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Question

What is formality? Explain with example.


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Solution

Formality:

  • “It is defined as the number of gram formula mass of solute dissolved in per litre of solution.” It is used for ionic compounds.
  • The formality of the solution is denoted by the symbol, F.
  • It is similar to normality.

Formula used to calculate the formality:

  • Formality=Number of gram formula mass of soluteVolume of solution in litre

Or,

  • Formality=Moles of soluteVolume of solution in litre

Or,

  • Formality=Given weight of soluteMolecular weight of solute×Volume of solution in litre

For example:

Calculate the formality of sodium chloride (NaCl) solution, 5.85g of which have been dissolved to form 250mL of the given solution.

Step 1: Given,

  • Weight of solute (NaCl)=5.85g
  • Molecular weight of solute (NaCl)=58.5gmol-1
  • Volume of solution =250mL=0.250L(1L=1000ml)

Step 2: To determine the formality of NaCl solution.

  • Formality=Given weight of soluteMolecular weight of solute×Volume of solution in litre
  • Formality=5.85g58.5gmol-1×0.250L
  • Formality=0.4F

Therefore, the formality of NaCl solution is 0.4F.


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