Question

\(0.45 \mathrm{~g}\) of an acid of mol. wt. 90 was neutralised by \(20 \mathrm{~mL}\) of \(0.54 \mathrm{~N}\) caustic potash \((\mathrm{KOH})\). The basicity of acid is : (a) 1 (b) 2 (c) 3 (d) 4

Short answer

The basicity of the acid is 2.

Step-by-step solution

Calculate moles of KOH

To find the basicity of the acid, first calculate moles of KOH used in the neutralization. Normality (N) is equal to molarity (M) multiplied by the equivalent factor (which is 1 for KOH because it has one acid-neutralizing H+ ion per molecule). Use the formula: Moles of KOH = Volume (in liters) × Normality. That is, moles of KOH = 0.020 L × 0.54 N.

Calculate moles of acid

The number of moles of the acid is calculated by using its given mass and molar mass. Moles of acid = mass / molar mass. In this case, it is 0.45 g / 90 g/mol.

Calculate basicity of the acid

The basicity of the acid is the number of moles of KOH that one mole of the acid can react with. Thus, basicity = moles of KOH / moles of acid. Substitute the values calculated in steps 1 and 2.

Key concepts

Neutralization Reaction

A neutralization reaction is a fundamental chemical reaction where an acid and a base react to form water and a salt. It generally involves the combination of H+ ions from the acid and OH- ions from the base to produce water. Understanding this process is crucial when studying acids, bases, and their interactions.

In the context of the textbook exercise, the acid's unknown basicity can be determined through a neutralization reaction with a known quantity and concentration of caustic potash (KOH), a base. The reaction goes on until the acid is completely neutralized, meaning all H+ ions have been reacted. This concept is not just theoretical; it's often used in laboratory work to determine unknown concentrations and in industries such as pharmaceuticals for drug formulation.

Chemistry Problem Solving

Solving chemistry problems involves a systematic approach to understanding the chemical principles and applying mathematical computations correctly. The step-by-step solution provided exemplifies this process in determining the basicity of an acid. The essentials of chemistry problem solving include careful reading of the problem, identification of known and unknown variables, and the stepwise use of chemical and mathematical formulas to find the solution.

In this exercise, the key to finding the basicity is interpreting the conditions of the neutralization reaction and then applying stoichiometry principles to calculate the moles of acid and base that are reacting. Each step builds upon the previous, showing how a well-structured approach makes even complex problems manageable.

Molarity and Normality

Molarity and normality are measures of the concentration of a solution. Molarity (M) is defined as the number of moles of solute per liter of solution. On the other hand, normality (N) measures the equivalent concentration, which depends on the equivalents of solute per liter of solution. Equivalents relate to the number of moles of the substance that can react with or supply one mole of hydrogen ions (H+) or hydroxide ions (OH-).

For a substance like KOH, which has one reactive hydroxide ion per molecule, the molarity and normality are the same. However, for acids, normality can vary based on the number of reactive hydrogen ions available for reaction, which is linked to the concept of basicity. The exercise provided uses normality to simplify calculations, because it directly reflects the acid-neutralizing capacity of the KOH solution.

Stoichiometry

Stoichiometry is the area of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. Through stoichiometry, we can predict the amounts of substances consumed and produced in a given reaction.

The problem provided steps through stoichiometric calculations to find the basicity of an acid. Basicity refers to the number of H+ ions that one mole of acid can donate in a reaction. Stoichiometry allows us to relate the moles of KOH used to neutralize the acid with the moles of acid present. By calculating the ratio of these moles, we can determine the basicity of the acid. This practical application of stoichiometry is pivotal for real-world chemistry tasks, from lab analysis to industrial production.