What do you understand by significant figures? Please answer this question urgently. Please. Please. Please.

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Significant figures are meaningful digits that are known with certainty. The uncertainty in experimental or the calculated value is indicated by mentioning the number of significant figures.

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Postulates of Dalton’s atomic theory –

1. Matter consists of indivisible atoms.

2. All the atoms of a given element have identical properties including atomic mass. Atoms of different element differ in mass.

3. Compounds are formed when atoms of different elements combine in a fixed ratio.

4. Chemical reaction involves reorganization of atoms. These are neither created nor destroyed

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Give one example each of a molecule in which empirical formula

and molecular formula are (i) same (ii) Different.

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(i) Same molecular formula and empirical formula. Carbon dioxide, both is CO2.

(ii) When molecular formula and empirical formula are different –

Hydrogen peroxide: molecular formula is H2O2 and empirical formula is HO

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The cathode ray discharge tube experiment performed by J.J. Thomson led to the discovery of negatively charged particles called electron.

A cathode ray tube consists of two thin pieces of metals called electrodes sealed inside a glass tube with sealed ends. The glass tube is attached to a vacuum pump and the pressure inside the tube is reduced to 0.01mm. When fairly high voltage (10, 000V) is applied across the electrodes, invisible rays are emitted from the cathode called cathode rays. Analysis of this rays led to the discovery electrons.

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The canal ray experiment led to the discovery of –

(i)The anode rays, travel in straight line

(ii)They are positively charged as they get deflected towards the –ve end when subjected to an electric and magnetic field.

(iii)They depend upon the nature of gas present in the cathode tube.

(iv)The charge to mass ration (e/m) of the particle is found to depend on the gas from which they originate.

(v)They are also material particles

The analysis of these proportions led to the discovery of positively charged proton.

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J.J. Thomson proposed that an atom consists of a spherical sphere (radius of about 10-10m)in which the positive charges are uniformly distributed the electrons are embedded into it in such a manner so as to give stable electrostatic arrangement.

This model is also called raisin pudding model.

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