CALCULATIONS INVOLVING COLLIGATIVE PROPERTIES

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CALCULATIONS INVOLVING COLLIGATIVE PROPERTIES. Section 18.4. AFTER READING SECTION 18.4, YOU SHOULD KNOW:. How to calculate molality (m) How to calculate the molar mass of a molecular compound from the freezing point depression or boiling point elevation. MOLALITY .

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### CALCULATIONS INVOLVING COLLIGATIVE PROPERTIES

Section 18.4

AFTER READING SECTION 18.4, YOU SHOULD KNOW:
• How to calculate molality (m)
• How to calculate the molar mass of a molecular compound from the freezing point depression or boiling point elevation
MOLALITY
• Molality (m) – the number of moles of solvent dissolved per kilogram of solvent
• m = mol solute / kg solvent
• Mole fraction

nA = moles of solute A nB moles of solvent B

• XA = nA XB = nB

nA + nB nA + nB

BOILING POINT ELEVATION
• Change in boiling point temperature
• ΔTb = Kb * m
• ΔTb = change in boiling pt temp
• Kb = molal boiling-point elevation constant
• Depends on the solvent
• Units (oC / m)
• m = molality (mole / kg)
FREEZING POINT DEPRESSION
• Change in freezing point temperature
• ΔTf = Kf * m
• ΔTf = change in freezing pt temp
• Kf = molal freezing-point depression constant
• Depends on the solvent
• Units (oC / m)
• m = molality (mole / kg)

AFTER READING SECTION 18.4, YOU SHOULD KNOW:

• How to calculate molality (m)
• How to calculate the molar mass of a molecular compound from the freezing point depression or boiling point elevation