2.1.4 Acid-base titrations

Question 1

what does a titration reaction involve

Answer 1

Titration reactions involve measured volumes of two solutions reacting in the exact mole ratio required by the balanced equations. If the consentration of one solution is known the concentration of the other reaction can be worked out.

Question 2

What is the process of a titration calculation

Answer 2

1. HIGHLIGHT
2. write balanced equation
3. mole ratio
4. calculate moles
5. mole ratio
6. calculate concentration (from moles)

Question 3

What apparatus is used in titrations

Answer 3

• volumetric flask
• (volumetric) pipette
• burette

Question 4

how is the volumetric flask used in titrations

Answer 4

• Used to prepare standard solutions

Question 5

how is a volumetric pipette used in titrations

Answer 5

• Filled so that the bottom of the meniscus is level with the line.
• The solution is transfered to a conical flask and the tip dipped into the solution to release the last drop.

Question 6

how is a burette used in titrations

Answer 6

• Fill from the top with the tap closed - below eye level
• Final level is adjusted by opening the tap ( which also fills below the tap)
• The solution doesnt need to be at 0.00cm3 before the titration ( but record regardless)
• Careful use of the tap allows one drop at a time to be added near endpoint.

Question 7

Describe how results are recorded with titrations

Answer 7

• All readings MUST end in .x0 or .x5 including 0.00
• The initial burette reading goes on the second line of the table. The final reading goes on the first line.
• The titre is just ‘volume added’ and is obtained by subtraction
• Titrations are recorded until concordant titres are recorded. The mean of these values are used for calculations.

Question 8

Define concordant

in terms of titre results

Answer 8

concordant results are titres that are within 0.1cm3 of each other.

Question 9

explain standard solutions

Answer 9

• a standard solution is a solution whose exact concentration is known.
• All measurements made during its preperation MUST be taken with high resolution apperatus. eg a 3d.p. balance, volumetric flask and pipette.

Question 10

what are some problems when preparing standard solutions

Answer 10

• The mass of some solids cannot be measured accurately.
  -sodium carbonate is a hydrated salt Na1CO3.xH2O where x = 10 but it loses some of this water of crystallisation on standing. The mass measured will be for a solid with more Na2CO3 present than the Mr of Na2CO3.10H2O suggests. The actual concentration of CO32- will be higher than the calculated value.
  -Sodium hydroxide absorbs water from the air (and CO2). The mass measured is for NaOH + H2O so there are fewer moles of NaOH present than calculated. The actual concentration of OH- is lower than the value calculated.

Question 11

State the method for preparing standard solutions

Answer 11

1. the solid is first weighed accurately.
2. The solid is dissolved in a beaker using less distilled water than will be needed to fill the volumetric flask to the mark.
3. This solution is transfered to a volumetric flask. The last traces of the solution are rinsed off into the flask with distilled water.
4. The flask is carefully filled to the graduation line by adding distilled water a drop at a time until the bottom of the meniscus lines up with the marker. If too much water is added, the solution will be too dilute and must be prepared again. You should view at eye-level for accuracy.
5. Finally the volumetric flask is slowly inverted several times to mix the solution thoroughly. If this stage is ommited, titration results are unlikely to be concordant.

Question 12

state the procedure of doing titrations

Answer 12

1. Add a measured volume of one solution to a conical flask using a pipette.
2. Add the other solution to a burette, and record the initial burette reading to the nearest x.05cm3.
3. Add a few drops of an indicator to the solution in the conical flask.
4. Run the solution in the burette into the conical flask, swirling the conical flask throughout to mix the two solutions. Eventually the indicator changes colour at the endpoint of the titration. The end-point is used to indicate the volume of one solution that exactly reacts with the volume of the second solution.
5. Record the final burette reading. The volume of the solution added from the burette is called the titre. Calculated by the final reading - initial reading.
6. A quick trial titration is carried out first to find the approximate titre.
7. The titration is then repeated accurately, adding the solution dropwise as the end point is approached. Further titrations are carried out until concordant results are obtained.