Chem Practical (volumetric Analysis)

Question 1

Why starch is added only when the soln turns pale yellow

Answer 1

If u do, then more iodine molecules will be strongly absorbed on the starch molecules to form the blue-black complex. As release of iodine molecules from the complex to react with S2O3 2- takes time, this causes the colour change to occur less readily and reduces the accuracy of the titration

Question 2

purpose of using starch solution

Answer 2

to have a more accurate detection of the end point colour change

Question 3

Why not force out the last drop in the pipette

Answer 3

Cuz it is calibrated during manufacture. Doing so will result in more soln than required

Question 4

Burette reading to what dp

Answer 4

2dp

Question 5

Why need continuous swirling

Answer 5

To ensure even mixing

Question 6

Pipette vol to what dp

Answer 6

1 dp

Question 7

Electronic Balance dp

Answer 7

Write everything so 3dp

Question 8

Purpose of draught protection shields

Answer 8

Ensure reading is not affected by wind or movement

Question 9

If u were given a empty bottle to find mass how to present?

Answer 9

Mass of FA1 + weighing bottle/g
Mass of empty weighing bottle/g
Mass of FA1/g

Question 10

If u were given bottle with mass already inside, how to present

Answer 10

Mass of weighing bottle + FA1/g
Mass of weighing bottle + residual/g
Mass of FA1/g

Question 11

Possible reasons for anomalous results

Answer 11

1. The titrant was not added drop wise near the end point, making it difficult to know endpoint
2. Soln in conical flask was not swirled continuously to ensure complete mixing
3. The standard soln prepared was not mixed thoroughly to ensure Homogenous mixture

Question 12

why crucible is preferable over boiling tube for thermal decomposition

Answer 12

1. An open crucible has a larger surface area which allows water vapour to be lost more easily than a boiling tube
2. Using a boiling tube for heating will be slower as the water vapour formed will condense on the cooler part of the boiling tube and escapes less easily

Question 13

How using a crucible for thermal decomposition have a significant source of error

Answer 13

solid may be lost via wind

Question 14

How to present reading for thermal decomposition:

Answer 14

mass of empty crucible/g
mass of crucible and impure __/g
mass of crucible after 10 min heating/g
after second heating/g
after thrid heating/g
mass of water lost/g