practical titration FAQs Flashcards
what is the error of apparatus with division markings?
half the smallest division
e.g. each reading in burette is 0.10cm3
error of burette reading = ±1/2 x 0.10 = ±0.05cm3
for apparatus with no division markings, error is given in qns
what is overall apparatus error?
sum of all percentage errors associated with each piece of apparatus
percentage error = ± (uncertainty or apparatus error)/volume used in apparatus x 100%
if 2 readings are to be taken (e.g. inital temp and final temp, apparatus error x2)
why titre volume cannot be too low/high and what is the ideal titre volume?
small titre volume –>larger percentage error
large titre volume –> tedious, time consuming, can cause larger percentage error due to refilling of burette
ideal titre volume: 10.00-40.00cm3
ideal titre volume: ensure concentration ratio of analyte and titrant is same as stoichiometric ratio
what is experimental error?
experimental error = |(exp value - actual value)|/actual value x 100%
exp error < apparatus error: accurate results
exp error > apparatus error: inaccurate results
why does the titration involving KMnO4 not need an indicator?
observable colour change at end point
if KMnO4 is in the burette, colour change is from colourless to pale pink
if KMnO4 is in the conical flask, the colour change is from purple to colourless
why must titration involving KMnO4 be conducted in an acidic medium?
brown ppt (MnO2) will be formed in neutral/alkaline medium
why should HCl not be used in titration involving KMnO4 and H2O2?
Cl- ions will be oxidised by MnO4- ions, so larger volume of KMnO4 is required to reach the end point and titre value would be higher
explain the colour changes observed during iodometric titration
solution turns from brown to yellow as I2 is reduced to I-
upon addition of starch, solution turns deep blue due to formation of I2-starch complex
end-point: deep blue solution turns colourless, as I2 is desorbed from the complex and reduced completely
why is the starch indicator only added when a small amt of iodine remains and not at the start of the titration?
if the starch is added at the start of the titration, large amount of iodine is trapped in the starch-iodine complex. Desorption of iodine takes place slowly and therefore larger volume of thiosulfate is required to reach end point
why must the I2 produced be titrated immediately with S2O32-?
I2 is volatile and may escape from the conical flask
why is H2O2 stored in brown bottles
prevent light from being absorbed by the solution, slowing down disproportionation
explain why in a iodometric titration, when the conical flask is left to stand after end point is reached, the colourless solution turns blue again.
the oxygen can oxidise the iodide in the conical flask to iodine again
