Acid-Base Titration (OCR A Level Chemistry)

• There are a series of suggested practicals for PAG 2 - Acid-base titration
  • PAG 2.1: Determination of the concentration of hydrochloric acid
    • This involves making a standard solution of sodium hydrogencarbonate and titrating this against a solution of hydrochloric acid in order to work out the concentration of the acid.
  • PAG 2.2: Determination of the molar mass of an acid
    • This involves making a solution of an unknown hydrated acid (e.g. citric acid monohydrate), then titrating this against a sodium hydroxide solution of known concentration. This allows the molar mass of the acid to be determined, and with additional information, the formula of the acid.
  • PAG 2.3: Identification of an unknown carbonate
    • This involves making a solution of an unknown Group 1 carbonate and then titrating this solution against hydrochloric acid of a known concentration to determine the molar mass of the carbonate and hence the identity of the metal.
• All three suggested practicals use volumetric analysis, which requires making standard solutions and completing titration experiments.
  • The main difference between the suggested practicals is how the titration results are processed

Volumetric Analysis

• Volumetric analysis is a process that uses the volume and concentration of one chemical reactant (a volumetric solution) to determine the concentration of another unknown solution
• The technique most commonly used is a titration
• The volumes are measured using two precise pieces of equipment, a volumetric or graduated pipette and a burette
• Before the titration can be done, the standard solution must be prepared
• Specific apparatus must be used both when preparing the standard solution and when completing the titration, to ensure that volumes are measured precisely

Some key pieces of apparatus used to prepare a volumetric solution and perform a simple titration 

1. Beaker
2. Burette
3. Volumetric Pipette
4. Conical Flask
5. Volumetric Flask

Making a Volumetric Solution

• Chemists routinely prepare solutions needed for analysis, whose concentrations are known precisely
• These solutions are termed volumetric solutions or standard solutions
• They are made as accurately and precisely as possible using three decimal place balances and volumetric flasks to reduce the impact of measurement uncertainties
• The steps are:

Volumes & concentrations of solutions

• The concentration of a solution is the amount of solute dissolved in a solvent to make 1 dm³ of  solution
  • The solute is the substance that dissolves in a solvent to form a solution
  • The solvent is often water

• A concentrated solution is a solution that has a high concentration of solute
• A dilute solution is a solution with a low concentration of solute
• Concentration is usually expressed in one of three ways:
  • moles per unit volume
  • mass per unit volume
  • parts per million

Answer:

Step 1: Find the number of moles of NaHCO₃ needed from the concentration and volume:

• • number of moles  = concentration (mol dm^-3) x volume (dm³)  
  • n = 0.200 mol dm^-3 x 0.250 dm³
  • n = 0.0500 mol

Step 2: Find the molar mass of NaHCO₃ 

• • M_r = 23.0 + 1.0 + 12.0 + (16.0 x 3) = 84.0 g mol^-1

Step 3: Calculate the mass of NaHCO₃ required

• • mass = moles x molar mass
  • mass =  0.0500 mol x 84.0 g mol^-1 = 4.2 g

Performing the Titration

• The key piece of equipment used in the titration is the burette
• Burettes are usually marked to a precision of 0.10 cm³
  • Since they are analogue instruments, the uncertainty is recorded to half the smallest marking, in other words to ±0.05 cm³

• The end point or equivalence point occurs when the two solutions have reacted completely and is shown with the use of an indicator

The steps in a titration

• A white tile is placed under the conical flask while the titration is performed, to make it easier to see the colour change

Titrating 

• The steps in a titration are:
  • Measuring a known volume (usually 20 or 25 cm³) of one of the solutions with a volumetric pipette and placing it into a conical flask
  • The other solution is placed in the burette
    • To start with, the burette will usually be filled to 0.00 cm³

  • A few drops of the indicator are added to the solution in the conical flask
  • The tap on the burette is carefully opened and the solution added, portion by portion, to the conical flask until the indicator starts to change colour
  • As you start getting near to the end point, the flow of the burette should be slowed right down so that the solution is added dropwise
    • You should be able to close the tap on the burette after one drop has caused the colour change

  • Multiple runs are carried out until concordant results are obtained
    • Concordant results are within 0.1 cm³ of each other

Recording and processing titration results

• Both the initial and final burette readings should be recorded and shown to a precision of  ±0.05 cm³, the same as the uncertainty

A typical layout and set of titration results

• The volume delivered (titre) is calculated and recorded to an uncertainty of ±0.10 cm³
  • The uncertainty is doubled, because two burette readings are made to obtain the titre (V final – V initial), following the rules for propagation of uncertainties

• Concordant results are then averaged, and non-concordant results are discarded
• The appropriate calculations are then done

Answer

Step 1: Calculate the average titre from concurrent titrations only (those within 0.10 cm³ of each other)

• • Average titre22.80 cm³

Step 2: Calculate the number of moles of sodium hydrogencarbonate

• • Moles =  x 0.200 = 4.56 x 10^-3 moles

Step 3: Calculate (or deduce) the number of moles of hydrochloric acid

• • The stoichiometry of NaHCO₃ : HCl is 1 : 1
  • Therefore, the number of moles of sodium hydrogencarbonate is also 4.56 x 10^-3 moles

Step 4: Calculate the concentration of hydrochloric acid

• • Concentration = 0.182 mol dm^-3 

   Answer

   Part i)

   Step 1: Calculate the number of moles of sodium hydroxide

• • Moles of NaOH1.638 x 10^-3 moles

   Step 2: State the number of moles of the unknown monoprotic acid

• • Moles of acid = 1.638 x 10^-3 moles since the stoichiometric ratio is 1HX : 1NaOH

   Step 3: Calculate the molar mass of the unknown monoprotic acid

• • 81.20 g mol^-1 

   Part ii)

   Step 1: Calculate the mass of the halogen

• • 81.20 - 1.0 = 80.2

   Step 2: Use the periodic table to identify the halogen

• • 80.2 ≈ Bromine (79.9)

   Step 3: State the final answer

• • The unknown monoprotic acid is HBr / hydrobromic acid

   Answer

   

   Step 1: Calculate the number of moles of hydrochloric acid

• • Moles of HCl2.2425 x 10^-3 moles

   Step 2: Calculate the number of moles of X₂CO₃

• • Moles of X₂CO₃1.12125 x 10^-3 moles (since the stoichiometric ratio is 1X₂CO₃ : 2HCl)

   Step 3: Calculate the mass of X₂CO₃ in the 25.0 cm³ aliquot

• • = 0.119 g

Step 4: Calculate the molar mass of X₂CO₃

• • 106.132 g mol^-1 

 Step 5: Calculate the mass of X in X₂CO₃ 

• • = 23.066

   Step 6: Use the periodic table to identify the Group 1 element in X₂CO₃ 

• • 23.066 ≈ Sodium/ Na (relative mass of 23.0)