Walter Scott Hendrixon.

A laboratory manual of general chemistry online

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(d) To dilute solutions of copper, cadmium and zinc preferably
chlorides add H 2 S in excess. Let settle and pour off most of the liquid
in each case and add equal volumes of dil. HC1. Are the reactions
with H 2 S reversible? How may Cu and Zn be separated? To solu-
tions of the same metals add an excess of NaOH and heat to boiling.
For explanation see 127a and 128c. How may copper be separated
from the other two elements?

To solutions of copper and cadmium add an excess of ammonia.
Add to the copper tube KCN (dangerous) drop by drop till the blue
color disappears. Add the same volume of KCN to the tube contain-
ing Cd. Now pass H 2 S into each tube. How may Cu and Cd be separ-


127. (a) To a little silver nitrate solution add drop by drop am-
monia solution till the small amount of silver oxide at first formed is
dissolved, forming the complex ion Ag(NH 3 ) 2 + . Add a little more
ammonia and try to precipitate silver chloride with a small amount of


NaCl solution. Now add an excess of dilute nitric acid. What is pre-
cipitated and why?

To another portion of silver nitrate solution add sodium hydroxide
solution which will precipitate mainly Ag 2 O, but it is alkaline and be-
haves as though partially hydrated. A similar copper hydrated oxiclo
is formed by adding an excess of NaOH to a solution of copper sulfate
and heating to boiling. Try it. The compound is Cu(OH) 2 .2CuO. Add
ammonia to the tube containing the silver oxide till it just dissolves
and then H 2 O 2 , which will give metallic silver. Prepare the same sort
of a solution of silver oxide in ammonia, that is, containing the ion
Ag(NH 3 ) 2 + , add 1 gram sodium potassium tartrate dissolved in a little
water, warm the test tube and let stand. If silver is not deposited on
the tube warm again. Note analogy of the complex silver ammonia ion
to that of copper, Cu(NH 3 )4 ++ which is deep blue.

(b) To four tubes containing silver nitrate add respectively a
solution of a chloride, a bromide, an iodide and to the fourth drop by
drop a solution of KCN (caution) , till the silver cyanide at first formed
is dissolved. Treat a little of each of the halides of silver with an ex-
cess of ammonia. Which are dissolved? Treat other small portions
with a solution of sodium thiosulfate and shake till dissolved. Treat
yet other portions with a solution of KCN till dissolved. How 1 could
you distinguish the three halides of silver by their color? How dis-
tinguish by their solubility in ammonia? What is formed when sil-
ver chloride dissolves in ammonia? What when it dissolves in KCN?
Expose a little chloride to sunlight and observe color after a few min-
utes. Compare cuprous chloride and silver chloride as to the effects
on them of sunlight and ammonia solution.

(c) To solutions of sodium phosphate, potassium chromate, sodium
arsenite and sodium arsenate add silver nitrate. Try to dissolve por-
tions of each precipitate in ammonia and in dil. nitric acid. Make
careful records of all results for they are to be used in the qualitative
testing for acid radicals.


128. (a) To solutions of magnesium, zinc and cadmium salts,
preferably sulfates or chlorides add an excess of ammonium hydrox-
ide. Some zinc and cadmium hydroxide are precipitated but in an ex-
cess of ammonia they form the complex ions Zn(NH 3 )4 ++ and
Cd(NH 3 )4 4+ which are soluble. To show that only a part of the Mg is
precipitated as hydroxide, filter it off and add acid sodium phosphate
to the filtrate when more magnesium as NH 4 MgPO4 will be precipi-
tated. The prevention of complete precipitation as Mg(OH) 2 by am-
monium hydroxide is due to the necessary accumulation of highly ion-
ized ammonium salt as the reaction progresses. To show this first


add to a solution of magnesium salt ammonium chloride solution and
then ammonium hydroxide. Also, precipitate Mg(OH) 2 with ammonia
and then add ammonium chloride. In the first case no magnesium
hydroxide was precipitated and in the second it dissolved.

The action of the ammonium salt which is highly ionized is to
supply the common ion NH 4 which forces the already slight dissociation
of ammonium hydroxide to the left in NH 4 OH=(reversibly)NHr+OH-
till there are not enough OH ions to form sufficient Mg(OH) 2 to exceed
its solubility limit. Furthermore, the NH 4 ions from the ammonium
chloride or other ammonium salt unite with the OH ions associated
with the Mg in solution to form the undissociated NH 4 OH. This latter
action is of the same sort as in the neutralization of a base by an acid
in which the ion H of the acid unites with the OH of the base to form
undissociated water.

This explanation applies to the solubility of several other hydrox-
ides when ammonium salts are added and will be referred to later.

Prove that acid sodium phosphate alone will not completely pre-
cipitate Mg by adding to a magnesium solution an excess of phosphate,
filtering and then adding to the filtrate ammonium chloride and am-
monia. Make the corresponding salt of zinc, NH 4 ZnPO 4 .

(b) Burn a little Mg ribbon, place some of the oxide on moist
turmeric paper and state whether it is alkaline. Burn a little zinc
dust by heating and stirring in an iron crucible, and determine wheth-
er it has any alkaline property.

(c) Precipitate the hydroxides of Mg, Zn and Cd by adding to
their solutions NaOH a few drops at a time. Now add more NaOH to
half of each and determine which are soluble in an excess. Dissolve
the other half of each hydroxide by adding any dilute acid. With so-
dium hydroxide in excess zinc and cadmium hydroxides form Na 2 Zn02
and Na 2 CdO 2 , which resemble ordinary salts in form and properties.
The hydroxide of Zn and Cd and many other elements are "ampho-
teric"; that is, they act like bases toward strong acids, and like weak
acids toward strong bases. On the basis of the ion theory explain the
dissolving of Mg(OH) 2 by HC1.

(d) To solutions of Mg, Zn, and Cd add H 2 S for several minutes.
Which give sulfides? Shake and filter off half of each precipitate
and to the filtrates add ammonia. Did the H 2 S completely precipitate
both the Zn and Cd as sulfides? To the other half of each precipitate
obtained with H 2 S alone add dilute HC1 and finally con. HC1 if needed
to dissolve all the sulfides. Are the reactions of Cd and Zn salts with
H 2 S both reversible? Which one is most easily reversed? How may
Zn, Mg and Cd be separated?



129. Place 10 grams mercury in 4 c.c. con. nitric acid diluted with
the same volume of water and let stand a day. Pour off liquid and
dissolve the solid in water adding a little dilute nitric acid, letting the
metallic mercury remain. This is a solution of mercurous nitrate, (a) .
Dissolve about 2 grams mercury in a few c.c. con. nitric acid, heat and
if necessary add more nitric acid and heat till a drop of the solution in
water gives no precipitate with HC1. Now dilute with about 50 c.c.
water. This is mercuric nitrate, (b).

Treat small portions of (a) and (b) with NaOH in excess which
gives Hg 2 O and HgO and not the hydroxides. Compare the result with
that obtained with silver nitrate and NaOH, with a cupric compound
and NaOH cold and after boiling. Try ammonia on the solutions (a)
and (b). No oxides or hydroxides are formed but with (a) Hg and
Hg 2 N(NO 3 ). With (b) the same compound but no free Hg.

Treat a little of (a) with dil. HC1 and determine whether the HgCl
is soluble in an excess of HC1 or in nitric acid. Try the action of am-
monia which gives black Hg. HgNH 2 Cl. How may mercury in the mer-
curous condition be separated from silver? Oxidize a little of solution
(a) to (b) by adding Br water till the red color persists, boiling out
excess of Br. Prove that only mercuric mercury is present by adding
dil. HC1.

To small portions of (a) and (b) add a solution of KI drop by drop
giving green Hgl and red HgL. To the latter add an excess of KI
which will dissolve forming a double salt, HgI 2 2KI.

To a very dilute solution of stannous chloride add a solution of
mercuric chloride which gives HgCl while SnCh, stannic chloride, re-
mains in solution. This is a good test for either Hg or Sn. What must
be the valence of each ion when tested for?

To (a) and (b), and to solutions of Pb, Cu, Cd add an excess of
H 2 S. Let settle, pour off the liquid in each case and try to dissolve the
precipitates by boiling with dilute nitric acid. How may mercury be
separated from the other metals? Determine whether HgS will dis-
solve in ammonium sulfide. How may mercury be separated from As,
and Sb.


130. Dissolve most of 2 grams of tin by heating with 10 c.c. con.
HC1, best in a small flask on the water bath. Pour off half of the so-
lution into a dish and add 50 c.c. of water to the flask and use the so-
lution as stannous chloride, (a). Heat to boiling the solution in the
dish and add con. nitric acid a few drops at a time till a drop in a


little water gives no precipitate with mercuric chloride. Add 50 c.c. of
water forming a suitable solution of stannic chloride, (b).

To small portions of (a) and (b) add a few drops of NaOH, then
add in excess. Are these hydroxides "amphoteric" ? To fresh portions
of the tin solutions add yellow ammonium sulfide, at first only a few
drops, then an excess with heating, but do not boil. The SnS and SnS 2
at first formed, should dissolve. Now add HC1 in excess which will
reprecipitate the tin sulfide, SnS 2 from each solution. What other sul-
fides dissolve in ammonium sulfide? How may the tin sulfides be sep-
arated from those of copper, lead, mercury?

To a little of (a) add a solution of mercuric chloride which will
give insoluble HgCl. Try (b) with mercury chloride. Now reduce to
the stannous condition the Sn in a portion of (b) by persistent heat-
ing with finely divided iron, filter and add mercuric chloride. This is
a good test for either Hg or tin. In what condition of oxidation must
each be?

To a portion of (a) add a few drops of gold chloride which will
give colloidal gold, the purple of Cassius, a good test for gold.


131. To a dilute solution of lead acetate or nitrate add an excess
of NaCl solution. Before the lead chloride settles pour off one half
and boil, adding more water if necessary and boiling till it all dis-
solves. Let the other half stand 5-10 minutes, filter and to the filtrate
add dil. sulfuric acid. What proof here that lead sulfate is less soluble
than lead chloride? Filter off the lead sulfate and pass into nitrate
H 2 S. Which is less soluble, lead sulfate or lead sulfide? From a dilute
solution precipitate lead chloride and prove that it is soluble in HC1 if
added in large excess. Prove in the same way that lead sulfate is
soluble in nitric acid. These facts must be kept in mind in the analysis
of Group II. Try to dissolve lead chloride in ammonia. Devise a
scheme for the separation of Ag, Hg (mercurous) and Pb.

132. To a solution of lead acetate add a solution of sodium carbon-
ate till alkaline. The precipitate is a basic carbonate similar to "white
lead." Shake a solution of lead acetate with PbO, filter and pass
through carbon dioxide which will give much the same compound.
Mix 5 c.c. con. nitric acid and 5 c.c. water, heat and add a little at
a time about 2 grams red lead, PbaO^ The brown product is lead di-
oxide, PbO 2 . Filter a few drops of the liquid and add dil. H 2 S0 4 . Is
there lead in solution? To show the oxidizing power of lead dioxide
add to it and the rest of the liquid a few drops of any salt of manga-
nese and boil persistently. Let the solid matter settle and note deep
red color due to permanganic acid, HMn0 4 .


133. To 20 c.c. of the lead acetate solution of the laboratory add
80 c.c. pure water, place in flask and with a thread suspend in it a
folded strip of zinc, about 5 grams, and let stand till next period. Ex-
amine the "lead tree," and test the solution for lead and zinc. Which
metal is most electropositive?


134. Dissolve a few tenths of a gram of Al in con. HC1 and dilute
to a test tube full, and use where A1C1 3 is required (a). Dissolve an-
other small amount of Al in a few c.c. of NaOH warming, which gives
sodium aluminate, Al (ONa) 3 , (b). What gas is given off in each case?

To a little of (a) add ammonia in excess. To (a) add drop by drop
NaOH till a permanent precipitate is obtained. Dissolve one-half of it
by adding HC1 and the other half with an excess of NaOH. Is A1(OH) 8
amphoteric? In what two ways may it ionize? To (b) add ammonium
chloride till the precipitate is permanent, and account for its formation.
How could you separate Al and Zn?

To portions of (a) add in excess sodium carbonate, and ammonium
sulfide and account for the precipitation of Al (OH) 8 in each case?

135. By heating dissolve 15 grams of aluminium sulfate,
A1 2 (SOOs 18H 2 O in 50 cc. water, stir in the calculated weight of am-
monium sulfate to make ammonium aluminium alum (NH^aAMSOO*
and when all is dissolved filter into a crystallizing dish while hot. Let
a drop or two of nitrate fall on a watch glass, let the water evaporate
and examine the crystals with a microscope. Examine the crystals in
dish, dissolve a few and test the solution with blue litmus paper. Why
are the solutions of Al salts acid? See 63.


136. Chromate Ion to Chromium Ion: Dissolve in a dish 10
grams potassium dichromate in 50 c.c. water with heat and let cool.
Add 10 c.c. con. sulfuric acid and then a little at a time 50 per cent
alcohol heating if necessary to start the reaction. Continue the addi-
tion of alcohol till the solution has a green color, a few drops in much
water showing no brown. Avoid a large excess of alcohol. Set aside
two-thirds of the solution and examine the crystals of chrome alum
at the next laboratory period. Dilute the other third with 20 times its
volume of water and use it below. One molecule of the dichromate is
reduced by the oxidation of three molecules of alcohol to aldehyde,
CH 3 COH. With aid of text write the equation. What was the action
of H 2 S and of SO 2 on acidified solutions of K 2 Cr 2 O 7 ?

Treat a portion of your solution of chrome alum with an excess of
ammonia. How separate Cr and Zn? Try to dissolve a little of the
remaining precipitate with NH 4 C1. How separate Cr from Mg? To an-


other portion of chrome alum solution add an excess of NaOH and boil.
How separate Cr and Al? Is Cr(OH) 8 amphoteric? Try portions of
your chrome alum with solutions of sodium carbonate and ammonium
sulfide. In each case the hydroxide is precipitated as in the case of Al.

137. Chromium Ion to Chromate Ion: To a few c.c. of chromium
nitrate or chloride solution in a dish add an equal volume of NaOH.
Stir into it gradually 2 grams sodium dioxide and heat to boiling. Fil-
ter if necessary, acidify a little of the filtrate with acetic acid being
sure of an excess by testing, and add barium chloride. To a little di-
lute solution of sodium dichromate add acetic acid and barium chloride
and compare precipitates.

13^. Dichromate to Chromate: (a) Dissolve 5 grams sodium di-
chromate in 25 c.c. water in a dish, add slowly with stirring NaOH till
the solution becomes yellow. Evaporate till the salt crystallizes on

(fo) Chromate to Dichromate: Dissolve 10 grams sodium chro-
mate in 25 cc. water and add the calculated weight of con. sulfuric
acid, that is, one molecule of acid to two of the chromate. Evaporate
till the dichromate crystallizes out on cooling.

(c) Chromium Trioxide: Dissolve 2 grams potassium dichromate
ii 5 c.c. water by heat, cool till it begins to separate then drop con.
dlfuiic acid directly upon the surface of the liquid in test tube till the
precipitate formed does not quite dissolve. Heat to dissolve most of
it, note the escape of some oxygen, set in rack to cool and observe
crystals of CrO 3 in an hour.

139. Chromium Oxychloride: In a retort place 3 grams dichro-
mate, 2 grams NaCl and 15 cc. con. H 2 SO 4 . Heat and collect the oxy-
chloride as you did nitric acid. What does it look like? Dissolve some
of it in water, add an excess of NaOH, then an excess of acetic acid and
finally barium chloride. What is the precipitate?

140. Test for Chromate Ion : To two small portions of a chromate
solution add acetic acid then barium chloride and lead acetate respec-
tively. To a neutral solution of a chromate add -silver nitrate. Divide
into two portions. In one try the solubility of the silver chromate in
ammonia and in the other with dil. nitric acid. A third test is the re-
duction of chromate ion to chromium ion with change from yellow or
red to green as in 136.


141. Dissolve most of about 1 gram of card teeth in con. HC1 di-
luted with an equal volume of water. Pour one half in a dish. Dilute
the other half to a test tube full leaving in it the undissolved iron. This


is ferrous chloride, FeCl 2 , solution (a). Heat the half in the dish and
with stirring add a little con. nitric acid at a time till the black pre-
cipitate at first formed dissolves and a drop of the solution in a little
water gives no blue color with potassium ferricyanide. Dilute to a
test tube full. It is ferric chloride, FeCls (b).

Treat small portions of (a) with an excess of ammonia, NaOH, so-
dium carbonate, H 2 S, ammonium sulfide. The first and second give
ferrous hydroxide Fe (OH) 2 , changing in air to ferric hydroxide Fe (OH) 3 ;
the third gives ferrous carbonate. Hydrogen sulfide has no effect, but
ammonium sulfide gives black FeS. Treat small portions of (b) with
the same reagents in excess. Ammonia, NaOH and sodium carbonate
give ferric hydroxide. H 2 S reduces ferric to ferrous iron with the sep-
aration of sulfur. Why does it not precipitate iron sulfide? Is either
hydroxide amphoteric? Try to dissolve ferrous and ferric hydroxides
with ammonium chloride. Compare the action of sodium carbonate
and ammonium sulfide on ferric iron, aluminium and chromium solu-
tions. Devise ways to separate iron from Al, Cr, Cu, Mg, As.

142. Treat portions of (a) and (b) with solutions of potassium
ferrocyanide, potassium ferricyanide, ammonium sulfocyanide and
tabulate results as tests for ferrous and ferric iron.

The following is a fine example of a reversible reaction and illus-
tration of the influence of the common ion: To a test tube nearly full
of water add about 5 drops of ammonium sulfocyanide and the same
amount of ferric chloride. The reaction is,

FeCl 3 +3NH 4 CNS= (reversibly) Fe (CNS) 3 +3NH 4 Cl.

Divide the red solution in four test tubes. To one add more of the
ferric chloride, to the second more of the sulfocyanide, to the third
ammonium chloride, and compare colors with that of the fourth. Re-
fer to 62 and to text book and explain fully.

143. Double Salts: (a) Dissolve in 50 cc. water by' heat 10
grams ferrous sulfate and the calculated amount of ammonium sulfate
to make ammonium ferrous sulfate and filter hot into a crystallizing
dish. Examine crystals in dish, also let a drop cool on watch glass
and use microscope. This is Mohr's salt, (NH 4 ) 2 Fe(SO 4 ) 2 6H 2 O.

(b) To make iron alum dissolve 15 grams ferrous suifate in 25 c.c.
water in dish, also the calculated amounts of ammonium sulfate and
con. sulfuric acid. Now heat and add slowly with stirring, con. nitric
acid till a drop diluted shows no ferrous iron. Set aside to crystallize,
giving (NH 4 ) 2 Fe 2 (S0 4 ) 4 24H 2 0.


144. From nickel and cobalt chlorides or nitrates precipitate their
hydroxides with NaOH in excess. Try to dissolve portions of the hy-
droxides in excess of NaOH with heating. Are they amphoteric? Try


to dissolve other portions of the hydroxides with ammonium chloride.
How may these metals be separated from ferric iron, aluminium, chro-

Try to precipitate the sulfides of Ni and Co with H 2 S, then with
H 2 S and ammonia or with ammonium sulfide. Try to dissolve the sul-
fides with dil. HC1. How may Ni and Co be separated from Zn, Fe, Sb
and the metals of groups I and II?

To 1 to 2 c.c. of solutions of Ni and Co add NaOH with shaking till
the hydroxides are just permanent. Add to each tube acetic acid in
slight excess, then to each 10 cc. of a solution of potassium nitrite and
let stand. The precipitate is K 3 Co(NO 2 )e. How may Co be separated
from Ni?

Again precipitate the hydroxides of the two metals from 1 to 2 cc.
of their solutions avoiding a large excess of NaOH. Add to each a so-
lution of KCN (dangerous) till the precipitates just dissolve. Now add
to each about 1 c.c. of NaOH and bromine water till it colors them per-
manently red. A black precipitate of nickelic hydroxide should be ob-


145. To a few cc. of a solution of manganous salt, as MnCl 2 , add
ammonia in excess and to another portion NaOH in excess. Does the
Mn(OH) 2 redissolve? How may Mn be separated from Zn, Al, Cu, Ag?

Add to one fresh portion of the solution ammonia in excess then
ammonium chloride, and to another portion add ammonium chloride
then ammonia. Compare the results with those obtained with Mg and
the same reagents. To one of these solutions add ammonium sulfide
and to the other hydrogen sulfide. How could you separate Mn and
Mg? Try to dissolve the sulfide in dil. HC1. How may Mn be separ-
ated from Cu, Hg, As, Sb?

To a manganese solution add ammonium chloride, ammonia, and
sodium phosphate. Compare result with the action of these reagents
on solutions of Zn and Mg.

146. Permanganic Acid and Permanganate: To about 2 c.c. of
a solution of any manganous salt add an equal volume of con. nitric
acid, then about 1 gram of red lead or lead dioxide, and heat some time
at the boiling point. Let the undissolved matter settle and note red
color of permanganic acid. This is a good test for Mn. Pour off a
little of the clear solution into water to see color better.

(b) Melt in an iron crucible 5 grams solid KOH and 2.5 grams
potassium chlorate, and stir in gradually 2 grams MnO 2 . Heat with
stirring till the mass turns solid and raise the temperature with full
burner flame, and continue 5 minutes. When the mass is cold dis-
solve out by heating with the crucible nearly full of water. Pour the
solution into a large test tube and let settle. To one portion of the


green solution of potassium manganate, K 2 MnO4, add dilute sulfuric
acid till* it just turns red forming potassium permanganate, KMnO<.
Try changing the manganate to permanganate in another portion by
pa-rsing through it carbon dioxide, and in a third by diluting it with
much water.

147. Dissolve about one-fourth gram of oxalic acid in water, add
5 c.c. dil. sulfuric acid, heat to about 80 degrees and add a little at a
time a solution of potassium permanganate till the color becomes per-
manent. Dissolve about one-half of a gram of ammonium ferrous sul-
fate (143a), add sulfuric acid and permanganate as above. These re-
actions illustrate oxidation by permanganic acid and are much used
in quantitative analysis. Write the equations, assuming that the oxalic
acid is oxidized to water and CO- and the FeSO 4 to Fe-CSO^s.

The following scheme of analysis is prepared for first year stu-
dents in chemistry to be used in the separation of the common metals.
Provision is not made for every contingency. For example, it is as-
sumed that the ions to form insoluble phosphates in Group III are
not present.

148. Group I; Ag, Pb, Hg: Determine with test paper whether
the solution is neutral or only slightly acid. If neutral add one-tenth
of its volume of HC1, sp. gr. 1.12. If strongly acid, neutralize with
ammonium hydroxide and then add the HC1. The purpose is to have
enough acid to prevent the precipitation of BiOCl in this group and
ZnS in the next, and not enough to prevent the precipitation of SnS
and CdS in group II.

If no precipitate is formed pass to group II. If one is formed let
remain a few moments and filter. Set aside the filtrate (1) for group
(II). Wash twice the precipitated chlorides of Ag, Pb, Hg 2 with small
portions of cold water. Now pour through the filter a half test tube
full of boiling water. Boil and pour through again. Add to one-half

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Online LibraryWalter Scott HendrixonA laboratory manual of general chemistry → online text (page 7 of 8)