chemistry

Class 9 Chemistry Fundamentals of Chemistry Important Q/A for Exams

Class 9 Chemistry Fundamentals of Chemistry Important Q/A for Exams:

Chemistry branch of science which deals with

  1. Composition of matter
  2. Properties of matter
  3. Changes in matters
  4. Different laws and principles which deals with these changes
  5. Internal Structure

Matter 

Anything in the universe which occupy space (having volume) and has mass is called matter.

  • Matters can neither be created nor destroyed, although one form can be changed into another form. This is called Law of conservation of matter.
  • E.g. Table, Chair, Ice, Sugar, etc.….

Physical states of Matter

Matter exists in 6 physical states:

  1. Gas: air, CO2, H2, O2.
  2. Liquid: H2O, Milk, oil.
  3. Solid: Ice, Sugar, Sand.
  4. Plasma e.g. Ionized gas Present in Electric Discharge
  5. Neon Signs (Street Lights).
  6. Liquid crystal
  7. Super atoms.

Substances:

A sample of pure matter is called a substance or anything whose composition is uniform throughout and has well-defined physical and chemical properties is called a substance.

E.g. Silver, Gold, Water, NaCl, etc.

They are of two types:

i) Elements        

ii) Compound

Element:

A substance which is made up of only one kind of atoms is called element e.g. O2, O3, Cl2, S8, He etc. There are 92 natural elements while total elements are more than 110.

Compound:

A substance which is made up of more than one kind of atoms chemically combined together, is called compound e.g. CO2, H2SO4, C6H12O6 etc.

Isotope:

Atoms having same atomic number but different atomic masses are called isotopes. e.g. Hydrogen has three isotopes protium 1H1, deuterium 1H2, tritium 1H3, Carbon has three isotopes

6C12, 6C13, 6C14, oxygen also has three isotopes 8O16, 8O17, 8O18 etc. More than 300 isotopes occur in nature they include 40 radioactive isotopes. Often, elements with even atomic number have more number of isotopes and are abundant.

Atomic Number (Z):   

“Number of protons in the nucleus of an atoms” or “Number of electrons in an atom” is called atomic number.

Number of electrons in an atom          = Z

Number of protons in an atom            = Z

Number of neutrons in an atom          = A – Z

Atomic masses:

Atomic masses are expressed in three ways:

(i)         Mass Number (A):       Sum of protons and neutrons in an atom is called its mass number or nucleon number.

                                    A = Protons + Neutrons

(ii)        Relative atomic mass (Ar):      The atomic mass of an atom as compared to mass of Carbon-12, is called relative atomic mass e.g. relative atomic mass of Hydrogen (protium i.e. 1H1) is 1.0078 amu.

(iii)       Average atomic mass (Ar):      The weight average of isotopic masses of an element is called average atomic mass of the element.   

e.g. (i)                          Relative isotopic mass             Relative abundance

            Carbon-12                   12.00000 amu                         98.89%

            Carbon-13                   13.00335 amu                         1.11%

Ions:

Charge carrying species is called ion. Ions bearing positive charge are called cations while ions bearing negative charge are called anions. Formation of cations is endothermic process while formation of anions is exothermic process.

Molecular ion:

When a molecule gains or loses electrons, the resulting species is called molecular ion e.g. CH4+, O2+, CO+, N2+ etc. Cationic molecular ions are more common than anionic molecular ions.

Chemical Formulae:

            The symbolic representation of a compound that shows elements in the compound and also the ratio of their atoms is called chemical formula. Chemical formulae are of three types:

(i)  Emperical formula or Simple formula:            

            The formula of a compound that shows elements in the compound and their simple atomic ratio is called empirical or simple formula.

(ii)  Molecular formula:      

            The formula of a compound that shows elements in the compound and actual number of their atoms is called molecular formula.

Formula unit:

            Simplest unit whose repetition builds crystal is called formula unit. Usually for ionic compounds their empirical formula is their formula unit while for covalent compound their molecular formula is their formula unit.

Mole and Avogadro’s Number (NA):

(1)        Mole like dozen is a number. As twelve things are called a dozen, 144 things are called a gross similarly:

“Avogadro’s number (6.02 x 1023) of things or particles is called a mole.”

e.g. (i)  6.02 x 1023 atoms of Hydrogen are called one mole or one gram-atom of Hydrogen.

e.g. (ii) 6.02 x 1023 molecules of Hydrogen are called one mole or one gram-molecule of Hydrogen.

e.g. (iii) 6.02 x 1023 electrons are called one mole of electrons.

Q. Vinegar is 5% acetic acid . This contains 2 carbon atoms ,  four hydrogen atoms and 2 oxygen atoms. Write its empirical and molecular formulas ?

Given:

Number of carbon atoms = 2

Number of hydrogen atoms = 4

Number of oxygen atoms = 2

Find out its empirical and molecular formulas

Solution:

Molecular formula      = C2H4O2          

 Empirical formula   =   CH2

Trick to Calculate Empirical Formula from Molecular formula is as follow:


Q. Caffeine ( C8H10 N4 O2) is found in tea and coffee . Write its empirical formula ?

Given:

molecular formula      = C8H10 N4O2 

Find out its empirical formula

Solution:

Empirical formula   =   C4H5 N2O


Q. Calculate the formula mass:

Formula mass of C3H6O

           = 12 x 3 + 1 x 6 + 16

           = 36 + 6 + 16

           = 58 amu


Formula mass of MgSO4

 = 24 + 32 + 16 x 4

 = 24 + 32 + 64

 = 120 amu

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