Question

How many gram of dibasic acid (mol. wt. 200 ) should be present in \(100 \mathrm{~mL}\) of the aqueous solution to give 0.1 normality? (a) \(1 \mathrm{~g}\) (b) \(1.5 \mathrm{~g}\) (c) \(0.5 \mathrm{~g}\) (d) \(20 \mathrm{~g}\)

Short answer

1 gram of dibasic acid is needed.

Step-by-step solution

Understand Normality

Normality is defined as the number of equivalents of solute per liter of solution. In the case of a dibasic acid, it can donate two protons (H⁺), so it has two equivalents per mole.

Calculate Required Equivalents

To find the equivalents required for 0.1 normality in 100 mL (0.1 L) of solution, use the formula: \( \text{Normality} = \frac{\text{Equivalents}}{\text{Liters of solution}} \). Thus, \( 0.1 = \frac{\text{Equivalents}}{0.1} \) or \( \text{Equivalents} = 0.01 \).

Convert Equivalents to Mass

Since the dibasic acid is 2 equivalents per mole, the moles needed is \( \frac{0.01}{2} = 0.005 \) moles. Use the molecular weight to convert moles to grams: \( \text{mass} = \text{moles} \times \text{molecular weight} = 0.005 \times 200 = 1 \) gram.

Key concepts

Dibasic Acid

A dibasic acid is a type of acid that has the capacity to donate two protons (hydrogen ions, or H⁺) per molecule. This means that when a dibasic acid dissociates in solution, it can release two moles of protons per mole of the acid. Consequently, these acids can undergo two stages of ionization. For example, sulfuric acid (H₂SO₄) is a well-known dibasic acid.

When dealing with dibasic acids, it's crucial to recognize that they play a significant role in determining the normality of a solution. Because they donate two protons, their equivalents are higher compared to monobasic acids, which only donate one proton. Understanding how to handle dibasic acids is essential for correctly calculating normality in chemistry.

Equivalents in Chemistry

Equivalents in chemistry represent the amount of a substance that reacts with a defined amount of another substance. Understanding the concept of equivalents is crucial for titrations and many other chemical calculations.

• Defining Equivalents: An equivalent is calculated by dividing the amount of substance by its equivalent factor, which often relates to the number of electrons transferred, protons donated, or molecules reacted.


• Equivalents in Normality: In the context of normality, equivalents refer to the amount of solute that reacts completely in a specific reaction. For dibasic acids, this involves recognizing that they can donate two protons, thus have two equivalents per mole.

Using equivalents helps in simplifying many stoichiometric computations as it allows us to focus on the reactivity potential of ions or molecules rather than just their mass or volume.

Molecular Weight

Molecular weight, also known as molecular mass, is the sum of the atomic weights of all atoms in a molecule. It is usually expressed in atomic mass units (amu) or grams per mole (g/mol).

• Calculation of Molecular Weight: To calculate the molecular weight, sum up the atomic masses of all the elements present in the molecule. For example, in a molecule like H₂O, molecular weight is the sum of twice the atomic mass of hydrogen and once the atomic mass of oxygen.


• Role in Chemistry: Knowing the molecular weight of a substance allows chemists to convert between moles and grams, which is pivotal in laboratory calculations. This is crucial for determining how much of a substance is needed to achieve certain normality or molarity.

Molecular weights are a foundational concept in stoichiometry and help in ensuring that reactions have the correct proportions of reactants.