Chemistry

Atoms and Molecules

Question:

What is relative atomic mass of an element? How it is related to atomic mass unit?

Answer:

The relative atomic mass (Ar) of an element is the average relative mass of an element of the element with respect to an atom of Carbon-12 taken as 12u.

                                      Average mass of 1 atom of an element 
Relative Atomic mass of an element = -------------------------------------- 
                                      1/12th of Mass of one C-12 atom

The relative atomic mass is a pure number and has no units. It indicates number of times one atom of an element is heavier than 1/12th of C-12 atom.

Relative atomic mass is also called average atomic mass.
Atomic mass is expressed in amu or u while relative atomic mass is a number.

e.g. Atomic mass of one Hydrogen atom is 1u and its relative atomic mass is 1.
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Atoms and Molecules

Q 1.

How many moles are there in 4.6 gms of Sodium(Na)?

Q 2.

What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?

Q 3.

What is the numerical value of Avogadro number ?

Q 4.

Fill in the blanks:
According to law of definite proportions, in a chemical substance the elements are always present in __________ proportions by mass.

Q 5.

Fill in the blanks:
According to Dalton's atomic theory, atoms of different elements differ in ______, size and chemical properties.

Q 6.

Write the full form of IUPAC.

Q 7.

Name the unit in which the radius of an atom is usually expressed.

Q 8.

Calculate the molecular masses of the following compounds :
(a) Methanol, CH3OH
(b) Ethanol, C2H5OH

Q 9.

Calculate the molecular masses of the following compounds :
(a) Hydrogen sulphide, H2S
(b) Carbon disulphide, CS2
(Atomic masses :H = lu;S = 32u;C = 12u)

Q 10.

An element Z has a valency of 3. What is the formula of oxide of Z ?

Q 11.

Name the following compounds. Also write the symbols/formulae of the ions present in them :
(a) CuSO4
(b) (NH4)2SO4
(c)Na2O
(d)Na2CO3
(e)CaCl2

Q 12.

How many moles are 3.6 g of water ?

Q 13.

How many moles of calcium carbonate (CaCO3) are are present in 10 g of the substance ? (Ca = 40 u • C= 12 u-O = 16 u)

Q 14.

If one mole of nitrogen molecules weighs 28 g, calculate mass of one molecule of nitrogen in grams.

Q 15.

Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Q 16.

Give four examples of diatomic molecules.

Q 17.

An element X has a valency of 4 whereas another element Y has a valency of 1. What will be the formula of the compound formed between X and Y ?

Q 18.

What is an ion ? How is an ion formed ? Explain with the help of two examples of different ions.

Q 19.

(a) Define gram molecular mass of a substance. How much is the gram molecular mass of oxygen ?
(b) If sulphur exists as S8 molecules, calculate the number of moles in 100 g of sulphur.. (S = 32 u)

Q 20.

Is there any exception to law of conservation of mass?

Q 21.

What is relative atomic mass of an element? How it is related to atomic mass unit?

Q 22.

Give examples of triatomic molecules.

Q 23.

Name one element each which forms diatomic and tetra atomic molecule.

Q 24.

Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.

Q 25.

What is meant by atomicity ? Explain with two

Q 26.

Calculate the molecular mass of nitric acid,HNO3.
(Atomic masses :H =lu; N = 14u;0 = 16u)

Q 27.

What do we call those particles which are formed :
(a)by the gain of electrons by atoms ?
(b)by the loss of electrons by atoms ?

Q 28.

What is the difference between a cation and an anion ? Explain with examples. Using this information, write down the formulae of:
(i) Sodium sulphide
(ii) Copper nitrate

Q 29.

Fill in the following blanks :
(a)1 mole Contains...........atoms- molecules or ions of a substance
(b)A mole represents an..........number of Particles of a substance.
(c)60 g of carbon element are...........moles of carbon atoms.
(d) 0.5 mole of calcium element has a mass of..........
(e) 64 g of oxygen gas contains..........moles of oxygen atoms.

Q 30.

Calculate the number of molecules in 4 g of oxygen.

Q 31.

(a) Define gram atomic mass of a substance.How much is the gram atomic mass of oxygen ?
(b) How many moles of oxygen atoms are present in one mole of the following compounds ?
(i)Al2O3(ii) co2(iii) C12O7 (iv) H2SO4(p)A12(S04)3

Q 32.

What is the law of conservation of mass?

Q 33.

State Law of constant proportions. Explain with an example.

Q 34.

Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Q 35.

Name the international organization who approves names of elements.

Q 36.

Which element has the smallest atom in size?

Q 37.

Fill in the blanks:
The atomicity of sulphur S8 is ________.

Q 38.

Fill in the blanks:
One mole of water contains ______________ molecules.

Q 39.

Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.

Q 40.

Name one element which forms diatomic and triatomic molecule.

Q 41.

Define mole. What is its significance?

Q 42.

What is molar mass?

Q 43.

Find out number of atoms in 15 moles of He.

Q 44.

Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)

Q 45.

Calculate the mass percentage of Carbon(C) , Hydrogen (H) and Oxygen (O) in one molecule of glucouse (C6H12O6).
(Atomic mass of C = 12u, H = 1u and O = 16u)

Q 46.

What are the building blocks of matter ?

Q 47.

Dalton's atomic theory says that atoms are indivisible. Is this statement still valid ? Give reasons for answer.

Q 48.

Calculate the molecular mass of ethanoic acid, CH3COOH.
(Atomic masses :C = 12u;H = lu;0 = 16u)

Q 49.

Calculate the molecular mass of chloroform (CHC13).
(Atomic masses :C= 12u;H = lu;Cl = 35.5u)

Q 50.

State whether the following statements are true or false :
(a)A sodium ion has positive charge because it has more protons than a neutral atom
(b)A chloride ion has negative charge because it has more electrons than a neutral atom.