The concentration of a substance can be expressed in several quantities such as moles, percent (%) weight / volume,), molarity, molality, parts per million (ppm) or mole fraction. Concentration shows the amount of solute in a unit of the amount of solvent.
Molality shows how many moles are dissolved in every 1000 grams of solvent.
[tex]\tt molality(m)=\dfrac{mol~solute}{kg~solvent}=\dfrac{mol~solute\times 1000}{mass~solvent(g)}[/tex]
m = Molality
n = number of moles of solute
p = solvent mass (grams)
Given
25.03 g FeCl3 in 5.00 L of pure water
Required
The molality
Solution
mol solute = mol FeCl3 :
= mass : molar mass
= 25.03 g : 162.20 g/mol
= 0.1543
mass solvent=mass water :
= 5000 ml(5 L) x 1 g/ml
= 5000 g = 5 kg
The molality :
= 0.1543 mol : 5 kg( 4 sig fig : 1 sig fig)
= 0.03 m(the result must be 1 sig fig)
molality
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