Question

How to provide step by step calculations for percent ammonia by back titration

Answer 1

Her is the complete step by step calculations for the percent ammonia by back calculation

A back titration, or indirect titration, is generally a two-stage analytical technique:

1. Reactant A of unknown concentration is reacted with excess reactant B of known concentration.
2. A titration is then performed to determine the amount of reactant B in excess.Back titrations are used when:

• one of the reactants is volatile, for example ammonia.
• an acid or a base is an insoluble salt, for example calcium carbonate
• a particular reaction is too slow
• direct titration would involve a weak acid - weak base titration

• Determine the amount of ammonia in the cloudy ammonia solution

• Calculate the total moles of HCl originally added to the diluted cloudy ammonia solution:
  n(HCl_{total added}) = c x V
  c = concentration (molarity) = 0.100 mol L^-1
  V = volume = 50.00 mL = 50.00 x 10^-3 L
  n(HCl_{total added}) = 0.100 x 50.00 x 10^-3 = 5.00 x 10^-3 mol
• Calculate the moles of HCl that reacted with the ammonia in the diluted cloudy ammonia solution
  n(HCl_titrated) + n(HCl_{reacted with ammonia}) = n(HCl_{total added})
  n(HCl_{total added}) = 5.00 x 10^-3 mol
  n(HCl_titrated) = 2.150 x 10^-3 mol
  2.150 x 10^-3 + n(HCl_{reacted with ammonia}) = 5.00 x 10^-3
  n(HCl_{reacted with ammonia}) = 5.00 x 10^-3 - 2.150 x 10^-3 = 2.85 x 10^-3 mol
• Write the balanced chemical equation for the reaction between ammonia in the cloudy ammonia solution and the HCl_(aq).

  NH_3(aq) + HCl_(aq) → NH₄Cl_(aq)

• From the balanced chemical equation, calculate the moles of NH₃ that reacted with HCl.
  From the equation, 1 mol HCl reacts with 1 mol NH₃
  So, 2.85 x 10^-3 mol HCl had reacted with 2.85 x 10^-3 mol NH₃ in the cloudy ammonia solution.
• Calculate the ammonia concentration in the cloudy ammonia solution.
  c = n ÷ V
  n = moles (NH₃) = 2.85 x 10^-3 mol (moles of NH₃ that reacted with HCl)
  V = volume (NH₃(aq)) = 25.00 mL = 25.00 x 10^-3 L (volume of ammonia solution that reacted with HCl)
  c = concentration (molarity) = 2.85 x 10^-3 ÷ 25.00 x 10^-3 = 0.114 mol L^-1
• The concentration of ammonia in the cloudy ammonia solution was 0.114 mol L^-1
• From this data you can easily calculate the % ammonia