Question

The normality of orthophosphoric acid having purity of \(70 \%\) be weight and specific gravity \(1.54\) is (a) \(11 \mathrm{~N}\) (b) \(22 \mathrm{~N}\) (c) \(33 \mathrm{~N}\) (d) \(44 \mathrm{~N}\)

Short answer

The normality of the orthophosphoric acid is (c) 33 N.

Step-by-step solution

Calculate Mass of Orthophosphoric Acid in 1 Liter

Calculate the mass of orthophosphoric acid in 1 liter of solution using the specific gravity: \[ \text{Mass} = \text{Specific Gravity} \times 1000 \text{ g/L} = 1.54 \times 1000 \text{ g/L} = 1540 \text{ g/L} \]

Determine Mass of Pure Acid

Since the acid is 70% by weight, calculate the mass of pure orthophosphoric acid: \[ \text{Mass of Pure Acid} = 0.70 \times 1540 \text{ g/L} = 1078 \text{ g/L} \]

Calculate Moles of Orthophosphoric Acid

The molar mass of orthophosphoric acid \((\text{H}_3\text{PO}_4)\) is approximately 98 g/mol. Calculate the moles of orthophosphoric acid: \[ \text{Moles} = \frac{\text{Mass of Pure Acid}}{\text{Molar Mass}} = \frac{1078 \text{ g/L}}{98 \text{ g/mol}} \approx 11 \text{ mol/L} \]

Calculate Normality

The normality of orthophosphoric acid (\(\text{H}_3\text{PO}_4\)) is three times the molarity because it can donate three protons: \[ \text{Normality} = 3 \times 11 \text{ mol/L} = 33 \text{ N} \]

Select the Correct Answer

Based on the calculation, the normality of the orthophosphoric acid solution is 33 N. Therefore, the correct answer is (c) 33 N.

Key concepts

Orthophosphoric Acid

Orthophosphoric acid, also known as phosphoric acid, is a chemical compound with the formula \( ext{H}_3 ext{PO}_4\). It's a triprotic acid, which means it can donate three hydrogen ions (protons).
This characteristic makes it especially useful in a variety of chemical reactions and industrial applications.

• **Structure**: The molecule consists of one phosphorus atom, four oxygen atoms, and three hydrogen atoms.
• **Acidic Properties**: It acts as an acid in aqueous solutions, and its ability to donate three protons contributes to its high normality when concentrated.
• **Uses**: Commonly used in fertilizer production, food industry, and cleaning agents.

Understanding its properties, especially its ability to donate multiple protons, is crucial for calculations involving concentration measures, like normality and molarity.

Specific Gravity

Specific gravity is an important concept in understanding the concentration of a solution. It is defined as the ratio of the density of a given substance to the density of water at a specific temperature.
This concept is dimensionless, meaning it has no units, and provides a way to measure how heavy or light a liquid is compared to water.

• **Formula**: Specific gravity = \(\frac{\text{Density of substance}}{\text{Density of water}}\)
• **Relevance**: For the orthophosphoric acid solution in our exercise, a specific gravity of 1.54 means that the solution is 1.54 times as dense as water, which is a crucial step in determining the mass of the acid present in the solution.
• **Applications**: Widely used in industries to determine the concentration and quality of chemical solutions.

Specific gravity helps approximate the content of solutes in a liquid, aiding in calculating both mass and volume properties.

Molar Mass

Molar mass is a fundamental concept in chemistry, referring to the mass of a given substance (chemical element or chemical compound) divided by the amount of substance.
The molar mass of a compound is the sum of the atomic masses of all atoms in the molecule.

• **Unit**: Typically expressed in grams per mole (g/mol).
• **Calculation for \(\text{H}_3\text{PO}_4\)**: Each mole of orthophosphoric acid \(\text{H}_3\text{PO}_4\) contains: 3 hydrogen atoms (approximately 1 g/mol each), 1 phosphorus atom (approximately 31 g/mol), and 4 oxygen atoms (approximately 16 g/mol each).
  These add up to a molar mass of roughly 98 g/mol.
• **Purpose**: Knowing the molar mass is essential for converting between the mass of a substance and the amount of substance, enabling accurate calculation of moles needed in reactions.

A solid grasp of molar mass helps in understanding the stoichiometric relationships in chemical reactions and solution concentration calculations.

Molarity

Molarity, a commonly used measure of concentration in chemistry, is defined as the number of moles of solute per liter of solution.
This measurement helps scientists and engineers determine how much substance is present in a specific volume of liquid.

• **Formula**: \( ext{Molarity (M)} = \frac{\text{moles of solute}}{\text{liters of solution}}\)
• **Significance in Calculations**: For orthophosphoric acid, the molarity was calculated as approximately 11 mol/L based on a given mass of solute and its molar mass.
• **Applications**: Molarity is crucial in preparing solutions accurately in laboratories, avoiding errors in chemical reactions and studies.

Understanding molarity aids in understanding both the theoretical and practical aspects of chemical reactions, ensuring the correct proportions and outcomes.