Chemistry U6

Question 1

Safety Precautions: Flammable

Answer 1

Heat in HWB
Do not put X near naked flame

Question 2

Safety Precautions: Gases

Answer 2

Do experiment in fume cupboard

Question 3

Justify Safety Precautions: Hot Objects

Answer 3

Hold objects with tongs to avoid burns in hands.

Question 4

Justify Safety Precautions: Glasses

Answer 4

Safety glasses to avoid splashes in eyes.

Question 5

Justify Safety Precautions: Gloves

Answer 5

Safety gloves when handling corrosive substances.

Question 6

Answer 6

Toxic, Health Hazard, Flammable, Irritant

Question 7

Separation of immiscible liquids.

Answer 7

Use of a separating funnel.
Tap and Stopper closed.
The funnel was shaken and inverted several times.
Invert and open the tap to release pressure built up due to gas formation.
Layers are allowed to settle down.
Open the stopper to balance the pressure inside and outside the funnel.
Open the tap and allow the lower layer to flow out into the beaker, then the upper layer.

Question 7

Answer 7

Oxidising, Corrosive, Explosive, Environmental Hazard.

Question 8

Filtration: Under pressure: Curved

Answer 8

Filter paper should be flat to close the holes.
Curled up sides may allow solid to pass through and lost.

Question 9

Recrystalisation: Justification + Step

Answer 9

Dissolve the crystals and a small amount of hot solvent, then heat to boiling. -> Dissolve the impure crystals.

Filter hot mixture -> remove insoluble impurities.

Allow the solution to cool -> to form crystals.

Filter the crystals and wash with a small amount of ice-cold solvent, -> to remove soluble impurities.

Dry the pure crystals in the oven -> remove water present in the crystals.

Question 10

Why final yield < the theoretical yield?

Answer 10

Impure reactant.
Incomplete reaction
Loss of product during transfer and recrystalisation.

Question 11

Determine the melting point of the solid.

Answer 11

Side arm heated to distribute heat evenly by convection currents.

A stream of bubbles forms in a capillary tube the air trapped inside expands when heated.

A continuous stream of bubbles comes from capillary tube,

Heating is stopped, and the temperature recorded when the compound is drawn into the capillary tube.

Question 12

Determine the melting point of the solid:
Effect of impurities on the solid.

Answer 12

Pure solid have sharp melting point, same with data value.

Impure solid have lower melting point and melt over a range.

Question 13

Determine the melting point of the solid:
Results obtained may not be same why?

Answer 13

Changes in atmospheric pressure.

Question 14

Cool exothermic reaction

Answer 14

Put flash in Ice WB, cool down the reaction mixture.

Prevent side-reactions.

Question 15

Dry liquid

Answer 15

Use anhydrous CaCl2, stir.
Cloudy solution becomes clear.

Question 16

Simple Distillation Draw:

Answer 16

Question 17

Reflux Draw:
Anti-bumping granules, Condenser use?

Answer 17

Anti-bumping granules for smooth boiling
The condenser prevents the escape of volatile reactants.

Question 18

Steam Distillation Draw:
Purpose?

Answer 18

Liquid decomposes easily

Question 19

How to show the presence of optical isomers?

Answer 19

Use polarimeter.
Optical isomers rotate Plane of PBL in opposite direction.

Question 20

U-tube.

Answer 20

first Anhydrous CaCl2,
second KOH soda lime

Question 21

Determine mass of water in hydrated salt.

Answer 21

Weigh a certain mass of hydrated salt in an evaporating dish using an electron balance.

Heat to evaporate the water.
Allow it to cool. Weigh.

Reheat and reweigh until constant mass.

The difference in mass is the mass of water.

Question 22

Reading burette techniques:

Answer 22

Eye same level as the meniscus.
Read the bottom of the meniscus

Question 23

Titration techniques justify.

Answer 23

Burette rinsed with water -> To remove traces of the previous solution.

Rinsed with the solution used. -> to move water that would change the concentration of the solution.

Question 24

How to prepare a standard solution.

Answer 24

Weigh the required mass of solid using an electron balance. Put mass into the beaker. Add distilled water, stir with a glass rod to dissolve the solid. Transfer the solution to a volumentric flask using a funnel. Wash the beaker with distilled water, and transfer the washing to the volumetric flask. Add distilled water to the volumetric flask slowly until the bottom of the meniscus is at the graduation mark. Close the stopper and shake and invert several times to mix the solution.

Question 25

Determine the concentration of acid using titration.

Answer 25

Place 25cm3 of NaOH solution to flash using a pipette. Add three drops of phenolphtalein, solution turns pink. Wash the burette with distilled water, and watch the solution with dill. HCL. Fill the burette with dill. HCl using a funnel. Read the initial reading by looking at the bottom of the meniscus at the same level as eye level. Add dill. HCl from the burette until the pink colour (pink -> pale pink) Use white tile to more easily observe sharper colour changes. Record the amount of acid used to estimate roughly the endpoint. Repeat the above steps, and add acid drop by drop near the endpoint. Repeat the process until the results are concordant. Calculate the mean titre and use it to determine the concentration of acid.

Question 26

Prevent heat loss

Answer 26

Use a polystrene cup, Less heat is lost to the surroundings.

Question 27

Determine the temperature rise when solid dissolves.

Answer 27

Use a stopwatch. Measure the temperature of water every 30s, for 3 mins. Add the solid to the water. Stir and continue recording the temperature every 30s, until no more change in temperature. Plot a graph of temperature vs time. Join two sets of points with a straight line, extrapolate -> Determine maximum temperature at 3 minutes.

Question 28

Why calculated Delta H different from the value given?

Answer 28

Heat is lost to the surroundings and, apparatus. Incomplete combustion. Evaporation of fuel after weighing.

Question 29

Test for CO3 2-

Answer 29

Add acid, effervescence, and gas turn limewater milky.

Question 30

Test acid:

Answer 30

Add Na2CO3 effervescence, and gas turn limewater milky.

Question 31

Silver nitrate test: Step and result

Answer 31

Add dill. nitric acid. Add aqueous silver nitrate. Observe the color of precipitate. Add dillute NH3 to see if dissolves or not. Cl white ppt Soluble in dill NH3 Br Cream ppt Soluble in conc NH3 I Yellow ppt Insoluble in conc NH3

Question 32

Test SO4 2-

Answer 32

Dill. Nitric acid, Barium nitrate White ppt

Question 33

Test NO3 -

Answer 33

Add NaOH, add Al, warm gently. Gas bubbles turn damp red litmus paper to blue. OR Heat solid strongly. Metal nitrate -> Metal oxide, NO2, O2 Bubbles of colorless gas relights glowing splint.

Question 34

NH4 +

Answer 34

Warming, bubbles of NH3 gas given on turn damp red litmus paper blue.

Question 35

Cl2

Answer 35

Insert damp blue litmus paper. Bleaches

Question 36

H2, O2

Answer 36

Insert burning split, pops Insert glowing splint, relights

Question 37

Flame test:

Answer 37

Clean Pt wire dip in conc HCl. Dip the wire into salt. Expose the wire to a non-luminous flame in a Bunsen burner. Observe the colour of the flame.

Question 38

Flame test, air hole, why open?

Answer 38

The flame is not hot enough.

Question 39

Flame test results: Lithium, Sodium, Potassium, Calcium, Strontium, Barium

Answer 39

Red, golden yellow, lilac, brick red, red, apple green

Question 40

Test water: (Physical and Chemical)

Answer 40

Use anhydrous cobalt cl paper Blue to pink Anhydrous CuSO4 Colorless to blue. Heat the liquid to a boil. It should boil at 100C

Question 41

Test C=C alkene

Answer 41

Add Br orange to colourless Add KMnO4 purple to colorless

Question 42

Test OH

Answer 42

Add PCl5 Steamy fumes turn damp blue litmus paper red. (HCl) K2Cr2O7 orange to green

Question 43

Ch3CO iodoform

Answer 43

CHI3, NaI, salt, H2O

Question 44

Color of complexes

Answer 44

Incompletely filled d oribtals Splitting of d orbitals. d-d transitions. Absorption of light. Complementary colour observed