molality
Defination : Total moles of a solute contained in a kilogram of a solvent that’s called molality.
It is a measure of the concentration of a solute in a solvent.
It is defined as the total number of moles of the solute divided by the mass of the solvent in kilograms.
Unlike molarity, which is based on the volume of the solvent, molality takes into account the mass of the solvent, making it a more accurate measure of concentration especially when dealing with solutions that undergo significant changes in volume with temperature.
The unit is mol/kg.
molality Formula
\[m=\frac{Moles\;Of\;Solute}{Mass\;of\;solvent\;in\;Kg}\]
\[m=\frac{Solute\;in\;gm/Molecular\;Weight}{Mass\;of\;solvent\;in\;gm/1ooo}\]
\[m=\frac{Solute\;in\;gm\times1000}{Mass\;of\;solvent\;in\;gm\times\;M.W}\;\;\]
This formula calculates the molality of a solution, given the mass of the solute, molar mass of the solute, and mass of the solvent.
\[m=\frac{Solute\;in\;gm\times1000}{Mass\;of\;solvent\;in\;gm\times\;M.W}\;\;\]
– gm of solute: mass of the solute in grams
– M.W: molar mass of the solute in g/mol
– 1000: conversion factor from grams to milligram
– solvent in gm: mass of the solvent in grams
The result, m, will be in moles per liter (mol/L), which is the unit for molarity.
molality Uses
Mainly used in concentrations of solutions related to vapor pressure and temperature changers.
Used in determining the boiling point or melting point.
molality Calculation
01) 2 gm NaOH dissolve in 4kg Solvent then define molality ?
NaOH = 2 gm
NaOH Molecular Weight = 40 gm/moles
Solvent = 4 kg = 4000 gm
M = ?
\[m=\frac{Solute\;in\;gm\times1000}{Mass\;of\;solvent\;in\;gm\times\;M.W}\;\;\]
\[m=\frac{2\times1000}{4000\times\;40}\;\;\]
m= 0.125
02) 5 gm of Toluene is Dissolve in 300 gm Benzene. Define the molality ?
Toluene M. W = 92 gm/mole
Toluene = 5 gm
Solvent in gm = 300 gm
\[m=\frac{Solute\;in\;gm\times1000}{Mass\;of\;solvent\;in\;gm\times\;M.W}\;\;\]
\[m=\frac{5\times1000}{300\times\;92}\;\;\]
m = 0.18
03) How Much gm Methanol Require 1m Methanol 5kg water of solution ?
Methanol gm = ?
Methanol M.W = 32 gm /mole
m= 1
Solvent =5 kg = 5000 g
\[m=\frac{Solute\;in\;gm\times1000}{Mass\;of\;solvent\;in\;gm\times\;M.W}\;\;\]
\[1=\frac{Solute\;in\;gm\times1000}{5000\times\;32}\;\;\]
Methanol in gm =160
04) What is the molality when 1.5 mol is dissolved in 2.75 kg of solvent ?
Mass of solute = 1.5 mol
mass of solvent =2.75
\[m=\frac{Moles\;Of\;Solute}{Mass\;of\;solvent\;in\;Kg}\]
\[m=\frac{1.5}{2.75}\]
m = 0.55
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