Solutions of various substances find considerable application in medicine, mechanical engineering and chemical production. As they consist of two components - solvent and the dissolved substance, during the work with them it is necessary to deal with such size as a mass fraction of substance. It is the integral characteristic of any solution, irrespective of its components.
Instruction
1. At the solution of tasks in which solutions of some substances appear in others it is necessary to deal with the size called a mass fraction. It expresses amount of the dissolved substance in relation to amount of solvent. A mass fraction - the dimensionless size equal to the relation of mass of the dissolved substance to the mass of all solution in general. It is usually expressed as a percentage or unit shares. She is found as follows:? in =m in / solution m, where m in - the mass of the dissolved substance, solution m - the mass of solution. The mass of solution consists of two components: the mass of the dissolved substance and mass of solvent. Solution of sulfuric acid can be an example. Oxide of SO3 sulfur is dissolved in H2O water and receive sulfuric acid. It looks as follows: =m solution m in +m H2O=m SO3+m H2O=m H2SO4. The formula given below is used most often when in a task the mass of solution is not specified, and there is only a weight of the dissolved substance and water. In a different way the mass fraction is expressed as follows:? in =m in / (m in +m H2O).
2. At known to the mass of solution and mass of the dissolved substance the mass fraction is determined by the formula given in the previous step. At the same time there is a question: in what units to express a mass fraction of the dissolved substance? In case it needs to be expressed as a percentage, the result of calculations is multiplied by hundred:? in =m v*100/m solution. In case it is necessary to express result of calculation in shares of unit, additional calculations are not carried out.
3. There are tasks in which on the contrary, the mass fraction is given, and it is required to determine the mass of substance which is required for solution preparation. In that case the mass of the dissolved substance is found on a formula: m in-va =m *? in / 100.
4. For preparation of some solutions use crystalline hydrates. As a rule, they represent complex chemical structures of a look: FxNOy*5H2O. A technique of finding of a mass fraction of the dissolved substance in this case other. First, in any task where crystalline hydrate appears, are specified the mass of the crystalline hydrate m kr and the mass of the waterless substance FxNOy. The relation of mass of crystalline hydrate to its molar weight is equal to the relation of mass of waterless substance to its molar weight increased by FxNOy coefficient: m kr/M kr =mv/x*mv. The mass fraction of waterless substance is equal to its weight divided into the mass of solution:? in = to m in / m of river. The formula of crystalline hydrate needs to be transformed as follows: m kr/M kr =? v*mr/x * MV, where m kr - the mass of crystalline hydrate, M kr - the molar mass of crystalline hydrate? in - a mass fraction of the waterless dissolved substance, m r - the mass of solution, x - coefficient of waterless substance, MV - the molar mass of waterless substance. From here the mass fraction of the dissolved substance will be equal:? in =m r*m kp/m kr*kh * MV.