atoms and molecules class9 che t2

7/28/2019 Atoms and Molecules Class9 Che t2

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BAL BHARATI PUBLIC SCHOOL,PITAMPURA

CHEMISTRY NOTES(2013-2014)

CLASS9_CH: ATOMS&MOLECULES_(TERM2)

Q1 State law of conservation of mass.Ans The mass can neither be created nor destroyed in a chemical reaction.

Q2 State law of constant propotion.

Ans A pure chemical compound always consists of the same elements that are combinedtogether in a fixed proportion by mass.

Q3 Write name and symbol of first 20 elements of the periodic table.Ans Atomic number Name of the elementSymbol1 Hydrogen H2 Helium He3 Lithium Li4 Beryllium Be

5 Boron B6 carbon C7 Nitrogen N8 Oxygen O9 Flurine F10 Neon Ne11 Sodium Na12 Magnesium Mg13 Aluminium Al14 Silicon Si15 Phosphorus P16 Sulphur S

17 Chlorine Cl18 Argon Ar19 Potassium K20 Calcium Ca


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Q4 Define atomic mass unit.Ans The mass of one twelfth (1/12) of the mass of one atom of carbon taken as 12.

Q5 What are molecules?Ans Molecules represents a group of two or more atoms(same or different) chemicallybonded to each other and held tightly by strong attractive forces.

Molecules are of two types(a) Molecules of elements

(b) Molecules of compound

Q6 What do you mean by molecular mass?Ans The average relative mass of the molecule as compared to the mass of carbon takrnas 1u.Q7 Write the relationship between number of moles and atomic mass.

Ans Number of moles=givenmass/gram atomic mass.

Q8 Why are chemical reactions according to law of conservation of mass?Ans In all chemical reactions, there is only exchange of reactant taking place when productsare formed tants Since there is no loss or gain of mass,the chemical reactions are,thechemical reactions are according to law of conservation of mass.

Q9. What is the difference between the mass of molecule and molecular mass?Ans Mass of a molecule is that of a single molecule also known as its actual mass. Butmolecular mass is the mass of Avagadros number of molecules.

Q12 Where do we use the words mole and mol?Ans In the text part we use the word mole while as a unit ,we call it mol.

Q13 How many moles are present in 11.5 g of sodium?Ans Gram atomic mass of Na =23g23g of Na represents = 1 mol11.5g of Na represents= (1mol)*(11.5g)/23.5g)=0.5 mol

Q14 Explain why the number of atoms in one mole of hydrogen gas is double thenumber of atoms in one mole of helium gas?Ans Hydrogen gas is a diatomic in nature(H2)

while helium gas is monoatomic (He).As aresult, the number of atoms in one mole of hydrogen(2*NA)

are expected to be double as compared to number of atoms in one mole of helium

Q15An element Z forms an oxide with formula Z2O3


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What is its valency?Ans Valency of Z=3

Q16 The valency of an element A is 4 . Write the formula of it oxideAns The formula of its oxide is A2O4 or AO2.Q17 An element X has valency 3 while the element Y has valency 2.Write the formulaof the compound between XandY .

.Ans X2Y3

Q18 What are ions?

Ans Ions are of two types(a)Cation- positively charged

(b)Anion-negatively charged

Q19 Which postulate of Daltons Atomic theory is the basis of law of conservation of mass?

Ans Atoms can neither be created nor destroyed during a physical or a chemicalchangeQ20 Write the formulae of sodium oxide and aluminium chlorideAns Sodium oxide = Na2O

Aluminium chloride =AlCl3

Q21 Find out the ratio by mass of the combining elements in the following compounds(a) MgCO3

(b) CH3OH

(c) CaCl2

Ans21(a)MgCO3

Mg :C:O = 24:12:48 = 2:1:4

(b)CH3OHC : H : O =12:4:16 =3:1:4(c) CaCl2

Ca :Cl=40:71:

Q22 Define atomicity of the molecule.

Ans Atomicity of the molecule is the number of atoms present in the molecule.Q23 What is the atomicity of oxygen,ozone,neon and sulphur?


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Ans Oxygen O2=2

Ozone O3=3

Neon Ne =1Sulphur S8=8

atoms and molecules class9 che t2

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