development of practical skills

Question 1

why might a student obtain a smaller titre when compared with another student

Answer 1

• the pipette is rinsed with water before filling with HCl

Question 2

why might a student obtain a larger titre when compared with another student

Answer 2

• the jet space of the burette is still filled with air before starting the titration

Question 3

why would rinsing the conical flask with distilled water before completing a titration not affect the titre

Answer 3

• it will not affect the number of moles that react during the titration

Question 4

when making a standard solution, how can it be ensured that the final concentration of the solution is accurate

Answer 4

• using a dropping pipette to add water up to the graduated mark

Question 5

what assumptions are made when CALCULATING enthalpy change

Answer 5

• the specific heat capacity of the solution is the same as water
• the density of solution is the same as water
• all reactants have reacted completely

Question 6

in an experiment to determine ef of magnesium oxide a student tapped the residue with tongs to break it up
why?
how would this affect the ef?

Answer 6

• to ensure all of the magnesium had reacted
• it would be more accurate

Question 7

what would the effect be when filling the volumetric flask to the top of the flask, not the graduation line

Answer 7

• the titre would be greater than expected
• as the solution is less concentrated
• so more solution is required to reach the end point

Question 8

what piece of apparatus should be used to measure solutions the most accurately

Answer 8

• volumetric pipette

Question 9

when finding enthalpy change of neutralisation, how can apparatus be improved to improve accuracy

Answer 9

use a lid/ more insulation around the cup
- therefore, temp change would be greater
- thus, calculated temp would be closer to literature value

Question 10

what apparatus is the most accurate for collecting gas over water

Answer 10

inverted burette

Question 11

when heating in a crucible, why should a lid be used

Answer 11

• improves accuracy as prevents loss of solid
• should also be loose fitting to allow gas to escape/ enter

Question 12

why does a crucible need to be dry before heating

Answer 12

• would cause mass loss to be too large as the water would be lost when heating

Question 13

what are the potential errors of using a gas syringe

Answer 13

• gas escape before bund inserted
• syringe sticks
• some gases like co2 and so2 are soluble in water so the true amount of gas is not measured

Question 14

why is a volumetric pipette more accurate than a measuring cylinder

Answer 14

smaller uncertainty

Question 15

phenolphthalein colour

Answer 15

if ACID is added from the burette the colour change is pink(alkali) to colourless (acid)

Question 16

methyl orange colours

Answer 16

red in acid
yellow in alkali
orange at end point

Question 17

when heating in a crucible, why would the tongs be used to lift the crucible lid slightly during heating

Answer 17

to make sure enough o2 is present for the reaction to occur

Question 18

what would be the effect of filling the volumetric flask to the top and not to the graduated line?

Answer 18

• titre would be greater
• solution less concentrated
• more solution required to reach the end point

Question 19

what piece of apparatus can be used to collect GAS OVER WATER that would improve accuracy

Answer 19

inverted burette

Question 20

state two assumptions made when using the q=mc△t equation

Answer 20

• solutions have the same specific heat capacity as water
• density of the solution is the same as water

Question 21

how to make up a standard solution

Answer 21

1. weigh sample bottle containing required mass of solid on a 2dp balance. transfer to a beaker and record the difference in mass
2. add 100cm3 of distilled water to the beaker. use a glass rod to dissolve it.
3. pour solution into a 250cm3 graduated flask via a funnel
4. rinse beaker and funnel and add washings from the beaker and glass rod to the flask
5. make up the graduated line with distilled water using a dropping pipette
6. invert flask to ensure uniform solution and concentration

Question 22

what acid should be used for manganate titrations

Answer 22

dilute sulfuric acid

Question 23

how to reduce uncertainties in a titration

Answer 23

• replace measuring cylinders with pipettes or burettes
• increase volume and conc of substance in the conical flask
• decrease conc of substance in the burette