Which of the following is more reliable?

Molality (m)

The correct answer is option 3. Molality (m).

The most reliable concentration unit is Molality (m).

% (w/v): This represents percent by weight per volume. It depends on both the solute and solvent masses and their densities. Changes in temperature can affect the density of solutions, making % (w/v) less reliable for precise concentration measurements across varying temperatures.

Molarity (M): This represents moles of solute per liter of solution. Similar to % (w/v), temperature variations can affect the solution volume due to thermal expansion/contraction, impacting the accuracy of Molarity at different temperatures.

Molality (m): This represents moles of solute per kilogram of solvent. Molality is independent of temperature changes because the mass of the solvent (in kg) remains relatively constant. This makes Molality the most reliable unit for concentration measurements across varying temperatures.

Strength (S): This term is a general descriptor of concentration and doesn't have a universally defined unit. It can be expressed in various ways like % (w/v) or Molarity, inheriting their limitations.

Therefore, for reliable concentration measurements that are independent of temperature variations, Molality (m) is the preferred choice.