Molarity (M) is defined as number of moles of solute dissolved in:

One cubic decimeter of solution

The correct answer is option 1. One cubic decimeter of solution.

Molarity (M) is a way to express the concentration of a solution, and it is defined as the number of moles of solute dissolved in one liter of solution.

Molarity (M) is given by the formula:

\(\text{Molarity} (M) = \frac{\text{Number of moles of solute}}{\text{Volume of solution in liters}} \)

Moles of Solute: A mole is a standard unit in chemistry that represents \(6.022 \times 10^{23}\) particles (atoms, molecules, ions, etc.). The number of moles of solute refers to the quantity of the substance dissolved in the solution.

Volume of Solution: The volume is measured in liters. When we say “one cubic decimeter,” we mean one liter (1 dm³ = 1 L).

Why it’s Not Defined in Terms of Solvent:

Volume of Solvent: If molarity were defined in terms of the volume of just the solvent (the liquid that dissolves the solute), it would not account for the volume change that occurs when the solute is added. Molarity is concerned with the final volume of the solution, which includes both the solute and the solvent.

Moles of Solvent: Molarity does not involve the number of moles of solvent. Instead, it focuses on the total volume of the solution after mixing the solute with the solvent.

Molal Solution: Molality (m) is another concentration measure, defined as the number of moles of solute per kilogram of solvent. It is different from molarity because it does not depend on the volume of the solution but rather the mass of the solvent.

Example: If you dissolve 1 mole of sodium chloride (NaCl) in water and make the total volume of the solution up to 1 liter, the molarity of the sodium chloride solution is 1 M (1 mole per liter of solution). In summary, molarity is defined based on the final volume of the solution (including both the solute and solvent), and thus it is measured in terms of liters of solution.