of Solutions For example, % ethanol solution in water means that mL of ethanol is dissolved in water such that the total volume of the solution is mL. Solutions containing liquids are commonly expressed in this unit. For example, a % ( v / v ) solution of ethylene glycol, an antifreeze, is used in cars for cooling the engine. At this concentration the antifreeze lowers the freezing point of water to .4K (– .
°C). (iii) Mass by volume percentage ( w /V) : Another unit which is commonly used in medicine and pharmacy is mass by volume percentage. It is the mass of solute dissolved in mL of the solution. (iv) Parts per million : When a solute is present in trace quantities, it is convenient to express concentration in parts per million (ppm) and is defined as: Parts per million = Number of parts of the component × Total number of parts of all components of the solution ( .
) As in the case of percentage, concentration in parts per million can also be expressed as mass to mass, volume to volume and mass to volume. A litre of sea water (which weighs g) contains about × – g of dissolved oxygen (O ). Such a small concentration is also expressed as . g per g ( .
ppm) of sea water. The concentration of pollutants in water or atmosphere is often expressed in terms of m g mL – or ppm. (v) Mole fraction : Commonly used symbol for mole fraction is x and subscript used on the right hand side of x denotes the component. It is defined as: Mole fraction of a component = Number of moles of the component Total number of moles of all the compone nts ( .
) For example, in a binary mixture, if the number of moles of A and B are n A and n B respectively, the mole fraction of A will be x A = A A B ( . ) For a solution containing i number of components, we have: x i = i i .......