Define molality and molarity. Which is better and why?
Answers
Answer:
Molarity can be defined as the number of moles of a substance (known as the solute) that is dissolved in precisely 1 liter of a solution (solvent and solute combined).
Molarity (M) =
Litreinsolution
moleofsolute
Molality can be defined as the number of moles of substance (known as the solute) that is found in a certain mass of solvent given in kg, that it is dissolved in.
Molality(m) =
Massofsolvent(kilogram)
molesofsolute(moles)
Difference between Molarity and Molality
1) Molarity is concentration of a substance that is calculated as the number of moles of solute that is dissolved in 1 liter of solution while molality is concentration of a substance that is calculated as the number of moles of solute found in 1 kg of solvent.
2) The symbol for molarity is M, while that for molality is m (sometimes written as –m or m to distinguish it from mass).
3) The formula for molarity is moles / liter while the formula for molality is moles / kg.
4) Molarity is affected by changes in temperature while molality is unaffected by changes in temperature.
5) Molarity is affected by changes in pressure while molality is unaffected by changes in pressure.
6) Molarity may result in an imprecise and inaccurate concentration, while molality results in an accurate precise measurement of concentration.
Molality is considered better for expressing the concentration as compared to molarity because the molarity changes with temperature because of expansion or contraction of the liquid with temperature.But molality does not changes with temperature because the mass of the solvent does not change with change in temperature.