Define molality and explain its difference from molarity.

Define molality and explain its difference from molarity.

• A. Molality is moles of solute per kilogram of solvent; molarity is moles of solute per liter of solution
• B. Molality is moles per liter; molarity is moles per kilogram
• C. Molality is moles of solute per kilogram of solvent; molarity is moles per liter of solution
• D. Molality is mass of solvent per mole; molarity is volume of solution per mole

Answer: (A)

Molality and molarity are both ways to express how much solute is present, but they use different bases for the amount of solvent or solution. Molality is defined as the number of moles of solute per kilogram of solvent, while molarity is defined as the number of moles of solute per liter of solution. The crucial distinction is that the denominator for molality is mass of solvent, whereas the denominator for molarity is volume of the entire solution. Because mass stays essentially constant with temperature but volume changes with temperature, molality is largely temperature-independent, which is why it’s preferred in many thermodynamic and colligative-property calculations. In contrast, molarity can vary with temperature due to changes in solution volume. For example, dissolving 1 mole of solute in 1 kilogram of solvent gives 1 molal, but the resulting molarity depends on the final solution’s volume, which may not be exactly 1 liter.

Molality and molarity are both ways to express how much solute is present, but they use different bases for the amount of solvent or solution. Molality is defined as the number of moles of solute per kilogram of solvent, while molarity is defined as the number of moles of solute per liter of solution. The crucial distinction is that the denominator for molality is mass of solvent, whereas the denominator for molarity is volume of the entire solution. Because mass stays essentially constant with temperature but volume changes with temperature, molality is largely temperature-independent, which is why it’s preferred in many thermodynamic and colligative-property calculations. In contrast, molarity can vary with temperature due to changes in solution volume. For example, dissolving 1 mole of solute in 1 kilogram of solvent gives 1 molal, but the resulting molarity depends on the final solution’s volume, which may not be exactly 1 liter.