Planning Experiments Flashcards
State the suitable indicators to be used for a strong acid-strong base titration and their colours in ‘acidic’, ‘alkaline’ and at the end-point of the titration.
Strong acids include HCl, H2SO4 and strong alkali include NaOH, KOH and their pH changes occur between 4 and 10. Suitable indicators to be used include:
(acidic, end-point, alkaline)
Acidic indicators such as Methyl orange (red, orange, yellow) and screened methyl orange (violet, grey, green)
Alkaline indicators such as Thymol blue (yellow, green, blue) and Thymolphthalein (colourless, light blue when the acid is in the conical flask, blue).
State the suitable indicators to be used for a strong acid-weak base titration and their colours in ‘acidic’, ‘alkaline’ and at the end-point of the titration.
(acidic, end-point, alkaline)
Acidic indicators such as Methyl orange (red, orange, yellow) and screened methyl orange (violet, grey, green)
State the suitable indicators to be used for a weak acid-strong base titration and their colours in ‘acidic’, ‘alkaline’ and at the end-point of the titration.
Alkaline indicators such as Thymol blue (yellow, green, blue) and Thymolphthalein (colourless, light blue when the acid is in the conical flask, blue).
State the suitable indicators to be used for a weak acid-weak base titration and their colours in ‘acidic’, ‘alkaline’ and at the end-point of the titration.
Not tested.
Explain why methyl orange (your choice of indicator) is useful for the titration involving a weak base and a strong acid (planned titration).
This titration involves the reaction between a strong acid and a weak base. Methyl orange is a suitable indicator since its pH range coincides with the region of rapid pH change in this titration.
State the three types of redox reactions.
Manganate (VII) titration where KMnO4 which is a purple oxidising agent is reduced to colourless Mn2+ in an acidic medium when reacted with a reducing agent.
Dichromate (VI) titration where K2Cr2O7 which is an orange oxidising agent is reduced to green Cr3+ in an acidic medium.
Iodine-thiosulfate titration/Iodometric titration is used to determine the amount of an oxidising agent such as IO3- or BrO3- where the iodide ions are first oxidised to iodine by aqueous KI, forming iodine and the amount of iodine formed is then found by titrating agents standard aqueous Na2S2O3 which is oxidised by iodine into S4O62-
State the end-point colour change during a manganate (VII) titration.
When H2O2 (in the conical flask) is titrated against acidified KMnO4, the end-point colour change is from colourless to pale pink.
When Fe2+ (in the conical flask) is titrated against acidified KMnO4, the end-point colour change is from yellow to pale orange/pink.
MnO4- + 5Fe2+ + 8H+ -> Mn2+ + 5Fe3+ + 4H2O
Explain why hydrochloric acid cannot be used to provide the acidic medium for the titration between KMnO4 and H2O2.
The chloride ion would be oxidised by either H2O2 or MnO4- to Cl2 (g). Dilute sulfuric acid is commonly used to provide the acidic medium for redox reactions and is usually added in excess using the measuring cylinder.
State the end-point colour change for iodometric titration.
I2 (aq) and Na2S2O3 (aq)
With iodine (and no starch indicator) in the conical flask, the end-point colour change is from pale yellow to colourless. With the starch indicator placed in the conical flask, the end-point colour change is from blue-black to colourless.
The brown iodine solution is usually titrated against a standard solution of sodium thiosulfate placed in the burette. When the solution in the conical flask becomes pale yellow, the starch indicator is added and a blue-black starch-iodine complex is formed. The titration is continued until the blue-black solution turns colourless (more obvious colour change).
However, at the start of the titration, the concentration of iodine is relatively high, if the starch indicator is added at the start of the titration, many iodine molecules will be trapped in the starch molecules. Since the liberation of the iodine molecules from the starch molecules takes time, the titration results will be inaccurate.
How do you ensure the reliability of results?
Stopper and shake the volumetric falsk well to ensure a homogeneous solution is obtained.
Swirl the solution in the conical flask continuously during titration to ensure that the reactants are well mixed.
Repeat the titration to get at least two consistent titration readings that are at least within 0.10cm^3 of each other.
Repeat the experiment to check for the reliability of the titration results.
State an assumption about impurities.
They are water-soluble and do not react with dilute acids.
Describe the steps involved in the preparation of 250cm3 of standard solution from a solid. (2m)
1) Using an electronic balance, weigh accurately about 2.48 g of Na2S2O3.5H2O solid in a clean and dry weighing bottle.
2) With the aid of a filter funnel and glass rod, transfer the Na2S2O3.5H2O solid from the weighing bottle to a 100 cm3 beaker quantitatively and dissolve it in some deionised water. Rinse the weighing bottle with small volumes deionised water several times, adding each rinsing to the 100 cm3 beaker.
3) Transfer the solution quantitatively into a 250 cm3 volumetric flask. Rinse the beaker a few times with deionised water and transfer the washings into the volumetric flask. Use the dropping pipette to add the deionised water drop-wise when nearing the mark.
4) Stopper the volumetric flask and shake the solution thoroughly to ensure that it is homogeneous. Label this solution as FA3.
Note: Weighing by difference method is used when the sample is insoluble in water and the reagent such as FA1 cannot be transferred quantitatively from the weighing bottle to the desired vessel. It involves finding the mass of the weighing bottle and FA1, mass of weighing bottle after emptying FA1 (reweighed as some residual FA1 will stay in the weighing bottle) and finding the actual mass of FA1 used in the experiment by finding the difference between those two readings. The mass of the empty weighing bottle is not needed.
Describe the steps involved in the dilution of a solution.
Using a burette, add 50.00 cm3 of FA2 into 250 graduated flask. Fill the graduated flask to the 250cm3 mark with deionised water and use a dropping pipette to add deionised water drop by drop when nearing the mark. Stopper the graduated flask and shake the solution to ensure that it is homogeneous. Label the solution as FA3.
Describe the steps involved in titration.
1) Fill another burette with FAX
2) Pipette 25.0cm3 of FA1 into a 250cm3 conical flask and add 2 drops of XXX indicator. Titrate this against FAX.
3) Stop the titration when the end-point is reached and the colour of the solution in the conical flask changes from [end-point].
4) Record the titration results in an appropriate table.
5) Repeat the titration until at least two consistent results are obtained (i.e. two titre values which are within 0.10cm3 of each other).
