ANS:A - molal

"Molal" refers to a concentration unit used in chemistry to express the concentration of a solution in terms of the number of moles of solute per kilogram of solvent. It is denoted by the symbol "m." Here's how it works:

1. Definition: Molality (molal concentration) is defined as the number of moles of solute (𝑛n) dissolved in one kilogram of solvent (𝑚m).

Molality (m)=Number of moles of solute (n)Mass of solvent (kg)Molality (m)=Mass of solvent (kg)Number of moles of solute (n)​

2. Units: Molality is expressed in units of moles per kilogram (mol/kg).
3. Difference from Molarity: Molality differs from another common concentration unit, molarity (M), which expresses the concentration of a solution in terms of the number of moles of solute per liter of solution. Molality is preferred over molarity in certain situations, particularly when working with solutions at extreme temperatures or when comparing the concentrations of solutions with different densities.
4. Temperature Dependence: Unlike molarity, which can change with temperature due to the change in volume of the solution, molality remains constant with changes in temperature because it is based on the mass of solvent, which typically does not change significantly with temperature.
5. Example: For example, if you dissolve 0.5 moles of sodium chloride (NaCl) in 1 kilogram of water, the molality of the solution would be 0.5 mol/kg.

Molality is a useful concentration unit, especially in certain areas of chemistry such as colligative properties (where the behavior of solutions depends on the concentration of solute particles) and in applications where precise control of solution composition is required.