Question

Define the following modes of expressing the concentration of a solution.

Which of these modes are independent of temperature and why?

$A$ $w/w$(mass percentage)

$B$ $V/V$(Volume percentage)

$C$ $w/V$(mass by volume percentage)

$D$ ppm (parts per millions)

$E$ x(mole fraction)

$F$ $M$(Molarity)

$G$ $m$(Molality)

Answer 1

Part (A):

$w/w$ or mass percentage of a component of a solution is defined as the percentage mass of the solute dissolved in total mass of the solution. The formula is given as:

$w/w\left(\text{mass percentage}\right)=\frac{\text{mass of solute in the solution(g)}}{\text{Total mass of the solution(g)}}\times 100$

Part (B):

$V/V$ or volume percentage is defined as the percentage of the volume occupied by the solute in the total volume of the solution. The formula is given as:

$V/V\left(\text{Volume percentage}\right)=\frac{\text{Volume of the solute in the solution}}{\text{Total volume of the solution}}\times 100$

Part (C):

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

$w/V\left(\text{mass by volume percentage}\right)=\frac{\text{Mass of the solute(g)}}{\text{Total Volume of the solution (mL)}}\times 100$

Part (D):

$ppm$ or parts per million is a convenient method to express the concentration when a solute is present in trace quantities. It is defined as $1$ part of solute is dissolved in a million parts of the solution.

$ppm\left(\text{parts per million}\right)=\frac{\text{Number parts of the components}}{\text{Total number of parts of all components}}\times {10}^6$

Part (E)

Mole fraction or ${}^{\prime }{x}^{\prime }$ is defined as the ratio of the number of moles of the component in a solution to the total number of moles of all the components in the solution.

$x\left(\text{Mole fraction}\right)=\frac{\text{Number of moles of the components}}{\text{Total number of moles of all the components}}$

Part (F):

Molarity $\left(M\right)$ of a solution is defined as the number of moles of solute dissolved in 1 litre of the solution.

$M\left(\text{Molarity}\right)=\frac{\text{Number moles of the solute(mol)}}{\text{Volume of the solution(L)}}$

Part (G):

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

$m\left(\text{Molality}\right)=\frac{\text{Number of moles of solute(mol)}}{\text{Mass of the solvent(kg)}}$

Mass percentage, ppm, mole fraction and molality are only dependent on the mass or the number of moles, which makes them independent of temperature, as only volume gets affected by a change in temperature. Since the given quantities do not involve any volume term, they are temperature independent.