Question

The equivalent weight of phosphoric acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\) in the reaction: \(\mathrm{NaOH}+\mathrm{H}_{3} \mathrm{PO}_{4} \longrightarrow \mathrm{NaH}_{2} \mathrm{PO}_{4}+\mathrm{H}_{2} \mathrm{O}\) is (a) 89 (b) 98 (c) 59 (d) 29

Short answer

The equivalent weight of phosphoric acid is 98. Answer: (b) 98.

Step-by-step solution

Determine the Molecular Weight of Phosphoric Acid

Calculate the molecular weight of phosphoric acid \(\text{H}_3\text{PO}_4\) by adding the atomic masses: \(3 \times 1\) (for H) + \(31\) (for P) + \(4 \times 16\) (for O) which equals 98 g/mol.

Understand the Equivalent Weight Formula

The equivalent weight of an acid in a reaction is calculated using the formula \(\frac{\text{Molar Mass}}{\text{Basicity}}\), where basicity is the number of replaceable hydrogen ions (H\(^+\)) resulting from the acid in the given reaction.

Identify Basicity in the Reaction

In the reaction \(\text{NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{NaH}_2\text{PO}_4 + \text{H}_2\text{O}\), phosphoric acid \(\text{H}_3\text{PO}_4\) gives out only one hydrogen ion to form \(\text{NaH}_2\text{PO}_4\). Therefore, its basicity in this reaction is 1.

Calculate the Equivalent Weight of Phosphoric Acid

Using the formula \(\frac{\text{Molar Mass}}{\text{Basicity}}\), substitute the values: \(\frac{98}{1} = 98\). Thus, the equivalent weight of phosphoric acid is 98.

Key concepts

Molecular Weight Calculation

Understanding how to calculate the molecular weight is crucial when working with chemical compounds. The molecular weight, sometimes referred to as molar mass, is determined by summing up the atomic masses of all the atoms in a molecule. For phosphoric acid \(\text{H}_3\text{PO}_4\), its molecular weight calculation is done as follows:

• There are 3 hydrogen (H) atoms, each with an atomic mass of approximately 1 g/mol, so \(3 \times 1 = 3 \text{ g/mol}\).
• There is 1 phosphorus (P) atom with an atomic mass of about 31 g/mol.
• There are 4 oxygen (O) atoms, with each having an atomic mass of approximately 16 g/mol, leading to \(4 \times 16 = 64 \text{ g/mol}\).

Adding these values gives: \(3 + 31 + 64 = 98 \text{ g/mol}\). This value is the molecular weight of phosphoric acid, which is essential for further calculations such as determining its equivalent weight in reactions.

Basicity in Chemistry

In chemistry, basicity refers to the number of hydrogen ions (H\(^+\)) an acid can donate when it reacts. It essentially tells us how many protons an acid can release in a reaction. For example, phosphoric acid \(\text{H}_3\text{PO}_4\) is known as a triprotic acid, meaning, in different reactions, it can donate up to three protons. However, its basicity in a specific reaction depends on the number of hydrogen ions actually replaced.In the given reaction, \(\text{NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{NaH}_2\text{PO}_4 + \text{H}_2\text{O}\), phosphoric acid reacts to form \(\text{NaH}_2\text{PO}_4\). During this reaction, only one hydrogen ion is given up by phosphoric acid to form the salt, which means its basicity in this reaction is 1. This specific basicity is crucial because it directly influence how we calculate the equivalent weight of the acid in the reaction.

Acid-Base Reactions

Acid-base reactions are fundamental in chemistry, involving the transfer of protons (H\(^+\)) from an acid to a base. These reactions often result in the formation of water and a salt. Looking at the reaction \(\text{NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{NaH}_2\text{PO}_4 + \text{H}_2\text{O}\), we have sodium hydroxide, a strong base, reacting with phosphoric acid.This specific reaction is a classic example of an acid-base reaction, where the hydrogen ion from phosphoric acid is transferred to sodium hydroxide. The result is the formation of sodium dihydrogen phosphate \(\text{NaH}_2\text{PO}_4\) and water. Key characteristics of acid-base reactions include:

• Neutralization: the acid donates a proton to the base to form water and potentially a salt.
• Product formation: creation of products such as water and a salt in the reaction.
• Transfer of protons: central to defining this type of chemical reaction.

Understanding the nature of these interactions helps in grasping the underlying principles of chemical reactions and their stoichiometry.