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Answers to Problems of the A Groups
(o) Heat metallic K in a stream of H 2 in a tube well protected from the air
2K + H 2 -4 2KH
5 (a) Al will not dissolve directly in HNO 3 Therefore, dissolve Al in HC1, neutralize
with NH 3 , and with excess NH 3 precipitate A1(OH) 3 NaOH can t be used
Filter off the A1(OH) 3 and then/wi/ neutralize it with HNO 3 (add HNO 3 until the
precipitate just dissolves) Evaporate off water
2A1 + 6H
+ -> 2A1
3+ + 3H 2 |
A1
3+ + 3NH, + 3H 2 O -> Al(OH)iJ, + 3NH+
A1(OH) 3 + 3H
+ + 3N0 3 - -> A1
J+ + 3NO 3 + 3H 2 O
(b) Dissolve Cu in HNO 3 , then make basic with NaOH to precipitate Cu(OH) 2
NH 3 can t be used Filter off Cu(OH) 2 and then just neutralize with H 3 PO 4
3Cu + 8H
+ + 2NO 3 ~ -> 3Cu+ + 2NOf + 4H 2 O
Cu" + 2OH~ -* Cu(OH) 2 |
3Cu(OH) 2 + 2H
+ + 2H 2 PO 4 - -» Cu 3 (PO 4 ) 2 + 6H 2 O
(c) Dissolve Ni in HC1, then make basic with excess NaOH NH 3 can t be used
Ni + 2H
+ -» Ni
2+ 4- H 2 f
Ni
2+ + 2OH- -* Ni(OH) 2 |
(d) Dissolve Hg in concentrated HNO 3 , then make basic with excess NaOH NH 3
can't be used Just dissolve the precipitate with HC1, then evaporate off the
water
Hg + 4H
+ + 2N0 3 - -> Hg+ + 2NO 2 | + 2H 2 O
Hg
2+ + 20H -* Hg(OH) 2 j
Hg(OH) 2 + 2H
+ + 2C1- -» Hg
2+ + 2C1- + 2H 2 O
(e) Dissolve Zn in HC1, then make slightly basic with NH 3 and saturate with H 2 S
Filter off ZnS
Zn + 2H
+ -> Zn
2+ + H 2 |
Zn
2+ + H 2 S -+ ZnS| + 2H
+
(f) Dissolve Fe in HNO 3 to get Fe in 3+ state, then add excess NH 3 (or NaOH) to
precipitate Fe(OH) 3 Filter off Fe(OH) 3 and just neutralize with H 2 SO 4 , then
evaporate off water
Fe + 6H
+ + 3NOj -^ Fe
3+ + 3NO 2 t + 3H 2 O
Fe
3+ + 3NH 3 + 3H 2 O -^ Fe(OH) 3 | + 3NH 4
+
2Fe(OH) 3 + 6H
+ + 3SO 4
2 ~ -^ 2Fe
1+ + 3SO 4
2 ~ + 6H 2 O
6 In each of these problems, it is assumed that the calculated amount of water has been
added or removed in going from one step to the next In many cases (when volumes
are small, or when some of water must be removed), it is easier to evaporate off all
the water, then add back the desired amount
(a) Step I Treat the mixture with enough water (W) to dissolve 74 6 g (1 mole) KC1
(K) at 0 0°C
Answers to Problems of the A Groups
(o) Heat metallic K in a stream of H 2 in a tube well protected from the air
2K + H 2 -4 2KH
5 (a) Al will not dissolve directly in HNO 3 Therefore, dissolve Al in HC1, neutralize
with NH 3 , and with excess NH 3 precipitate A1(OH) 3 NaOH can t be used
Filter off the A1(OH) 3 and then/wi/ neutralize it with HNO 3 (add HNO 3 until the
precipitate just dissolves) Evaporate off water
2A1 + 6H
+ -> 2A1
3+ + 3H 2 |
A1
3+ + 3NH, + 3H 2 O -> Al(OH)iJ, + 3NH+
A1(OH) 3 + 3H
+ + 3N0 3 - -> A1
J+ + 3NO 3 + 3H 2 O
(b) Dissolve Cu in HNO 3 , then make basic with NaOH to precipitate Cu(OH) 2
NH 3 can t be used Filter off Cu(OH) 2 and then just neutralize with H 3 PO 4
3Cu + 8H
+ + 2NO 3 ~ -> 3Cu+ + 2NOf + 4H 2 O
Cu" + 2OH~ -* Cu(OH) 2 |
3Cu(OH) 2 + 2H
+ + 2H 2 PO 4 - -» Cu 3 (PO 4 ) 2 + 6H 2 O
(c) Dissolve Ni in HC1, then make basic with excess NaOH NH 3 can t be used
Ni + 2H
+ -» Ni
2+ 4- H 2 f
Ni
2+ + 2OH- -* Ni(OH) 2 |
(d) Dissolve Hg in concentrated HNO 3 , then make basic with excess NaOH NH 3
can't be used Just dissolve the precipitate with HC1, then evaporate off the
water
Hg + 4H
+ + 2N0 3 - -> Hg+ + 2NO 2 | + 2H 2 O
Hg
2+ + 20H -* Hg(OH) 2 j
Hg(OH) 2 + 2H
+ + 2C1- -» Hg
2+ + 2C1- + 2H 2 O
(e) Dissolve Zn in HC1, then make slightly basic with NH 3 and saturate with H 2 S
Filter off ZnS
Zn + 2H
+ -> Zn
2+ + H 2 |
Zn
2+ + H 2 S -+ ZnS| + 2H
+
(f) Dissolve Fe in HNO 3 to get Fe in 3+ state, then add excess NH 3 (or NaOH) to
precipitate Fe(OH) 3 Filter off Fe(OH) 3 and just neutralize with H 2 SO 4 , then
evaporate off water
Fe + 6H
+ + 3NOj -^ Fe
3+ + 3NO 2 t + 3H 2 O
Fe
3+ + 3NH 3 + 3H 2 O -^ Fe(OH) 3 | + 3NH 4
+
2Fe(OH) 3 + 6H
+ + 3SO 4
2 ~ -^ 2Fe
1+ + 3SO 4
2 ~ + 6H 2 O
6 In each of these problems, it is assumed that the calculated amount of water has been
added or removed in going from one step to the next In many cases (when volumes
are small, or when some of water must be removed), it is easier to evaporate off all
the water, then add back the desired amount
(a) Step I Treat the mixture with enough water (W) to dissolve 74 6 g (1 mole) KC1
(K) at 0 0°C