Molality and Composition Notes

Molality

Molality is defined as the number of moles of solute per kilogram of solvent. It is different from molarity, which is defined as the number of moles of solute per liter of solution. The unit used to measure molality is mol/kg or m.

$Molality = \frac{moles \ of \ solute}{kilograms \ of \ solvent}$

Difference between Molality and Molarity

• Temperature Dependence: Molality is temperature independent because it is based on mass, which doesn't change with temperature. Molarity, however, is temperature dependent because the volume of a solution can change with temperature.
• Use Cases: Molality is often used in colligative properties calculations, where the number of solvent molecules is important. Molarity is more commonly used in titrations and other applications where the concentration of the solute is important.

Percent Composition

Percent composition refers to the percentage by mass of each element in a compound or each component in a mixture.

Calculating Percent Composition

1. Percent Composition by Mass: To find the percent composition by mass of an element in a compound, you divide the mass of the element in one mole of the compound by the molar mass of the compound, and then multiply by 100%.

   $Percent \ Composition = (\frac{Mass \ of \ element \ in \ 1 \ mole \ of \ compound}{Molar \ mass \ of \ compound}) \times 100$

2. Percent Composition in Mixtures: To find the percent composition of a component in a mixture, divide the mass of the component by the total mass of the mixture, and then multiply by 100%.

   $Percent \ Composition = (\frac{Mass \ of \ component}{Total \ mass \ of \ mixture}) \times 100$

Example

For example, to find the percent composition of carbon in methane (CH4), you would divide the molar mass of carbon (approximately 12.01 g/mol) by the molar mass of methane (approximately 16.04 g/mol), and then multiply by 100%.

$Percent \ Composition \ of \ C = (\frac{12.01 \ g/mol}{16.04 \ g/mol}) \times 100$

Crystallization

Crystallization is a separation technique used to obtain pure solid substances from a solution. It involves dissolving a solid in a solvent at an elevated temperature to create a saturated solution, then cooling the solution to reduce the solute's solubility, causing crystals to form. These crystals are then separated from the remaining solution, washed, and dried to obtain the pure solid.

Process of Crystallization

1. Dissolving: The solid is dissolved in a suitable solvent at or near its boiling point to create a saturated solution.
2. Filtration (Optional): If the solution contains insoluble impurities, it is filtered while hot to remove them.
3. Cooling: The solution is allowed to cool slowly. As the temperature decreases, the solubility of the solute decreases, causing it to come out of solution and form crystals.
4. Crystal Formation: Seed crystals may be added to initiate crystallization. Slow cooling promotes the formation of larger, purer crystals.
5. Filtration: The crystals are separated from the remaining solution (mother liquor) by filtration.
6. Washing: The crystals are washed with a small amount of cold solvent to remove any remaining impurities.
7. Drying: The purified crystals are dried to remove any residual solvent.

Factors Affecting Crystallization

• Solubility: The solute must be more soluble at high temperatures than at low temperatures.
• Solvent: The choice of solvent is crucial. It should dissolve the solute well at high temperatures and poorly at low temperatures. It should also be easily removed from the crystals.
• Cooling Rate: Slow cooling generally leads to the formation of larger, purer crystals, while rapid cooling results in smaller, less pure crystals.