1973-11-17 — Page 15

育教儒華頁三第張四號 日廿月十年丑癸腦夏 WAH KIU YAT PO

報日僑華

31.2.

1974中學會考試題預習專欄

arated.

圣道書院主

1. of solvent in solution

化學(課程乙)

CHEMISTRY (2)

Suggested answers for last

Iron filings are first obtained by the attraction of

a

magent.

Carbon disulphide is poured on the mixture to dissolve sulphur, which is the only constituent soluble in this solvent. After stirring for some times the undis- solved constituents are filtered off and left to dry in air. The filtrate but aside in a cool place evaporates slowly to dryness, leaving sulphur crystals, because carbon disulphide is volatile.

The residue from above is then heated in a beaker. Sal-ammoniac sublimate on the bottom of the flask, which is half filled with cold water and placed on the top of the beaker. The apparatus is set up as shown:

white sublimatı

heat.

col

water

ixtur

After the removal of sal-ammon ac by sublimation, only table sal

and chalk are left in the baaker. Water as a solvent is added to the. beaker, dissolving table salt but not the chalk. The mixture is then heated and stirred to ensure com- plete solution of salt. Chalk is separated from table salt by falte. ing the suspension, and is dried 1 air. Pure salt can be obtained fro the filtrate by evaporating to dryness.

At 80°C.

164 g of saturated solution contain

n 64 § of solid

..80 g of saturated solution contain 80

164

31.2 g solid

80 g of saturated solution contain 48.8 g water

At 25°C.

9 g of water dissolve 4.5 g of solid

48.8 € of water will dissolve

4.5 x 48.8

g of solid

24.4 5

of solid

Amount of solid senarating

其府

說現色犧尤青一西數

6.8

of Yin 5.5

Enlution

9.1

(45.55.Se saturated

0.1)

36.4 of solvent will dissolve

1 x 100

E of

36.4

25.0 € of Y Solubility or y at 15°C

25.0 per 100 g of solvent Theoretical chemistry

A. Elements, Compounds, Mixtures.

An element is a substance which cannot, by any known chemical process, be split up into two or more

coppepler substances, eg,

A compound is a substance made up of two or more elements chem ically combined together, e.g. conper sulphate..

A mixture

may consists of any number of elements or compounds, or elements and compounds, mixed together, and not chemically combined, e.g., air.

Differences between mixtures

and compounds

Mixture

Variable

composition.

Compound

Composition fix- ed and definite.

2. Constituents can Chemical means

be separated by necessary for physical means. Sebaration.

No heat change when made.

Properties are those of its constituents.

Heat usually

evolved or taken in when made..

Possesses a new set of properties

Consists of two Every particle

or more kinds

of particles, i.e, is hetero- geneous,

B. Chemical and

the same, 1.e. is homogeneous.

Chemical cha sicar

changes

New substances with different. properties are

produced, There is usually evolution of heat, and sometimes of light; the change. is not easily reversed by mere change of external conditions unless the system is in a state of equilibrium, the

the weight of a riven material usually changes when it is converted into the new substance.

Examples When an iron pen-knife is 1ersed in blue copper sulphate solution the iron comes coated with a laver of rod commer.

Fe + CuSO,

Physical changes: No new substance s produced: if anything more than

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more heating or cooling is involund, it is usually merely a charge of state, and heat supplied to or re-. moved from a body, there is no mark- ed evolution or absorption of heat; the change is easily reversed by appropriate alteration in the exter- nal conditions; the weight of the material remains unchanged.

Example: Sulphur contained in test tube 1s heated.

The Chemical Lave

1. The law of conservation of

masst.

The total amount of matter in the universe is maltered, what. ever changes take place in its dis- tribution.

There are many reactions which can be used to prove this, e.g.., Wher silver nitrate solution and hydrochloric acid are mixed, a white not will be formed in solution.

AɛNOŽ + HC1

AgCl + HNO

The law of constant commosit- ion:

The elements combin in difin- ite ration by weight, so that the composition of a pure chemical compound is independent of the way in which it is prepared.

Example: Three specimens of cupri nxide made by different methods were reduced to coprer in a steam of pure dry hydrogen. The following results were obtained:

1. 1.1 g of copper oxide yielded

0.91 g copper

2. 1.25 g of conper oxide yielded

1.00 g copper

1.35 g of copper oxide yielded 1.08 g copper

Solution:

Wt. of

of copper oxide 1.14 1.25 1.35 of copper

0.91 1.00 1.08

wt. of oxygen

0.23 0.25 0.26 0.91 1.00 1.08 0.23 0.25 0.27 3.95.4.00 4.00

of copper vt. of oxygen

Thus the oxides all contain the same elements combined together in the. same proportions by weight within experimental error.

3. The law of multiplevroportion

When two elements combine to form more than one compound, the weihgt of one element which unite with indenti-

cal weights of the whole number, usually small.

Example: Three oxides of lead contain 92.8, 90.61, and 36.56% of lead respectively. Show by calculation that these figures are in accordance with the law of multiple proportions.

Solution:

Oxide of lead

% of lead

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Charlie Chaplin

(a) (b) (c) 92.80 90.41 86.56

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門:

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MODERN TIMES

of oxygen

7.20 9.39 13.44 of lead 92.80.90.61 85.56

Wt. of oxygen92.80 90.61 86.56 9.39 13.44 Wt. of oxygen 7.20

2

This the weights of lead which combine with a fixed weight(=) of oxygen are in the retin 4:3:2, which is a simple ratio.

4. Diffusion and Brownian movemen.

Diffusion of gases:

If a light gas brought into contact with a heavier gas, the two gases mix together despite the differ ence in their densities.

KEN

Graham's law of diffusion:

The relative rates at which two

diffuse are inversely propor- gases tional to the square roots of their densities. That is

R1D

Molecular Wt. of gas 1

RD2 Molecular Wt. of gas Example: The rate of diffusion of carbon dioxide and ozone are approxi mately as 2.9:2.71. If the molecular what is. wt. of carbon dioxide is 44, the molecular wt. of ozone?

Then

2.9 - [D] (V.D. Of

2.71 122

2.9 x 2.1

2.71 x 2.

25:2

Moleculer wt. of ozone = 25.2 x

50.

Brownian movement in liquids:

mat

Then very tiny particles of tter are suspended in a liquid, the particles engage in ceaseless activity. Each particles moves first one way and then another in a random or disirganized way, never really getting anywhere, but always on the

move.

Questions for this week

5.85 g of sodium chloride is found it contain 2.30 g of sodium. 3 g. of sodium reacts with water to form sodium hydroxide which is further added to hydrochloric acid after neutrisation, and heated, the weight of the residue is 7.63: g. Use these figures to illustrate the law of constant compósition.

One oxide of carbon contains 57.1 4% of oxygen, whereas the reduc- tion of 5 g of another oxide leaves 1.363 g of carbon. Do these figures illustrate the law of multiple proportions?

In 40 seconds, 120 cm of mathans diffused through a porous plate. How long will it take 30 cm of hydrogen bromide to diffuse

through the same plate under the same conditions

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