the molar quantities given in most chemistry textbooks, it is simply necessary to multiply by the Avogadro constant: and . The entropy of a molecule is defined as where is the number of states available to the molecule. The number of states must vary linearly with the volume of the system (here an idealized system is considered for better understanding, so that activities are posited very close to the true concentrations. Fundamental statistical proof of the mentioned linearity goes beyond the scope of this section, but to see this is true it is simpler to consider usual isothermal process for an ideal gas where the change of entropy takes place. It follows from the definition of entropy and from the condition of constant temperature and quantity of gas that the change in the number of states must be proportional to the relative change in volume . In this sense there is no difference in statistical properties of ideal gas atoms compared with the dissolved species of a solution with activity coefficients equaling one: particles freely "hang around" filling the provided volume), which is inversely proportional to the concentration , so we can also write the entropy as The change in entropy from some state 1 to another state 2 is therefore so that the entropy of state 2 is If state 1 is at standard conditions, in which is unity (e.g., 1 atm or 1 M), it will merely cancel the units of . We can, therefore, write the entropy of an arbitrary molecule A as where is the entropy at standard conditions and [A] denotes the concentration of A. The change in entropy for a reaction is then given by We define the ratio in the last term as the reaction quotient: where the numerator is a product of reaction product activities, , each raised to the power of a stoichiometric coefficient, , and the denominator is a similar product of reactant activities. All activities refer to a time . Under certain circumstances (see chemical equilibrium) each activity term such as may be replaced by a concentration term, [A].In an electrochemical cell, the cell potential is the chemical potential available from redox reactions (). is related to the Gibbs free energy change only by a constant: , where is the number of electrons transferred and is the Faraday constant. There is a negative sign because a spontaneous reaction has