# Using Freezing Point to determine Molar Mass Objectives: To measure freezing points of known concentrations of solutions To use freezing point to determine.

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Using Freezing Point to determine Molar Mass Objectives: To measure freezing points of known concentrations of solutions To use freezing point to determine molar mass of compounds

DataSolnFP ∆T fp Molality Molar Mass exp Molar Mass theor. % Error Water C 6 H 12 O 6 KNO 3

Class Data SolnFPFPFPFPFPFP C 6 H 12 O 6 KNO 3

The Set-up

Notes continued Solutions extend the liquid state of the solvent Solutions extend the liquid state of the solvent ID of solute is not important. NUMBER of particles in solution determines the effect ID of solute is not important. NUMBER of particles in solution determines the effect 1 mole of particles reduces the freezing temp of 1 liter of water by 1.85°C = “molal freezing point constant” K fp 1 mole of particles reduces the freezing temp of 1 liter of water by 1.85°C = “molal freezing point constant” K fp Same type of relationship exists for BP Same type of relationship exists for BP m (molality) = mols solute/kg solvent m (molality) = mols solute/kg solvent m can be solved by this formula: m=  T fp /K fp m can be solved by this formula: m=  T fp /K fp

Examples of how K fp is used Soluble ionic solids dissociate Soluble ionic solids dissociate C 12 H 22 O 11 C 12 H 22 O 11 NaCl NaCl CaCl 2 CaCl 2 Kfp is per particle (moles of particles), so must be multiplied by # of particles Kfp is per particle (moles of particles), so must be multiplied by # of particles

Calculations Use this equation to determine molar mass (exp): Use this equation to determine molar mass (exp): g solute (1000 g) (1 kg) g solvent (1 kg) (mol solute)  determine theoretical molar mass

Whiteboard none none

Freezing Point Lab Conclusion 1. On a particle level, why do you think FP is depressed in solutions? 2. If you tested for BP, why would you have to boil pure water first to determine water’s BP? Note, we assumed FP of water to be zero celsius. 3. CaCl 2 is widely used to melt ice. a.Why does ice melt under the freezing point? b.Why is one mole of CaCl 2 more effective than one mole of NaCl? Write Summary....

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