Solubility is found by determining the concentration of a substance in a saturated solution at a specific temperature and pressure.
Understanding Solubility Measurement
Solubility isn't directly "found" like a hidden object. Instead, it's measured experimentally by preparing a saturated solution. A saturated solution contains the maximum amount of a solute that can dissolve in a given solvent at a specific temperature and pressure.
Steps to Determine Solubility
Here's a breakdown of how solubility is typically found:
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Preparation of a Solution: An excess amount of the solute (the substance being dissolved) is added to the solvent (the substance doing the dissolving).
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Achieving Saturation: The mixture is stirred or agitated continuously at a controlled temperature until no more solute dissolves, ensuring that the solution reaches saturation.
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Separation of Undissolved Solute: Any undissolved solute is carefully removed from the solution. This is often achieved through filtration or decantation. It's crucial to ensure that no dissolved solute is lost during this process.
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Determining Concentration: The concentration of the dissolved solute in the saturated solution is then determined. This can be done using various analytical techniques, such as:
- Evaporation: A known volume of the saturated solution is evaporated to dryness, and the mass of the remaining solute is measured. This method is suitable for non-volatile solutes.
- Titration: If the solute is a titratable substance (e.g., an acid or base), titration with a standard solution can be used to determine its concentration.
- Spectrophotometry: If the solute absorbs light at a specific wavelength, spectrophotometry can be used to determine its concentration based on its absorbance.
- Other Analytical Techniques: Depending on the solute and solvent, other techniques like gravimetric analysis, chromatography, or atomic absorption spectroscopy can be employed.
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Expressing Solubility: The solubility is then expressed as the concentration of the solute in the saturated solution. Common units for solubility include:
- grams of solute per liter of solvent (g/L)
- grams of solute per 100 grams of solvent (g/100g)
- moles of solute per liter of solution (mol/L or Molarity)
Factors Affecting Solubility
It's crucial to note that solubility is influenced by several factors:
- Temperature: The solubility of most solids in liquids increases with increasing temperature. However, the solubility of gases in liquids generally decreases with increasing temperature.
- Pressure: Pressure has a significant effect on the solubility of gases in liquids, but a minimal effect on the solubility of solids or liquids in liquids. Henry's Law describes the relationship between gas pressure and solubility.
- Nature of Solute and Solvent: The "like dissolves like" rule generally applies. Polar solvents tend to dissolve polar solutes, while nonpolar solvents tend to dissolve nonpolar solutes. Intermolecular forces between solute and solvent molecules play a key role.
Example
Imagine you want to find the solubility of sugar in water at 25°C. You would:
- Add a lot of sugar to water.
- Stir the mixture at 25°C until no more sugar dissolves.
- Filter out the undissolved sugar.
- Evaporate a known volume of the filtered solution.
- Weigh the remaining sugar.
- Calculate the mass of sugar per volume of water, which is the solubility at that temperature.
In conclusion, solubility is experimentally determined by measuring the concentration of a solute in a saturated solution at a specified temperature and pressure using various analytical methods.
