What will be the ionisation constant of formic acid if its \(0.01 \mathrm{M}\) solution is \(14.5 \%\) ionised? (a) \(2.1 \times 10^{-4}\) (b) \(14.5\) (c) \(0.145\) (d) \(1.45 \times 10^{-4}\)

Formic acid, also known by its chemical formula HCOOH, is the simplest carboxylic acid and is found in nature, such as within ant venom. This organic acid is weak and does not fully dissociate in a solution. Understanding its dissociation involves recognizing that it separates into hydrogen ions (H+) and formate ions (HCOO-). The behavior of formic acid in a solution is essential for students studying acid-base chemistry and is a classic example when learning about chemical equilibrium and the concept of the ionisation constant.

The acid dissociation or ionization is a reversible reaction, which means formic acid can recombine with the hydrogen ions to form non-ionized molecules again. This reversible nature leads to the establishment of a chemical equilibrium between the ionized and non-ionized forms of the acid in a solution.

The degree of ionisation represents the extent to which an acid dissociates into ions in a solution. It's a crucial concept in acid-base chemistry because it tells us how strong or weak an acid is in a given solution. The degree of ionisation is typically expressed as a percentage, providing a straightforward way to understand the fraction of the original substance that has become ions.

Mathematically, it can be depicted as a ratio of the number of moles of the substance that has ionised to the total number of moles originally present. To convert this percentage into a usable number for calculations, it must be divided by 100. For instance, if formic acid is 14.5% ionised, as in our exercise, it means that 14.5% of the formic acid molecules have split into H+ and HCOO- ions, giving us a decimal of 0.145 for use in further calculations.

Chemical equilibrium is the state in which the rate of the forward reaction equals the rate of the reverse reaction. In the case of weak acids like formic acid, this means that the rate at which the acid molecules dissociate into ions is the same as the rate at which the ions recombine to form the acid molecules.

In a solution at equilibrium, the concentrations of the reactants and products remain constant over time, even though the reactions continue to occur. The equilibrium constant (Ka for acids) quantifies the concentrations of the products and reactants at this state. For ionisation reactions, the Ka is known as the ionisation constant, which provides insight into the strength of the acid. The higher the Ka value, the stronger the acid (greater degree of ionisation). The ionisation constant is determined by the concentrations of the ions and the remaining non-ionised acid at equilibrium. This constant is invaluable for various applications, including pH calculations and predicting the behavior of acids in reactions.