Molality of a solution in which 0.00585g of NaCl is dissolved in 500g of water.
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★ Solution:
Using molality formula
m = w/GMW × 1000/W(g)
Where
- m : molality = ?
- w : mass of solute = 0.00585 g
- GMW : Gram molecular weight of solute = NaCl = 23 + 35.5 = 58.5
- W(g) : weight in gram of solvent = 500g
Substituting the values we have
→ m = 0.00585/58.5 × 100/500
→ m = 0.0001 × 0.2
→ m = 0.00002 = 2 × 10-⁵
Hence , molality of the solution = 2 × 10-⁵ mol/g
★ Concept:
Molality : The molality of a given solution (m) is defined as "the number of moles of the the solute present in one kg or 1000g of solvent"
In other form:
m = n/W(kg)
Here,
n = mass of solute(w)/GMW of solute
Substituting we have
m = w/GMW × 1/W(kg)
Into grams
m = w/GMW × 1000/W(g)
m = n/W(kg) = w/GMW × 1/W(kg) = w/GMW × 1000/W(g)
SI ( Standard international ) unit : mol/kg
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