Solubility and ion concentration

In this lesson, we will learn:

• To recall the definition of electrolyte and non-electrolyte and define solubility in terms of equilibrium.
• How to identify electrolytes and non-electrolytes using molecular formulae and predict ionic or molecular solutions based on this.
• How to measure solubility of substances in solution.

• To begin this chapter, let’s recall our definitions and knowledge about solubility and how solutions are made:
  • A solution is made of:
    • A solvent, the liquid substance in excess that does the dissolving, such as water.
    • A solute, the substance in a smaller quantity that is being dissolved, such as table salt.
    Together, these make a solution.
  • Our definition of solubility from Solution chemistry and solubility: introduction is “the extent that a substance can dissolve in a specific amount of another substance at a specific temperature”.
  • From this we learned that like dissolves like, which means substances similar to each other will interact favorably and dissolve in one another, whichever one is the solute or solvent.
  • Saturation is when a solution has dissolved the maximum amount of solute possible, given the solute’s solubility. When more solute was added after this point it simply collected, undissolved, at the bottom of the container.
  • Polarity is a very important property of chemicals when discussing solubility. Polarity (see Polarity) in a molecule exists because electrons (and charge) is unequally distributed in a molecule. This gives molecules a permanent ‘partial charge’ as there is greater electron density in some regions of the molecule than in others.
  • When comparing chemical substances and their solubility, chemicals are studied in terms of hydrophilic ("water-loving") or lipophilic ("fat-loving") nature.  
    • Hydrophilic molecules are molecules with high polarity due to a large gap in electronegativity between atoms bonded together. Hydrophilic molecules are soluble in water, and generally insoluble in organic compounds like simple hydrocarbons. Hydrophilic compounds are polar.
    • Lipophilic molecules are molecules with low polarity in the molecule – there is no significant gap in electronegativity between the atoms that form the molecule. This means the molecule has little to no partial charge, which makes it insoluble in water, and generally soluble in organic compounds like simple hydrocarbons. Lipophilic compounds are non-polar.
  
  
• The ‘like dissolves like’ rule is important to remember for solubility. Hydrophilic compounds dissolve in polar solvents and lipophilic compounds dissolve in non-polar solvents.
  However, these two cases will make solutions with different properties:
  • Some compounds are electrolytes – substances that produce ions when dissolving to make a solution that conducts electricity. Electrolytes make ionic solutions.
    • Salt compounds made of metal and non-metal atoms are electrolytes and become ionic in solution. This is because salts have atoms or groups that have a very large difference in their electronegativity or electron withdrawing/donating properties. Salts therefore form oppositely charged ions in solution.
  • Some compounds are non-electrolytes – these compounds remain neutral when dissolved and the resulting solution does not conduct electricity.
    • Lipophilic compounds are generally non-electrolytes because the molecule likely doesn’t contain partial charges between the atoms bonded together, so ions would not form when it is dissolved in water, for example.
  
  
• By studying the molecular formula of a compound, you can normally predict whether a compound is an electrolyte or not. Equations can be written to describe this.
  • Compounds that have both metal and non-metal atoms will likely be ionic in solution and are therefore electrolytes.
    • Salts fall into this category as mentioned already.
    • For example iron (iii) chloride, FeCl₃, has a molecular formula that shows an iron (metal) atom and three chlorine (non-metal) atoms. This suggests it would be an electrolyte and form an ionic solution. In an equation:
      
      FeCl₃ $\enspace$→$\enspace$ Fe³⁺_(aq) + 3Cl^-_(aq)
  • Covalent compounds, made of only non-metal atoms, are generally non-electrolytes and will form molecular solutions.
    • Most organic compounds fall into this category unless it is an organic acid, which may be ionic to some degree.
    • For example hexane, C₆H₁₄, is made of just carbon and hydrogen. Both are non-metals and it is an organic compound. Both of these facts are strong evidence that it is a non-electrolyte and forms a molecular solution.
    • For non-electrolytes pay attention to state symbols. If it is not dissolved in water, do not call it aqueous or label the compound _(aq)!
  
  
• Now that we have covered dynamic equilibrium, our definitions of solubility and saturation can be adjusted to be more accurate:
  • Solubility is the equilibrium concentration of a substance dissolved in solution. This is sometimes measured in grams per liter (g / L) but also often measured in moles per liter or moles per cubic decimeter (mol dm^-3) so may be called the molar solubility.
  • Saturation is when a substance that is dissolved is in equilibrium with its undissolved state.
  
  
• Using these two ‘updated’ definitions we say that saturation occurs when two things are happening:
  • There is some undissolved material.
  • The undissolved material is in equilibrium with its dissolved state.
  
  
• Take an example of a salt e.g. sodium chloride, NaCl, in equilibrium between the solid and aqueous phases:
  
  NaCl_(s) $\enspace\rightleftharpoons \enspace$ Na⁺_(aq) + Cl^-_(aq)
  This can be separated into its two individual reactions – the dissolving (forward) process and the crystallization (backward) process:
  
  Dissolving: NaCl_(s) $\enspace$→$\enspace$ Na⁺_(aq) + Cl^-_(aq)
  Crystallization: Na⁺_(aq) + Cl^-_(aq) $\enspace$→$\enspace$ NaCl_(s)
  
• Calculating solubility is usually done by adding the solute to 1 liter of solvent (assumed to be at 25^oC) , and with the mass of solute recorded it is fairly simple to calculate:
  • If a saturated 1 L solution of NaCl _(aq) contains 360g of NaCl. Then the solubility of the solution is 360 g / L. This can be expressed as molar solubility:
    
    $[NaCl] = \frac{360g \enspace NaCl}{1 \enspace L} * \frac{1\, mol \enspace NaCl}{58.4g \enspace NaCl} = 6.16 M$
    
  • Converting from molar solubility to solubility in grams per liter is possible too, for example with copper sulfate, CuSO ₄ which has molar solubility of 0.877 M.
    
    $Solubility = \frac{0.877 \enspace mol}{1 \enspace L} * \frac{159.6g}{1 \enspace mol} = 140 \frac{g}{L}$
    
    When finding solubility of a substance, you can assume the solution is at 25^oC, but pay attention to what volume of solution is quoted – if it is not in liters, convert it into liters first.