Fractional Crystallization
421
and monohydrogen salts will yield pyro salts, such as sodium /ryrophosphate,
2NaHSO 4 4. Na2S 2 O 7 + H 2 Ot
The formulas of meta and pyro acids are related to the ortho acids that you
learned on p 107 (the exception is HNO 3 , which is meta nitric acid), as follows:
ortho acid — H 2 O — » meta acid
2 ortho acid - H 2 O — » pyro acid
These statements show the relationship between the formulas; the acids cannot
be made this way.
5. Metal hydrides. These compounds usually can be prepared by heating the
metal in an excess of H 2 and cooling it in H 2 . The temperature of treatment
varies from about 150. 0°C for some alkali metals to 700. 0°C for others. Extreme
care must be used when heating with H 2 : the system should be absolutely
leakproof against air; flames should not be used for heating or be in the vicinity;
and the H 2 flow should be safely vented. Further, some hydrides are spontaneously inflammable in air; they should all be handled with extreme care, and
never be thoughtlessly added to water, with which they react to form H 2 . The
alkali and alkaline earth metals form straightforward hydrides (MH and MH 2 )
composed of M
+ (or M
2+ ) and H~. The hydrides of most other metals are
nonstoichiometric (such as CeH 2 . 69 or TaH 0 . 76 ), with the H atoms occupying
interstitial positions in the metal's crystal lattice.
FRACTIONAL CRYSTALLIZATION
The preceding methods emphasize procedures resulting in a single product that
can be removed as an insoluble compound or a gas, or that will be present by
itself in water solution. When such a procedure cannot be used (or found), it
may be necessary to separate the desired compound from a mixture of two
soluble substances by a process known as fractional crystallization. This
method is based on the differences in solubility that the two substances possess
at high and low temperatures. The following problem illustrates the method.
PROBLEM:
The solubility of KNO 3 is 13.3 g/100 ml water at 0.0°C, and 247 g/100 ml at
100.0°C. The solubility of NaCl is 35.7 g/100 ml water at 0.0°C, and 39.1 g/100 ml
at 100.0°C. What weights of the pure substances can be obtained from a mixture
that contains 500 g each of KNO 3 and NaCl, using but one cycle of fractional
crystallization (that is, using one high-temperature and one low-temperature crystallization).
421
and monohydrogen salts will yield pyro salts, such as sodium /ryrophosphate,
2NaHSO 4 4. Na2S 2 O 7 + H 2 Ot
The formulas of meta and pyro acids are related to the ortho acids that you
learned on p 107 (the exception is HNO 3 , which is meta nitric acid), as follows:
ortho acid — H 2 O — » meta acid
2 ortho acid - H 2 O — » pyro acid
These statements show the relationship between the formulas; the acids cannot
be made this way.
5. Metal hydrides. These compounds usually can be prepared by heating the
metal in an excess of H 2 and cooling it in H 2 . The temperature of treatment
varies from about 150. 0°C for some alkali metals to 700. 0°C for others. Extreme
care must be used when heating with H 2 : the system should be absolutely
leakproof against air; flames should not be used for heating or be in the vicinity;
and the H 2 flow should be safely vented. Further, some hydrides are spontaneously inflammable in air; they should all be handled with extreme care, and
never be thoughtlessly added to water, with which they react to form H 2 . The
alkali and alkaline earth metals form straightforward hydrides (MH and MH 2 )
composed of M
+ (or M
2+ ) and H~. The hydrides of most other metals are
nonstoichiometric (such as CeH 2 . 69 or TaH 0 . 76 ), with the H atoms occupying
interstitial positions in the metal's crystal lattice.
FRACTIONAL CRYSTALLIZATION
The preceding methods emphasize procedures resulting in a single product that
can be removed as an insoluble compound or a gas, or that will be present by
itself in water solution. When such a procedure cannot be used (or found), it
may be necessary to separate the desired compound from a mixture of two
soluble substances by a process known as fractional crystallization. This
method is based on the differences in solubility that the two substances possess
at high and low temperatures. The following problem illustrates the method.
PROBLEM:
The solubility of KNO 3 is 13.3 g/100 ml water at 0.0°C, and 247 g/100 ml at
100.0°C. The solubility of NaCl is 35.7 g/100 ml water at 0.0°C, and 39.1 g/100 ml
at 100.0°C. What weights of the pure substances can be obtained from a mixture
that contains 500 g each of KNO 3 and NaCl, using but one cycle of fractional
crystallization (that is, using one high-temperature and one low-temperature crystallization).