the coefficients in the balanced equation. For gaseous reactions, pressure is an important factor, as it influences the volume and number of moles of gas present, which can be crucial when applying the ideal gas law in stoichiometric calculations. Similarly, concentrations of solutions are vital for determining the number of moles of solutes in reactions occurring in solution. On the other hand, while the net charge of ions is an important consideration in certain contexts, such as balancing redox reactions or understanding ionic species in solution, it does not directly influence the stoichiometric calculations concerning the amounts of substances involved. Stoichiometry focuses primarily on the quantity of substances and their ratios, rather than their charges. Thus, the net charge of ions does not factor