Define the following modes of expressing the concentration of a solution. Which of these modes are independent of temperature and why?

(i) w/w (mass percentage)


(ii) V/V (volume percentage)


(iii) w/V (mass by volume percentage)


(iv) ppm. (parts per million)


(v) x (mole fraction)


(vi) M (Molarity)


(vii) m (Molality)


(i) w/w or mass percentage of a component of a solution is defined as the mass of the individual component in the solution in the total mass of the solution in percent. The formula is given as


w/w = (Mass of the component of the solution)/(Total Mass of the solution) x 100


(ii) V/V or volume percentage is defined as the percentage of the volume occupied by the component in the total volume of the solution. The formula is given as


V/V = (Volume of the component)/(Total volume of the solution) x 100


(iii) Mass by volume percentage, w/V, is defined as mass of solute dissolved in 100mL of the solution. This can be calculated by measuring the mass of the solute in grams dissolved in milliliters of solute and converting it into the percent form.


(iv) Ppm or parts per million, it is a convenient method to express concentration when a solute is present in trace quantities. It is defined as:


Parts per million = (Number of parts of the component× 10^6)/(Total number of parts of all the components in the solution)


(v) Mole fraction or x is defined as the ratio of the number of moles of the component in a solution to the total number of moles of all components in the solution.


x = (Number of moles of a component)/(Total number of moles of all the components of the solution)


Mole fraction with a subscript on its right-hand side denotes its component.


(vi) Molarity (M) of a solution is defined as the number of moles of solute dissolved in 1 litre [or 1000mL or 10 cubics decimetre] of solution.


Molarity = (Moles of solute(n))/(Volume of solution in litre(V))


(vii) Molality (m) of a solution is defined the number of moles of the solute per kilogram (kg) of solvent. It is expressed as:


Molality = (Moles of solute(n))/(Mass of solvent in kg)


Out of these values, (i) mass percentage, (iv) ppm, (v) mole fraction and (vii) molality are independent of temperature, whereas (ii) volume percentage, (iii) mass by volume percentage, (vi) molarity is a function of temperature. This is because volume depends on temperature and the mass does not. For instance, heating a liquid substance will cause expansion of the constituent molecules and cause an increase in the volume. The mass is still the sum of the individual masses of the particles.


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