4.2 : Acid - Base Titrations Flashcards
What can titrations be used for?
Finding the concentration of a solution
Identifying unknown chemicals
Finding the purity of a substance
Why is finding the purity of a substance important ?
It is an important aspect of quality control for things like medicines, food and cosmetics.
It is essential that pharmaceuticals have a high level of purity - just a tiny amount of an impurity in a drug could cause a great deal of harm to a patient.
What are the typical tolerances of volumetric flasks?
a 100 cm3 volumetric flask : +/- 0.20 cm3
a 250 cm3 volumetric flask : +/- 0.30 cm3
How do you prepare a standard solution ?
Solid is weighed accurately.
Solid is dissolved in a beaker using less water than will be needed to fill the volumetric flask to the mark.
Solution is transferred to a volumetric flask. The last traces of the solution are rinsed into the flask with distilled water.
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 exactly with the mark. Too much water = solution is too dilute = prepared again.
Volumetric flask is slowly inverted several times to mix the solution thoroughly. If this stage is omitted, titration results are unlikely to be consistent.
Describe the procedure for an acid - base titration
Add a measured volume of one solution to a concise flask using a pipette.
Add the other solution to a burette, and record the initial burette reading to the nearest 0.05 cm3.
Add a few drops of an indicator to the solution in the concise flask.
Run the solution in the burette into the solution in the comical flask, swirling the conical flask throughout to mix the two solutions.
Eventually the indicator changes colour at the end point 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.
Record the final burette reading. The volume of solution added from the burette is called the titre, which is the final reading minus the initial reading.
A quick, trial titration is carried out first to find the approximate titre.
It is then repeated accurately, adding the solution drop-wise as the end point is approached. Further titrations are carried out until two accurate titres are concordant - agreeing to within 0.10 cm3
How do you work out the unknown?
Use your table method and ratio to find moles.
How can you identify a carbonate ?
Calculate the amount of HCl that reacted : she the mean titre and conc. of HCl
Determine the amount of X2CO3 that reacted : use the balanced equation and n(HCl)
Work out the unknown information :
scale up to find the amount of X2CO3 in the e.g. 250 cm3 solution prepared.
find the molar mass of X2CO3 : use the amount of n(X2CO3) in the 250 cm3 solution and the mass of the X2CO3 used to prepare this solution.
Use M(X2CO3) to identify x in the formula X2CO3
From the periodic table, find what most closely matches.
