Atoms and Molecules


Write the chemical formulae of the following. Also identify the ions present.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.


(a) Magnesium chloride: MgCl2 (ions present are: Mg2+, Cl1- )
(b) Calcium oxide: CaO (ions present are: Ca2+, O2- )
(c) Copper nitrate: Cu(NO3)2 (ions present are: Cu2+, NO31- )
(d) Aluminium chloride: AlCl3 (ions present are: Al3+, Cl1- )
(e) Calcium carbonate: CaCO3 (ions present are: Ca2+, CO32- )

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.

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

Q 3.

What is the numerical value of Avogadro number ?

Q 4.

Write the full form of IUPAC.

Q 5.

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

Q 6.

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

Q 7.

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 8.

What is meant by atomicity ? Explain with two

Q 9.

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 10.

Is there any exception to law of conservation of mass?

Q 11.

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 12.

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

Q 13.

What name is given to the number 6.022 x 1023

Q 14.

How many moles are 3.6 g of water ?

Q 15.

What is the mass of 0.2 mole of oxygen atoms ?

Q 16.

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 17.

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

Q 18.

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

Q 19.

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

Q 20.

Calculate the number of molecules in 4 g of oxygen.

Q 21.

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 22.

Explain with example that law of conservation of mass is valid for chemical reactions.

Q 23.

State Law of constant proportions. Explain with an example.

Q 24.

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

Q 25.

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

Q 26.

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

Q 27.

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

Q 28.

(a) What is meant by 'a mole of carbon atoms' ?
(b)(b) Which has more atoms, 50 g of aluminium or 50 g of iron ? Illustrate your answer with the help of calculations.
(Atomic masses : A1 = 27 u ; Fe = 56 u)

Q 29.

(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 30.

Fill in the blanks:
Atoms can be observed using ____________ Microscope.

Q 31.

Give four examples of diatomic molecules.

Q 32.

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

Q 33.

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

Q 34.

Name the elements water is made of. What are the valencies of these elements ? Work out the chemical formula for water.

Q 35.

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

Q 36.

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

Q 37.

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

Q 38.

What is the molecular mass of a substance?

Q 39.

What are the building blocks of matter ?

Q 40.

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.

Q 41.

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 42.

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

Q 43.

What are the postulates of Dalton's atomic theory?

Q 44.

Name the international organization who approves names of elements.

Q 45.

How will you define chemical symbol?

Q 46.

What is the atomic mass unit?

Q 47.

Fill in the blanks:
Hydrogen and oxygen combines to form H2O and H2O2. These two compounds obey the law of ____________________.

Q 48.

What are polyatomic ions? Give examples?

Q 49.

Write down the names of compounds represented by the following formulae:

(i) Al2(SO4)3
(ii) CaCl2
(iii) K2SO4
(iv) KNO3
(v) CaCO3

Q 50.

Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.