Final answer:
Reactant amounts are measured in molarity, which represents the moles of solute per liter of solution, facilitating accurate stoichiometry and reaction rate calculations in chemical reactions.
Step-by-step explanation:
The amount of reactants is measured in molarity because it provides a direct relation to the number of molecules involved in a chemical reaction. Molarity is defined as the number of moles of solute dissolved in one liter of solution. This measure is particularly useful in stoichiometry problems and when determining reaction rates, as it allows chemists to use volumes of solutions to directly relate to a known amount of substance that can participate in a reaction.
For example, to prepare a solution with a specific molarity, a chemist calculates the mass of the solute needed, so that when it's dissolved in a certain volume of solvent, the resulting solution contains the desired concentration. This is critical for reactions to occur correctly and is also essential when measuring reaction rates, as these are expressed as the change in concentration of reactant or product per unit time (M/s, M/min, or M/h).