Concentrations

Do you like your coffee thick or more ‘watery’? What do we mean by this? Maybe the ratio of water to milk? Or, maybe the concentration of milk in water! We always discuss a solution being diluted or concentrated. However, this is a qualitative way of expressing the concentration of the solution.

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Dilute and Concentrated Solutions

A dilute solution means the quantity of solute is relatively very small. On the other hand, a concentrated solution implies that the solution has a large amount of solute. But, unfortunately, these are relative terms. They do not give us an idea of the quantitative concentration of the solution.

So, to quantitatively describe the concentrations of various solutions around us, we commonly express levels of concentration in different ways. Let us look at a few of them below.

Quantitative Measurement of Concentration

1) Mass Percentage (w/w)

When we express the concentration of a solution as the percent of one component in the solution, we call it the mass percentage (w/w). Suppose, we have a solution containing component A as the solute and B as the solvent, then its mass percentage is expressed as:

Mass % of A = Mass of component A in the solution/Total mass of the solution×100

10% solution of sugar by mass means that 10 grams of sugar is present in 100 grams of the solution, i.e., 10 grams of sugar has been dissolved in 90 grams of water.

2) Volume percentage (V/V)

Sometimes, we express the concentration as a percent of one component in the solution by volume. In such cases, it is the volume percentage. It is given as:

Volume % of A = Volume of component A in the solution/Total volume of the solution×100

For example, if a solution of NaCl in water is said to be 10 % by volume that means a 100 ml solution will contain 10 ml NaCl.

3) Mass by Volume Percentage (w/V)

This unit is majorly used in pharmaceutical industry. It is the mass of a solute dissolved per 100 mL of the solution.

 4) Molarity (M)

One of the most commonly used methods for expressing the concentrations is molarity. It is the number of moles of solute dissolved in one litre of a solution. Thus, if one gram molecule of a solute is present in 1 litre of the solution, the concentration of the solution is said to be one molar. Unit of molarity: mol L^-1

Suppose a solution of ethanol is labelled as 0.25 M. By this, we mean that in one litre of the given solution 0.25 moles of ethanol is dissolved.

5) Molality (m)

Molality represents the concentration regarding moles of solute and the mass of solvent. It is given by moles of solute dissolved per kg of the solvent. The molality formula is given as:

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

Relationship Between Molality and Molarity

Let the density of the solution be ‘d’. Unit= g mL−1    Mass of solution = V × d The mass of solute = number of moles × molecular mass of solute   = n mA Mass of solvent, W = mass of solution – mass of solute   = V × d – n × mA Thus, Where mA is the molecular mass of solvent.

6) Mole Fraction

If the solution has a solvent and the a solute, mole fraction gives a concentration as the ratio of moles of one component to the total moles present in the solution. It is denoted by x.

The above-mentioned methods are commonly used ways of expressing the concentration of solutions. All the methods describe the same thing that is, the concentration of a solution, each of them has their own advantages and disadvantages. Molarity depends on temperature while mole fraction and molality are independent of temperature. All these methods are used on the basis of requirement of expressing the concentrations.

Solved Example for You

Q: What do you mean by the formality of a solution?

Answer: It is the number of formula mass in grams present per litre of solution. In case, formula mass is equal to molecular mass, formality is equal to molarity. Like molarity and normality, the formality also depends on temperature.