CBSE ADDA

Empirical formula  : The empirical formula is the simplest formula for a compound in which atoms of different elements are present in simple ratio. It shows the relative number of atoms of each element. For example CH 2 O is the empirical formula of Glucose C 6 H 12 O 6 Molecular formula   :  It is the formula in which   the actual number of atoms of  different element are present. For example, if the empirical formula of benzene is CH where as molecular formula  is C 6 H 6  , etc. An empirical formula is often calculated from elemental composition data. The weight percentage of each of the elements present in the compound is given by this elemental composition.  Let's determine the empirical formula for a compound with the following elemental composition:40.00% C, 6.66% H, 53.34% O. Element percentage Atomic mass Relative number of Atoms Dividing by least number Simple ratio C 40 12 40/12 = 3.33

  Ancient Greek philosophers – Democritus and Leucippus suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further. Democritus called these indivisible particles atoms (meaning indivisible). How and why elements combine and what happens when they combine.  Antoine L. Lavoisier  laid the foundation of chemical sciences by establishing  two important laws of chemical combination. (a)  The Law of conservation of mass was stated by Antoine L. Lavoisier as” Mass can neither be created nor destroyed in a chemical reaction” Verification of “Law of Conservation of mass”  A solution of sodium chloride and silver nitrate are taken separately in the two limbs  of an 'H' shaped tube. The tube is sealed and weighed precisely. The two reactants   are made to react by inverting the tube.   The following reaction takes place.  AgNO3(aq) + NaCl (aq) à AgCl (s) + NaNO3 (aq)  The whole tube is kept undisturbed for some

Page 47          1. What are canal rays? Ans: Canal rays are positively charged radiation s that can pass through perforated cathode plate. These rays consist of positively charged particles known as protons 2. If an atom contains one electron and one proton, will it carry any charge or not? Ans: An atom containing one electron and one proton will not carry any charge they make an atom neutral. Page 49 3. On the basis of Thomson’s model of an atom, explain how the atom is ne utral as a whole. Ans:  According to Thomson’s model of the atom, an atom consists of equal number of negatively and positively charged particles. The negatively charged particles are embedded in the positively charged sphere and balance their charges. Thus, they make an atom neutral. 4. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom? Ans: protons (positively-charged particles) 5. Draw a sketch of Bohr’s model of an atom w