Chemistry

Atoms and Molecules

Question:

What are the postulates of Dalton's atomic theory?

Answer:

Dalton's theory states that matter, whether an element, a compound or a mixture is composed of small particles called atoms.
The postulates are:
  1. All matter is made of very tiny particles called atoms.
  2. Atoms are indivisible particles i.e. can neither be created or destroyed in a chemical reaction. (Law of Conservation of mass)
  3. Atoms of a give element have same mass and identical chemical properties.
  4. Atoms of different elements have different masses and chemical properties.
  5. Atoms combine in the ratio of small whole numbers to form compounds. A chemical reaction is a re-arrangement of atoms. (Law of Multiple Proportions)
  6. The relative number and kinds of atoms are constant in a given compound. (Law of Definite Proportions)
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Atoms and Molecules

Q 1.

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

Q 2.

Write the full form of IUPAC.

Q 3.

What name is given to the number 6.022 x 1023

Q 4.

How many moles are 3.6 g of water ?

Q 5.

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 6.

What is the numerical value of Avogadro number ?

Q 7.

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

Q 8.

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

Q 9.

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

Q 10.

What is the mass of 0.2 mole of oxygen atoms ?

Q 11.

Is there any exception to law of conservation of mass?

Q 12.

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

Q 13.

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

Q 14.

The molecular formula of glucose is C6H12O6. Calculate its molecular mass. (Atomic masses : C = 12 u ; H =1 u ; O = 16 u)

Q 15.

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

Q 16.

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 17.

If 16 g of oxygen contains 1 mole of oxygen atoms, calculate the mass of one atom of oxygen.

Q 18.

Calculate the number of molecules in 4 g of oxygen.

Q 19.

Fill in the blanks:
The chemical symbol of mercury is _________.

Q 20.

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

Q 21.

What is meant by atomicity ? Explain with two

Q 22.

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 23.

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 24.

Calculate the mass in grams of 0.17 mole of hydrogen sulphide, H2S.

Q 25.

(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 26.

Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Q 27.

Fill in the blanks:
The _______________ of a compound is a symbolic representation of its composition.

Q 28.

Give four examples of diatomic molecules.

Q 29.

How many atoms are present in a
(i) H2S molecule and
(ii) PO43- ion?

Q 30.

Who introduced the term 'mole' in chemistry?

Q 31.

Define 'molecular mass' of a substance.

Q 32.

Calculate the molecular masses of the following :
(a) Hydrogen, H2 (b) Oxygen O2 © Chlorine Cl2 (d) Ammonia NH3 (e) Carbon dioxide, CO2
(Atomic masses : H=1 U;O=16 U; Cl=35.5 U;N=14 U;C=12 U)

Q 33.

Work out the formulae for the following compounds :
(a) Sodium oxide

Q 34.

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 35.

What are (ionic compounds, and (ii) molecular compounds ? Give two examples of each type of compounds.

Q 36.

What name is given to the amount of substance containing 6.022 x 1023particles (atoms, molecules or ions) ofa substance?

Q 37.

How many moles are there in 34.5 g of sodium ? (Atomic mass of Na = 23 u)

Q 38.

Show by means of calculations that 5 moles of CO, and 5 moles of H2O do not have the same mass. How much is the difference in their masses ?

Q 39.

If 12 g of carbon is burnt in the presence of 32 g of oxygen, how much carbon dioxide will be formed?

Q 40.

When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?

Q 41.

Fill in the blanks:
___________ is the combining capacity of an element.

Q 42.

Name the international organization who approves names of elements.

Q 43.

Write the chemical symbols of the following:
(i) Gold
(ii) Iron
(iii) Chlorine
(iv) Mercury

Q 44.

Can atoms of an element exist independently? Give examples of elements which exist in atomic form. Give examples of elements that do not exist in atomic form.

Q 45.

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

Q 46.

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

Q 47.

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

Q 48.

Fill in the blanks:
____________ is formed by the loss of one or more electrons by an atom.

Q 49.

How do we know the presence of atoms if they do not exist independently for most of the elements?

Q 50.

What is gram-molecular mass of a substance?