1968-01-10 — Page 11

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中文中學會方試題預習專欄

化學科

(+-)

·王錦釗

Chemistry (11)}

[These numbers are. proportional to.

114653英文中學會考試題預習專欄

Carbon (C)

Hydrogen (8)!

(26.7

26.7 x

• 12}

26. 12

2.2 x

0.989

物理科

26.7

(+)

陸永熾

Physics (11)

(üxygen (0)

(71.1]

71.1 x

-32.0

That 18,

19. Selution

The coefficient of linear expansion "or" 8.

substance is the fractional change in length_or_16]

per degras change in temperature.

We, of Ci wt. of H: wt, of o = 12.01 0.989 x 32.0

Assuming C 12

200 0-16

Brass rod

Migroreta” “

No. of C atoms

12.0 12

No, of H atoms: No, of O atoms

0.989 32 16

1 12

of the compound is }

tance the empirical formula

CHO2

期盘

日十月一年八六九一公年七十五國民等中

[Hence the true molecular formila

M8 CO

the compound

NaHCO

35.4

{(23+1+12+16x37

The equivalent weight of HC1 - 36,51 The sanius l'ant! weight of NaHCO2 - 84

Concentration or the given acia solution

* 17.3 gms. of the acid per litre. Normality or the given HCl solution

25 ml, of the given NaHCO, sölution

173

365

• 28.45 ml, or

17.3

N - HOT

173 28.45

365 25

x

the normality of NaHCO solution

- 0.5394 N

Thus concentration of samʊz solution.

•TAL % 0.5394) em. per litre

Weight of impure sodium bicarbonate in i litre of solution - 50 gms.

% of sodium bicarbonate in the solid sample

84 x 0.5394

* 100%

50

90.61 (to 3 sig. figures.)

whermomater

A length of metal rog, say 50 cm. Long, 12 enclosed in a steam jacket as shown in figure. 18 screwed in position of a microuter screw. The micrometer reading is recorded and also the temperature of the ros before steam is passed into. the jacket.

The micrometer 15 then unscrewed deveral turna for expansion. Steam from a boiler is passed in for some minutes. The micromëter is once more adjusted and a new reading taken. The temperaturej er ateam then is noted. Thus,

Original length of rod • lo cm.

Initial temperature of rod *

Final temperature of rod = t

laz micromer reading X

2nd micros ter reading

Increase in length - - x - y cm

Rias in temperature = '(tem

Cost of linear expansion

Initial temperature of the plate Initial diameter of bole - 40 cm. Final diameter of hole • 4064 arm Increase in diameter • 0.4 cm. Final temperature of plate - t Rise in temperature - (t = 0)°C

Coef of linear expansion of metal * 12.5x10

_ncrease in diameter) original dia” x coef. linear expansion.

Rise in tompoziture

0.4

40 x 12.5 x 10

Carbon (C7

Hydrogen (H)

Nitrogen (N)

by wt.

40.0

These numbers are. proportional to

12 40.0 X

12.0

13.3 x 2 •.3.99

FIRST AU J?

wer of Ciwe, of Assuming

12.0

and N

of Ce

ONG KONS

o. of N atoms}

compound

280 numbers are oportional

157.15 x

42.85

42.85

12

22.01

16,0

weight of o

12.0 16.0.

Method of carrying out the titration:

(a) Wash & Durette well with distilled water and then? with the acid solution: Fill the burette with acid solution to above the zero mark, Run out the auid solution quickly out of the tap to expel all the rinsings left in the jet below the tap and also to fill this part of the burette completely with the acid solution.

(b)

Then slowly run out acid to the sero mars. The burette is then set up ready for use,

Rinse out a 25 ml, pipette with the given bicarbonate solution. Discard all hè rinsinga. (a) Suck some more bicarbonate solution into the

pipette until the level of the solution is above the mark on the stem. Quickly place the forefinger over the open end of the pipette and allow the level of the solution to fall slowly until the bottom of the meniscus is just on the mark, (d) Then run the solution from the pipette into a

clean 150 ml. conical flask. The small amount of liquid that remains in the jet of the pipette must be left there, and must not be blown out. „Add two drops of methyl orange to the solution

the flask. Slowly add acid from the burette until the end-point is reached, that is, when the colour of the solution changes from yellow to orange. If too much acid is added, the solution will becoms pink....

f) This process is repasted two or three times and the average volume of acid required to ranog exactly with 25 ml. of sodium bicarbonate solution ia obtained

Side of plate becomes

12.5

- 100 (1 • 12.5

100 x 1,01 101

800)

At 800°C the diameter of the hole becomes 40.7 cm, and the side of the square plate is 101 cm.

The coefficient of real (or absolute) expansion of a liquid is the fraction of its volume by which it expanda per degree rise in temperature. The coefficient of apparent expansion of a liquid is the fraction of its volume by which the liquid appears to expand per degree rise in temperature when heated in an expansible vessel.

Coefficient of real expansion 19, in fact, the sum of the coefficients of apparent expansion and expansion of the vessel.

To determine the coefficient of apparent expansion of turpentine:

A dry empty S.U. DUGLLE is waigned (m). It is then weighed full with turpentine (m) at room temperature (t), The bottle, together with a' thermometer, is then placed in a beaker of water. The water is then heated slowly to some higher temperature (ty), stirring constantly to keep the temperature uniform. The bottle is then removed. from the water, dried on the outside, allowed to cool and finally weighed (m2). The coefficient of expansion is calculated as follow:

Thermometer

Thread

Vx (m

va (m,

#ight more

Assuming C - 12

0.16

of_t_atome No. or #LOMS.

-12

11

ne empirical formula of the compound is

the true formule or the compound ve unun. where n 18 any positive Enteger

Vapour density of Cnon 14 Molecular weight of Cnün 14 x Thus 12 x n

16 x n = 28. pr 28n

in 28 - 1

Coefficient of apparent expansion of turpentine

12

(¤2 = m)(t1 − t)

per degree V.

Initial temperature - 20°C. Initial volums of oil - 1 litre. Final temperature of oil - 15°C Cross-section of tube - 1 sq.cm. 011 level lowered hem. Apparent contraction = real contraction

fan of glass 1000 (1-5.44 x 10-4 x 71

- 1000 (1 - 24 x 10′′° x 5). 1000 [5 × 10-4(0,24 - 5.44)]

2.6

ans. The oil level lowers 2,6

contraction

Questions for caxt week (12)

ta) show by means of a labelled disgran now, starting

from common salt, you would make ► Sk by of

hydrochloric acid,-

(h) What is the action or hydrochloric acid oni (1) marble; (11) lead dioxide; (111) cubrie

oxide; (iv) bleaching powder?

(el Describe an experiment to show that hydrogen

chloride is a very soluble gas,

Describe briefly three methods that can be used for the preparation of metallic chlorides. In each case. give the name and formula of one chloride which can be prepared in that way.

temporarily to allow expansion or una gaboo, the flaska submerged in a tank of cold water, and the corks removed under the water. Water observed to enter both flasks, State whathar the effect is due, wholly or partly to (1) atmospheric pressure, (11) the contraction of the gas, (iii) solution of the gas in the water. How if at all, would you expect the volumes of water entering the flasks to differ? (o) Describe how the above experiment may be

modified to measure the coefficient of

Before heating After heating

Mass of bottle empty n gm.

Mass of bottle full of turpentine at room temperaturi

Masa or bottle and turpentine arter heating

Room temperature - t°C

Temperature after heating

Mass of turpentine left

Mass of turpentine expelled

Questions for next week?

21, (a) Explain why the definition of "coefficient of thermal expansion of gas" involves specifying the temperature of 0°C., but no temperature is specified for "linear coefflalenta" over ordinary temperature ranges.

Equal-sized corked flasks containing air and carbon dioxide respectively at room temperature are heated to 100% The corks are then removed

expansion of gas,

(d) 1000 c.c. of oxygen are collected at a pressure of 72 cm of mercury and at a temperature of 20°C. Calculate the mass of the oxygen, given that the density of oxygen at S.T.P. is 1.43 per litre.

2. (a) Define the terma British Thermal Unit, therml

capacity and water equivalent of a substance. (b) 1 gm of water, 1 gram of concrete (sp.ht. £),

and 1 gm. of iron (sp.ht. =) are each supplied with 1 calorie of heat.

9Calculate the rise)

in temperature of each.

A shallow iron tank containing water 23 embedded in a concrete surround, and is heated by the sun's ray. Explain the fact that a time the iron and the water are slightly warm and- the surface of the concrete is noticeably