Question

Number of moles of a solute per kilogram of a solvent is called: (a) Normality (b) Formality (c) Molality (d) Molarity

Short answer

The correct answer is (c) Molality.

Step-by-step solution

Understanding the question

The given question asks for the definition of a term that represents the number of moles of a solute per kilogram of a solvent.

Recall key definitions

It's important to recall the meanings of the terms: - Normality: Number of equivalents per liter of solution. - Formality: Number of moles of solute per liter of solution (similar to molarity but for formulas, not necessarily existing as distinct molecules). - Molality: Number of moles of solute per kilogram of solvent. - Molarity: Number of moles of solute per liter of solution.

Identify the correct definition

From the definitions, we observe that molality refers to the 'number of moles of solute per kilogram of solvent,' which exactly matches the given description in the question.

Key concepts

Moles of Solute

When we talk about moles of solute, we're referring to a unit of measurement used to express the amount of a chemical substance. A "mole" is a standard scientific unit for measuring large quantities of very small entities such as atoms, molecules, or other specified particles. One mole is generally comparable to Avogadro's number, which is approximately \(6.022 \times 10^{23}\) particles. This concept is central in chemistry as it allows chemists to quantify elements and compounds in chemical reactions and solutions.
Moles are used because working with actual individual molecules or atoms would be impractical due to their minuscule size. By using moles, chemists and students can easily convert measurements into relatable amounts that are easier to understand and use in calculations.

Kilogram of Solvent

The kilogram of solvent is a key component in determining the molality of a solution. Unlike other concentration units like molarity, which depend on the volume of the solution, molality focuses on the mass of the solvent. This is significant because the mass of a liquid doesn't change with temperature or pressure, whereas the volume can fluctuate. Therefore, molality is often considered a more stable measurement for concentration in varying conditions.
When calculating molality, one ensures precision by measuring in kilograms. This means that if you have, for instance, a solute dissolved in water, you should weigh the water (solvent) and express it in kilograms to ascertain the solution's molality precisely. This adherence to mass over volume is what gives the molality metric its robustness in scientific calculations.

Concentration Units

Concentration units are vital in chemistry as they describe how much of a solute is present in a solvent or solution. There are several concentration units, each serving different scientific needs and contexts. Here's a brief overview of some common concentration units:

• Molality: Calculated as the moles of solute per kilogram of solvent. It's particularly useful in situations where temperature and pressure vary as it is unaffected by those changes.
  
• Molarity: Measures the moles of solute per liter of solution. It's widely used but sensitive to changes in temperature and pressure, as it depends on the volume of the solution rather than its mass.
  
• Normality: Indicates the number of equivalents of the solute per liter of solution. It's often used in acid-base reactions and redox reactions.
  
• Formality: Similar to molarity, it refers to the number of moles of the formula unit per liter, typically used for substances with complex molecular makeup.

Understanding these units allows chemists and students to accurately prepare solutions and conduct experiments under precise conditions.