3 Part One. The Predicate

Chemical Symbols for Representing Compounds
It is a short step from using a symbol for an element to using a formula for a compound. However, it is often forgotten that the very act of writing the formula for a compound assumes the law of constant composition. HgO originally meant “some of a compound containing equal numbers of mercury and oxygen atoms”. A formula used in this way is referred to as an empirical formula, for it contains only the cxpcrimantally determined ratio o f the relative numbers of atoms of different elements in the compound, but makes no statement about how many atoms there are in one molecule. An empirical formula should be written with brackets (HgO)_x; to show that it makes no claim to represent one molecule of the substance; but for inorganic solids where there is seldom a method of determining x, the brackets are never used.

A molecule of a compound is defined as the smallest part of a compound that can exist as a free and separate substance. For crystalline solids in which one atom is surrounded by several equivalent neighbours, the word “molecule” has no well-defined meaning. Thus, the formula HgO refers to a gram-molecular weight of mercuric oxide, whereas the gram-molecular weight is the sum of gram-atomic weights of the elements in the proportions in which they occur in the compound. For example, zinc has a gram-atomic weight of 65 g and chlorine a gram atomic weight of 35.5 g, so the gram- molecular weight of zinc chloride (ZnCl₂) is 135 g.

For gases, the molecule is a perfectly real entity, and the molecular weight of any volatile substance defined as

can be measured experimentally. Therefore the formula NH₃ can either refer to one molecule of ammonia, containing one atom of nitrogen, and three atoms of hydrogen, or, alternatively, to one gram-molecular weight of ammonia.

Though the terms “gram-molecular weight” and “gram-atomic weight” are no longer used by chemists, it is interesting to know how the idea developed. At present, (he term “mole” (mot)is usually used to name the quantity of a substance.
Mashq 4. 12 B matni 12 A matnidan farqi haqidagi taxminlaringiz asosli bo'ldimi?

Mashq 5. Matnni tematik qismlarga ajrating.

Mashq 6. Matn nima haqida ekanligini ayting:

1) belgilar bilan bog'liqlik formulasini yozish uchun nimani bilish kerakligi haqida; 2) empirik formulaning ma'nosi; 3) empirik formulani yozishda qavslardan foydalanish to'g'risida; 4) uchuvchi moddalarning molekulyar massasi qanday aniqlanadi; 5) ammiak formulasini anglatadi.

Mashq 7. Qattiq moddalarni belgilashda molekulani aniqlovchi va formuladan foydalanish xususiyatlarini tavsiflovchi matndagi parchani toping. Ushbu parchani o’zbek tiliga tarjima qiling.