In practice, chemists rarely try to measure out a specific mass. Instead, they estimate the mass required and spoon out the mass approximately. Then they measure the mass of the sample precisely and convert it into an amount in moles to find the precise amount that they have obtained. Why do chemists typically not want to measure out very precise, predetermined masses when making up solutions?

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.