218. Constitutional Formulas. If we know the molecular formula of a compound, and the valence of its elements, we may represent the way in which the atoms are joined in the molecule by means of constitutional, or structural, formulas. When the molecules consist of two atoms each, as do those of hydrogen chloride and hydrogen, the atoms are assumed to be joined directly, and the constitutional formulas are H-Cl and H-H. Here the combining power (valence) of each atom is represented by a single line, or bond. In the case of the water molecule we assume that the two hydrogen atoms are united to the one oxygen atom. The oxygen atom seems to have its power of holding other atoms directed along two lines, or valences, each of which can hold the single valence of a hydrogen atom. In the same way the nitrogen atom of ammonia is united with each of the three hydrogen atoms, and the carbon atom of marsh gas with each of the four carbon atoms (cf. § 132). The constitutional formulas of these substances are:

Substances that have the same percentage composition, but different molecular weights, are said to be polymers of one an

other. Thus, formaldehyde, acetic acid, and grape sugar have the same percentage composition:

C=40.00%.

H= 6.67%.
0=53.33%.

The simplest formula for each is, therefore, CH2O. A substance having this formula must have a molecular weight of 30. This is the molecular weight of formaldehyde. The molecular weight of acetic acid is 60, and that of grape sugar, 180; hence these substances are the di-polymer and hexa-polymer, respectively, of formaldehyde, and their formulas are C2H4O2 and C6H12O6.

220. Isomers.

Many cases are known in which two or more compounds have the same composition and molecular weight, but different properties. Methyl ether and ethyl alcohol, for example, are both represented by the formula C2H6O; these substances are so different, however, that no one would mistake one for the other. Thus methyl ether boils at -24° C., at ordinary pressure, while ethyl alcohol boils at +78° C. Such compounds are said to be isomers of one another. Constitutional formulas represent the differences between isomers by different arrangements of the atoms in the molecule. Thus, the structural formula for methyl ether is,— H H

EXERCISES.

201

According to these formulas, all the hydrogen atoms of methyl ether have the same relation to the remainder of the molecule and should behave in the same way with reagents; while in the case of ethyl alcohol one hydrogen atom the one bound to oxygen - should be different from the other five. This is actually the case; for the atom of hydrogen bound to oxygen is the only one of the six that can be replaced by sodium and other metals.

221. Exercises.

1. Write the molecular equations for the following reactions: (a) Union of hydrogen and chlorine.

(b) Burning of hydrogen in air.

(c) Union of nitrogen and hydrogen to give ammonia.

(d) Action of sodium upon hydrogen chloride.

(e) Burning of ethane in air.

(f) Union of ammonia and hydrogen chloride.

(g) Burning of ammonia in oxygen.

(h) Decomposition of potassium chlorate by heat.

(i) Reaction of zinc with hydrochloric acid.

2. A liter of a gas weighs 1.96 g. under standard conditions, and its composition is: C, 27.27%; O, 72.73%. Find its formula.

3. A certain gas consists of N, 63.64%; O, 36.36%. 200 c.c. of it weigh 0.3932 g. under standard conditions. What is its formula?

4. A certain compound has the composition: C, 52.17%; H, 13.04%; O, 34.78%. Its vapor is 1.44 times as heavy as oxygen. Find its formula.

5. A compound has the composition: C, 10.04%; H, 0.84%; Cl, 89.12%. 0.26 g. of it had a volume of 70 c.c. at 99° C. and under 740 mm. pressure. What is its formula?

6. A compound has a molecular weight of 72. Its composition is: C, 83.33%; H, 16.67%. Find its formula.

CHAPTER XIX.

NITROGEN ACIDS AND OXIDES.

222. Nitric Acid. Nitric acid is one of the most important substances known to Chemistry. It has been in use since the time of the early alchemists, but its true nature was not understood until the latter part of the eighteenth century.

During the Middle Ages nitric acid was made by the distillation of a mixture of alum, blue vitriol, and niter.

Alum is potassium aluminum sulphate plus crystal water [K2SO4, Al2(SO4)3, 24 H2O], and blue vitriol is cupric sulphate plus crystal-water (CuSO4. 5 H2O); by the dry distillation of these substances sulphuric acid was set free. This with the niter (KNO3) gave nitric acid.

223. Commercial Preparation. At the present time nitric acid is made on a large scale by heating、 sodium nitrate with concentrated sulphuric acid. The operation is carried out in large iron retorts;* and the vapors evolved are condensed in a system of glass tubes (Fig. 50). The resulting liquid is redistilled.

LABORATORY METHOD

203

In this way there is obtained an acid of specific gravity 1.4; its boiling point is 120° to 121° C. This, the commercial grade of nitric acid, is only 68%, by weight, nitric acid. The remainder is water.

224. Laboratory Method. In the laboratory, nitric acid is commonly made by heating potassium

product of the reaction, potassium hydrogen sul