Maharashtra BoardEnglish MediumSTD 12 ScienceChemistrySolutions2 Marks
Question
Define the following: $(i)$ Molality $(ii)$ Mole fraction.
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Answer
Mole fraction is the ratio of the number of moles of a particular component of a solution to the total number of moles of the solution.
If a substance $‘A \ ’$ dissolves in substance $‘B \ ’$ and their number of moles are $n_A$ and $n_B,$ respectively, then the mole fraction of $A$ and $B$ are given as:
Mole fraction of $A=\frac{\text { Number of moles of } A}{\text { Number of moles of solution }}$
$=\frac{ n _{ A }}{ n _{ A }+ n _{ B }}$
Mole fraction of $B=\frac{\text { Number of moles of } B}{\text { Number of moles of solution }}$
$=\frac{n_B}{n_A+n_B}$ Molality:
Molality $(m)$ is defined as the number of moles of the solute per kilogram of the solvent. It is expressed as:
Molality $(m)=\frac{\text { Moles of solute }}{\text { Mass of solvent in } \ Kg }$
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