Write a statement comparing and/or contrasting the concentration units of molali

Question

Write a statement comparing and/or contrasting the concentration units of molality and molarity. Analyze your own and a neighbor’s statement in the following manner. Is it clear? (easily understood, free from the likelihood of misunderstanding) Is it relevant? (important to the topic) Is it accurate? (in accord with the findings of the discipline) If not, rewrite your statement! Write a statement comparing and/or contrasting the concentration units of molality and molarity. Analyze your own and a neighbor’s statement in the following manner. Is it clear? (easily understood, free from the likelihood of misunderstanding) Is it relevant? (important to the topic) Is it accurate? (in accord with the findings of the discipline) If not, rewrite your statement! Write a statement comparing and/or contrasting the concentration units of molality and molarity. Analyze your own and a neighbor’s statement in the following manner. Is it clear? (easily understood, free from the likelihood of misunderstanding) Is it relevant? (important to the topic) Is it accurate? (in accord with the findings of the discipline) If not, rewrite your statement!

Explanation / Answer

Molarity is defined as the number of moles of a solute dissolved in 1 L = 1000 mL of the solution. For example if 2 moles of NaOH are dissolved in 1 L of a solution of NaOH in water, then the molarity of the solution is 2. Molarity of a solution is denoted by the upper case M.

Molality of a solution is the number of moles of solute dissolved in 1 kilogram of the solvent. Mind that, here the weight of the solvent is taken and not the weight of the solution. Say, if we dissolved 1 mole of NaOH in 1 kg of water, the molality of the solution is 1. The weight of the solution is actually (1000 + 40) gm = 1040 gm (molar mass of NaOH is 40 gm/mole). Molality is denoted by the lower case m.

From these two definitions, we know that molarity involves the volume of the solution while molality involves the weight of the solvent. Volume of a solution is subjected to temperature changes. For most solvents, the volume increases with temperature, hence the molarity should decrease (this is because the moles of the solute remains constant while the volume increases; molarity is moles per liter and as volume increases, molarity should decrease). However, molality deals with weight of the solute and the solvent. Weight of a substance is independent of temperature; hence molality is independent of temperature. Thus for those solvents which shows a marked change in volume upon change in temperature, molality is the more correct way of expressing concentration.

Note that I do not have any other statement to compare, I could only explain what the two terms mean.

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