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EO英文中學會考試題預習專欄
堅道英 【院主編
化學科(二)
CHEMISTRY (2)
SOLUTIONS FOR LAST. BERICH
1. Atomic weights of elements are relative weights.
In the ohsmiozi sobie, the referenen ohonem in oxygen and itm usighid average of naturally. occurring isotopes being fized arbitrary se 16.0000. On this bamir atonio weight may be AA defined as the relative weight of one sten of an elenant senjared with the weight of one sion of oxygen takán au 16,
mutatónio number 18 the value of the positive" charge (pretons) on the nucleus; for neutral ator it is alta squal to the number of extra-nuclear olectrons. It is the ordinal number of that element in the periodio arrangement.
kocording to the law of constant composition, a compound is formed by elements in a fixed ratio by weight. The equivalent weight of an element is its weight that will combine with ez displace 8 of oxygen or 1.005 gs. of hydrogen, in element cad have more than one equivalent weight, if it combines with another element in more than one way, (a) The elestronie arrangement of the element with
atomic number 16 is 2,8,6; that with atonio: number 20 is 2.8,8.2. Thus the valence shells contain 6 and 2 eléctrons respectively. According to the ostet rula, the elementa nay stabilise their structure through gaining or losing slectrons. For atónio number 16, it ⠀ accepte two electrons to attain the outet so that the ion formed is a divalent anion. For atomic number 20, it loses two electrons for stabilisation and henɑs the ion is a divalent cation.
(b) The electronic jessement of element of
Watomie number 17
2.8.7. Thus its hydride
has the electronic structure
Here a pair of electrona is shared by the two atoms so that B attains a stable octet and a
a doublet. This type of bonding through electron sharing gives rise to compounds known as covalent compounds.
The solution of alum is precipitated by barium chloride solution according to the equations
K2SO4*X2 (904) 3*24K20 + 4BaCh2
4Bas0, + 2x+
+ 8C1 + 24,0
thus for every ga, molapule of barium malphate precipitated t gai molscule of the alum in removed.
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Carbonation. This is effected in a Solvay tower where carbon dioxide as bubblad upward against a downward stream of ammoniacal brine. Absorption of carbon dioxide firstly form ammonium carbonate and then ammonium bicarbonate. The reactione generate heat and have to be cooled by water coile. A maximum separation of sodium bicarbonate is effected by cooling the mixture to 15 0. The carbon dioxide is obtained by
heating calcium carbonate in special lima Kilns.. Filtration. The solid is filtered off, washed. with water and heated to sodium carbonate. The carbon dioxide evolved is returned to the reacting towers,
Recovery of ammonia. As this is the most costly. among the raw materials; and is also theoretically not consumed, great care is taken to recover it. The mother liquor from the filters contains the ammonia as bicarbonate and chloride. The former is decomposed by babbling steam into the solution, whilst the latter, may be treated with slaked lime from the quick line. of the kilns :-
Ca(OH)2NH C1 = CaC1, + 2NH | + 28_0↑ M.W. of washing soda Na2CO3.1020 - 2(23)
12 + 3(16) + 10(2 + 16)
its equivalent wt. 286 x 143 M.W, of baking soda
its equivalent wti
-84
23112 + 3(16)
Let fraction of baking soda by weight in the mixture be x. Thus fraction of washing soda 18 1-x.
Therefore total no. of go. equivalents in 2.34 gm. of the mixture 18
2.34 (143
However, the total no. of gm. equivalente is also given by
30(-0.7)
1000
Thus x 0.4034 and 1-x- 0.5966
Therefore comp. by wt. of washing, soda" 59.66 and baking soda 18 40.34.
The pyarogen üsy be collected
Kipp's apparatu
Wash bottle of NȧES
trough cooling: mixture
U-tube: containing
2.
flask with liquid HAS
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EXO#文中學會考試題預習專欄
茶堅道英文書院主編說
物理科(二)
Force C also resolves into 2 components
C:COB
- ◊ sin 60°
Resultant of I-components
B.cos.60°
C com 60o
Resultant of y-components
- B sin 60° ◊ sin 60°
Ans. The resultant
B and C : Zero) |
% comp. at.ut.
Carbon 57.7 12
Hydrogen
3.71
Oxygen 38.59 16
% comp
4.8
at.wt.
3.71
smallest. Whole
Therefore empirical formula of the orgánic acid ie СН302
At. Wt. of silver 108 thus its equivalent wt.
-108
Hence equivalent, wt. of the organic acid radical
100 56.8 - 56.8
x:108 82.0
No. of gm. mola. in 0.117 gm. 18
barium sulphate
0.117
0.0005011
-137.5 + 32 +4(16)
0.137
H.W. BaSO
Therefors no of ga, mole, in the 0.125 ga. alum is
0.0005011 x
0.0001253:
997.6
Hence K.M. of alum is
0.125
0.0001253,
But thie in also given as 2(39) + 4(32+64)
+ 24(2 + 16)+ 2 x N.N. of
894 + 2.x X., of X
Therefore M.W, of K is 997.6
894
2. Sodfum bisarbonats and carbonate are manufactured
industrially by the Solvay process. It utilizes the principle that sodium bicarbonate though -soluble in water becomes auch less soluble in a
medium of high sodium ion concentration (e.g. in brine), owing to the common ion effect.. Bohematically, a solution of ammonium bicarbonate
18 formed by bubbling excess of carbon dioxide into a solution of ammonias treatment of the solution with bring then yields the less soluble sodium bicarbonate:
NH3 + H2O + CO2 = NE HCO,
WE HOP, + NaCl »NH GI
+
HCC
Sodium carbonate is, then easily prepared from the bi-carbonate by heating s
2NaHCO3 = Ha, CO + CO2 ↑ + H201
For the actual process, the steps are similtaneouB and involvement
a. Preparation of ammoniacal brine. Brine is
pumped from a flooded salt mine to the top or a tower. It flows down through many partitions where a stream of ammonia gas is passing upward
Brine Ammonia
mushroom shaped baffle
ammonia
Solvay Tower
Hydrogen sulphide is usually prepared in the.... laboratory by the action of dilute hydrochloric aold on ferrous sulphide in a Kipp's apparatus:
FOS+ 2HC1E
The impurities present faolade xöid fumes, moisture, and hydrogen due to the presence of fri iron in the ferrous sulphide, It may be purified from sold spray by passing the gas through a saturated solution of sodium hydrogen sulphide, NaHS + HCl = NaCl + 8,3, and from moisture by fused calcium chloride or phosphoric oxide. The gas may then be liquefied using a cooling mixture of solid carbon dioxide and ethyl alcohol. The hydrogen passes on and pure liquid hydrogen sulphide is left. The hydrogen sulphide may then be collected over meZOWIE
(a). Zino sulphide as precipitated by passing
hydrogen sulphide into an alkaline solution zino sulphate
硒
223
this indicates that the hydrogen sul behaves as a vdak dibasio aoid.
(b) With dilute nitric acid, colloidal sulphur is formed and nitrogen dioxide is liberatedı ZENO3 + HSS↓ + 2R01 + 250
in this case the nitric acid servos AÐ ́IN oxidising agent reacting with the reducing hydrogen sulphide.
Concentrated sulphuric acid dan behave as an oxidising agent and thus cannot be used to dry hydrogen sulphide. :
+125
+ 50 ↑ + 31
(d) Bromine can ozidize hydrogen sulphide in the
presence of wateri
→ 2HBr
In (b), (e), and (d) bydrogen sulphide behaves as a reducing agent, the sulphur at an oxidation state of -2. becoming sulphur at the oxidation state of sero at the end of the redox reactions,
„A series of e-matar organic compounds in which
each member differs from the next by a constant amount of CH is called a homologous aeries, and ite members are homologues. The paraffina or alkanes constitate, such a family, the molecular formulae of which conform to the general form Co^2n+2* In such a series the physical properties such as melting point, boiling point and density change more or less regularly as the ́ si sa of the molecule inoreases, The chemical properties are also similar and are characterized by the particular functional group they possess. Thus alkenes, monohydric alcohols and monocarboxyllio" acido have general formula4 C 2
"n ̈2n+2′′ CHOH respectively. Their functional groups being the carbon-carbon double bond, the hydroxyl or the carboxyl groupe.
For an ethyl ester, every equivalent of the acid radical contains a corresponding equivalent of the ethyl group, i.e. C2 (of 29 waita)......
29
111
Thus equivalent wt. of the ester 82 + 29 The vapour density of the ester is 111, thus the
MW. of the ester 18 111 x 2 = 222. i.e. contains 2 equivalents/mole. Since the empirical formula of the acid contains
4(12)+3(1) + 2(16) - 83 unita
And the M.W. of the acid calculated from the
ester in 222 - 2(29) + 2(1)
166
Therefore the molecular formula of the acid is
and its a dibasic acid. 6 4
(the acid is actually phthalic acid
QUESTIONS FOR NEXT WEEK
Describe one method for the manufactura oz nydrogen Give an account of the reactions that can take place between hydrogen and (a) ohlorine, (b) nitrogen, paying particular attention to the effect of varying the conditions of reaction.
2. Outline methods adopted to remove hardness from
water. The following experiments were conducted tt determine the temporary and permanent hardness of a water containing only the sulphate and bicarbonate. of calcium,
100 cc. portions of the water were titrated with 0.06N acid using methyl orange as indicator. The mean titre was 5.0.0.0.
200 c.c. of the water were mixed with 25 o.c. of 0.12N sodium carbonate and boiled to dryness, The residue was extracted with distilled water ané the solution was exactly neutralized by 22.0-0 0. of deciñormal acid.
Calculate the temporary and permanent hardness of the water, and express the results as parts of calcium carbonate per 100,000 of water.
Give one method of preparing from metallic zino) (a) zino carbonate, (b) anhydrous sino chloride.
4.00 gm. of a powder known to contain only copper, zino and aluminium were treated with exo888 of warm dilute hydrochloric acid until reaotion was complete. The filtered solution and washings were treated with ammonium obloride. solution and ammonium hydroxide in slight excess. A colourless precipitate was produced. After filtration, washing, drying and ignition to constant weight, it left a product weighing 7-35 go. Calculate the percentage composztion of the original powder,
State the products of electrolysis of the following in aqueous solution with inert electrodes: (a) sulphuric acid, (b) silver nitrate, (o) copper sulphate. dive suitable. equations. What difference is made if (b) ia performed with a silver anode and (a) with a copper anode? Give a sketch of apparatus suitable to demonstrate (a);