Weight of 12 atom of carbon 12

  1. (Flyweight) - GoF

can be measured experimentally. Therefore the formula NHj can either refer to one molcculc 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, the term "mole" (mol) is usually used to name the quantity of a substancc.

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1) , , ; 2) ; 3) ; 4) ; 5) .

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Section

b)
c)

Ex. 1. Check up if you remember the ways of expressing time.

2.00 It is two (o'clock).
5.10 It is ten minutes past (after) five. It's ten past five. It's five-ten.
4.15 It's a quarter past (after) four. It's fifteen minutes past (after) four. It's four-fifteen.
It's half past two. It's two thirty.
7.45 It's a quarter to eight. It's seven fortv-five.
2.55 It's five (minutes) to three. It's two fiftv-five.
a. m. \ *
p. m.
to be fast
to be wrong [( )
to be slow J


Ex. 2. Answer the following questions, using expressions from exercise 1:

1. What time is it now? 2. When do your classes usually begin? 3. When did the lecture begin? 4.1 believe my watch is fast, what time is it by your watch? 5. At what time shall wc meet? 6. At what time are your classcs over? 7. When do you get up? 8. When will you be free? 9. When will you come to the laboratory?

Ex. 3. Make up sentences or short stories with the following words:

1) introduce, they, symbols, Swedish, begin, shorthand, chemist, simple, Bcrzelius, element, name; 2) phenomenon, allotropy, such, be, when, can, form, clement, exist, different, die same, state, physical; 3) act, formula, law, the very, write, assume, composition, constant; 4) gram, weight, molecular, be, clement, seem, atomic

Ex. 4. Give detailed answers to the following questions:

1. In what way were the modem symbols for the elements introduced? 2. What is the meaning of a chemical symbol for an element? 3. In what way do we usually write formulae for compounds? 4. What law is the basis for writing the formula of a compound? 5. What is an atom? 6. What is a molcculc?

Ex. 5. Discuss the following topics:

1. The History of die Introduction of Symbols.

2. Symbols for Representing Elements.

3. Symbols for Representing Compounds.

DO YOU KNOW THAT ...

Bcrzelius published his first paper on the new nomenclature in 1813. A year later, he published his second article in which he formulated the rules for writing formulae. The number of atoms was marked by a figure at the upper end of the symbol. If a compound contained two atoms of a particular clement, he suggested that the sign should be written in bold type. He marked an oxygen atom in oxides by a point or a comma. Thus, the formula for water was H 'instead of H20. New symbols for chemical elements were soon accepted by scientists, they proved very convenient, but as to formulae, they were acccptcd only in die middle of the 19th century.




Text 9 | Fluorine | Lesson 10 | Text 10 | Lesson 11 | Fascinating Phosphorus | Modifications of Phosphorus | Lesson 12 | Text 13 A The History of Chemistry | Text 13 |

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