Chemistry is the scientific study of the structure of substances and how they change under different conditions or when mixed together. It involves studying molecules and how they combine in fixed ratios according to scientific laws like the law of conservation of mass, the law of definite proportions, and the law of multiple proportions. The average atomic mass of an element takes into account the relative abundance of its different isotopes.

1. CBSE Class 11 Chemistry
SOME BASIC CONCEPTS OF
CHEMISTRY

2. What is Chemistry?
It is the scientific study of the structure of
substances and what happens to them in
different conditions or when mixed with
each other.
Molecules

5. (i.e., Imperial System)
The Metre Convention also known as the Treaty of the Metre, is
an international treaty that was signed in Paris on 20 May 1875
by representatives of 17 nations (Argentina, Austria-Hungary,
Belgium, Brazil, Denmark, France, Germany, Italy, Peru,
Portugal, Russia, Spain, Sweden and Norway, Switzerland,
Ottoman Empire, United States of America, and Venezuela).
The treaty created the International Bureau of Weights and
Measures (BIPM), an intergovernmental organization under the
authority of the General Conference on Weights and
Measures (CGPM) and the supervision of the International
Committee for Weights and Measures (CIPM), that
coordinates international metrology and the development of
the metric system.

6. According to
the new
convention of
2019

7. Hierarchy table of SI System

8. Volume
Density

9. SCIENTIFIC NOTATION RULES
Scientific notation is a standard way of writing very large and very small numbers so that they’re easier to both
compare and use in computations.
Every number in the scientific notation must be in the form of :
a x 10n
where 1 ≤ a < 10 and n must be a positive or negative integer.
To convert a number into scientific notation, first we have to identify where the decimal point and non zero digit
come.
There are two cases in it.
Case 1 :

10. To move the decimal point to the left, we have to count number of digits as explained in the example given below.
According to the example given above, we have to move the decimal point 3 digits to the left and exponent of 10
should be 3 (positive integer)
When we do so, we get the scientific notation of the given number.
So,
2301.8 = 2.3018 x 103

11. Case 2 :
To move the decimal point to the right, we have to count number of digits as explained in the example given below.
According to the example given above, we have to move the decimal point 5 digits to the
right and exponent of 10 should be -5 (negative integer)
When we do so, we get the scientific notation of the given number.
So,
0.000023 = 2.3 x 10-5

12. Important Note:
If we don't find decimal point at anywhere of the given number, we have to assume that there is decimal point at the
end of the number.
For example,
2300000 -------------> 2300000.
Here, the non zero digit comes first and decimal point comes next. So we have to apply case 1 to convert this number
into scientific notation.
Practice Problems :
Write the given numbers in scientific notation.
(1) 0.00006
(2) 5400000
(3) 71 x 102
(4) 33 x 10-3
(5) 0.63 x 10-3
Solutions
(1) 6 x 10-5
(2) 5.4 x 106
(3) 7.1 x 103
(4) 3.3 x 10-2
(5) 6.3 x 10-4

13. For Example,

14. Example
(1) 1.644 has four significant figures
(2) 1.6044 has five
(3) 0.0054 has two
(4) 1500. has four
(5) and 1500.0 has five
(6) (for number 1500, there are two as, can
be written as 1.5 × 103
)

15. Multiplication and Division
The result must contain the same number of significant figures as in the value having the least number of significant
figures.

16. 1.25 ft.

17. Laws of Chemical Combination
1. Law of Conservation of Mass - It states that matter can neither be created nor destroyed.
This law was put forth by Antoine Lavoisier in 1789.
2. Law of Definite Proportions - This law was given by, a French chemist, Joseph Proust. He
stated that a given compound always contains exactly the same proportion of elements by weight.

18. 3. Law of Multiple Proportions - This law was proposed by Dalton in 1803. According to this
law, if two elements can combine to form more than one compound, the masses of one element
that combine with a fixed mass of the other element, are in the ratio of small whole numbers.

19. 4. Gay Lussac’s Law of Gaseous Volumes - This law was given by Gay Lussac in 1808. He
observed that when gases combine or are produced in a chemical reaction they do so in a simple
ratio by volume provided all gases are at same temperature and pressure.

20. 5. Avogadro Law - In 1811, Avogadro proposed that equal volumes of gases at the same
temperature and pressure should contain equal number of molecules.

23. Average Atomic Mass
Many naturally occurring elements exist as more than one isotope. When we take into account
the existence of these isotopes and their relative abundance (per cent occurrence), the average
atomic mass of that element can be computed. For example, carbon has the following three
isotopes with relative abundances and masses as shown against each of them.

24. 48.00 u

25. u

26. (for gas)