pract - source of error

Question 1

metal + acid
volume gas collection

Answer 1

error
exothermic reaction
might raise temp, solution
rate of reaction faster than expected

improve
thermostatically controlled water bath

Question 2

metal + acid
rate of reaction

Answer 2

error
mass, metal ribbon different
even if same length
lighter mass, lesser no. of metal particles

improve
weigh each piece before use

error
metal ribbon floats on surface, not submerged, acid, difficult completely surround, acid

improve
use glass rod, completely submerge metal strip

Question 3

record temperature change expt.

Answer 3

error
heat produced, lost to surrounding
highest temperature measured, lower than expect

improve
place beaker, styrofoam cup (insulator, heat)
cover styrofoam cup
minimise heat lost, surrounding

Question 4

titration

Answer 4

error
difficulty reading mensicus (liquid i.e. potassium manganate (VII)), dark liquid

inaccurate reading

solution
reading, using top of meniscus, take multiple readings, use avg

error
burette (solution i.e. acid/alkali) impure, volune of (solution in burette) needed larger than expcted, neutralise

soln. flow out of pipette slowly, dk when to start stopwatch, inaccurate time readings

only blame burette soln.

Question 5

anything involving conc. of soln

Answer 5

evaporation, liquid
cover w/ petri dish

Question 6

does chem accept (end point not definitive due to color)

Answer 6

NO

Question 7

delay in human reaction time

Answer 7

• starting/stopping stopwatch
• time inaccurate

Question 8

heating of soln,
get solid (i.e. crystallisation/loss of mass)

Answer 8

sputtering
- mass of solid lower than expected
- consider lower heat of boiler/cotton wool if loss mass

Question 9

change of apparatus
improve expt

Answer 9

instead of measuring cylinder -> burette

water displacement to collect volume of gas -> gas syringe

Question 10

finding heat rise
what is advantage of using graph vs using experimental method

Answer 10

graph not affected by heat loss to surrounding

Question 11

how to do cation test on solid

Answer 11

acidify with dilute acid
only do cation test on soln.

Question 12

disadv, pipette/burette

Answer 12

slow for liquid to move out of pipette/burette, unsure when to start stopwatch

Question 13

disadv of using beaker vs conical flask

Answer 13

conical flask difficult swirl contents w/o spill

Question 14

h2o2, KI source of error

Answer 14

h2o2 decomposes in room temp, pressure

KI oxidised, iodine when exposed, o2, loss, chemical

Question 15

which salts can absorb moisture from atmosphere

Answer 15

zinc chloride, sodium hydroxide, absorb moisture, air, increase mass

and also all the gas dryers
conc. sulfuric acid
calcium oxide
(fused) calcium chloride

Question 16

methyl orange if pH7 why kinda sucks

Answer 16

methyl orange only change color, pH3-4

Question 17

last resort source of error

Answer 17

only one stopwatch used, many different expt.

repeat 3x, find avg

Question 18

impurities in solid P
measure %tage purity by titrate P w/ HCl
error

Answer 18

impurities i.e. carbonate might react w/ acid

higher %tage purity, expected

Question 19

salt prep source of error

Answer 19

• may not be completely dry, mass obtained high, expect
• some solids lost, filtration
• sputtering -> solids lost

Question 20

thermal decomposition source of error

Answer 20

• not completely thermal decomposed, some water/carbonate content still present
• anhydrous salt formed may absorb water, surroundings

Question 21

how to decompose hydrated CuSo4 that will decompose -> So2 gas also

Answer 21

weigh
heat gently
heat rim of mouth when condensation forms
let it cool, notice color changes
when cooled, heat gently again and see if there is further change in mass

There will be greater loss of mass in CuSo4.xh2o as sulfate will decompose as well as hydration

HEAT GENTLY -> remove water
HEAT STRONGLY -> decompose solid TEST GAS when solid melts

Question 22

when add acid (acidify) report

Answer 22

• any ppt
• solubility of ppt
• any gas

Question 23

when warm soln. with test tube what other precautions to take

Answer 23

• use boiling tube, larger cap, better control, small quantity soln may spurt out quickly, heating
• remove tube from flame time to time, prevent spurt
• apply heat w/ gentle/small flame

Question 24

deposit vs solid vs residue

Answer 24

deposit: metal
solid: colour change when heat
residue: phys separate

Question 25

soln vs suspension aka is there ppt or not.

Answer 25

soln: swirl, leave to stand, no solid particles, light can pass thru

Question 26

chromatography expt. full

Answer 26

- fill test tube, 2cm depth water - start line, chromatography paper, pencil, add spot - chromatography paper -> test tube - ensure solvent below start line - split cork, chromatography paper in place - wait 10min - take out chromato paper, mark solvent front immediately - spray locating agent, separate, x no of dots - measure distance from SL -> Solvent, measure distance from SL -> Spot - calculate Rf value, compare against known values, amino acids

Question 27

explain y water added each reaction mixture

Answer 27

keep total vol reaction mix constant

Question 28

why heating more NaHCo3 decompose further

Answer 28

compose further into Na2O and co2

Question 29

differentitate calcium carbonate and zinc carbonate

Answer 29

heat strongly, zinc carbonate will decompose

Question 30

record highest and lowest temp of exothermic reaction how to improve accuracy

Answer 30

draw graph - derive temperature rise from extrapolated reading - measure highest temp will have heat loss

Question 31

improve accuracy

Answer 31

- light sensory/colorimeter w/ data logger for reaction time - glass rod stir instead of thermometer - temperature sensor connect to data logger - cool test tube and its contents in a dessicator, use larger mass of solid when decomposing - heat until constant mass observed, collect co2 and record vol o fgas, no further change

Question 32

QA format

Answer 32

add few drops... add in excess..

Question 33

generic sources of error

Answer 33

- only one stopwatch used for multiple samples - same size dosent mean same mass

Question 34

porous pot diagram

Answer 34

https://docs.google.com/document/d/1Ep_FO9CxYSFbxmf3Zqg9mgm9NNyt2enpfMDr49gufrU/edit?tab=t.0 page 42

Question 35

when use large pieces of metal + acid what is disadv

Answer 35

dosent allow for fast reaction max heat recorded may be lower than usual