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Concentration of Solutions

Objectives:

    1.    Measure the concentrations in terms of molarity, molality, and mole fraction.

    2.    Differentiate between saturated, unsaturated, and supersaturated solutions.

Key Terms:

    concentration        molarity        molality        mole fraction        saturated        unsaturated        supersaturated

Notes: (15-2)

In chemistry there are many ways to describe the measurement of liquids.  With solutions we often talk about the concentration (amount of solute per solvent).  

Molarity describes the moles of solute per liter of solvent
Molarity = moles solute / liters solvent
ex: 73g of HCl powder is dissolved into 4L of water
(73g HCl / 4L) x (1 mole HCl /36.5 g HCl) = 2 mol HCl / 4L = 0.5M HCl
Molarity is expressed with a capital M
Affected by temperature

 

Molality describes the moles of solute per kilogram of solvent
Molality = moles solute / kilogram of solvent
ex: 117g of NaCl is dissolved into 1000g of water
(117g NaCl / 1000g) x (1 mole NaCl /58.5 g NaCl) x (1000g / 1kg)= 2 mol NaCl / 1kg = 2m NaCl
Molality is expressed with a lower case m
NOT affected by temperature

 

Mole Fraction
Mole Fraction solute = moles solute / moles solvent
Mole Fraction solvent = moles solvent / moles solute
ex: 117g of NaCl is dissolved into 1000g of water
(117g NaCl / 1000g) x (1 mole NaCl /58.5 g NaCl) x (18g / 1 mole H2O)= 2 mol NaCl / 55.6 mol H2O = 0.036 or 3.6%
Mole Fraction has no units
Not affected by temperature

 

Saturation

The point where a solution contains the maximum amount of solute
Dependant on temperature and pressure
Amount of solute increases with temperature and decreases with pressure

Supersaturated

Containing more solute than is normally possible in a solution
Can be made by cooling a saturated solution

Unsaturated

Any solution that has not reached the point of saturation
 

 

Last modified: March 25, 2003