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In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.
sodium carbonate + ethanoic acid sodium ethanoate + carbon dioxide + water

Mass of the reactants = Mass of sodium carbonate + Mass of ethanoic acid 

= 5.3+6.0=11.3 g

Mass of products = Mass of carbon dioxide + Mass of water + Mass of sodium ethonate

= 2.2 + 0.9 + 8.2 = 11.3 g

Thus, the total mass of the reactants is same as that of the products. Thus, the mass was neither created nor destroyed during the chemical reaction. Therefore, the mass was conserved during the reaction of sodium carbonate and ethanoic acid.


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?

X grams of hydrogen 8 x gram of oxygen will be required to form water.

Oxygen required to react with 3 g hydrogen to form water = 3 8 = 24 g.



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

Atoms are indivisible particles, which cannot be created nor destroyed in a chemical reaction.


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

The relative number and kinds of atoms are constant in a given compound.


Define the atomic mass unit.
The atomic mass unit (abbreviated as amu) is unit that is used to express the atomic masses of atoms and molecule at massed of compounds. One atomic mass unit is a mass unit equal to exactly one twelfth(1/12th) the mass of one atom of carbon-12. The atomic masses of all elements have been found relative to an atom of carbon-12.
1/12th of carbon atom is basically the one twelfth the mass of one atom of carbon-12 isotope.


 Why is not possible to see an atom with naked eye?

The size of an atom is very small. So it is impossible to see an atom with naked eye.


Write down the formulae of

(i) sodium oxide

(ii) aluminium chloride

(iii) sodium suphide

(iv) magnesium hydroxide

The formulae are:

 (i) sodium oxide- Na2O

aluminium chloride-AlCl3

(iii) sodium suphide-Na2SO3

(iv) Magnesium hydroxide - Mg(OH)2


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

(i) Al2(SO4)3

(ii) CaCl2

(iii) K2SO4

(iv) KNO3

(v) CaCO3.


The names of compounds are as follows:

(i) Al2(SO4)3 – aluminium sulphate

(ii) CaCl2 – Calcium chloride

(iii) K2SO4 – potassium sulphate

(iv) KNO3 – potassium nitrate

(v) CaCO3- calcium carbonate



 What is meant by the term chemical formula?

The chemical formula of a compound is a symbolic representation of its composition.
Chemical formulas such as HClO4 can be divided into empirical formula, molecular formula, and structural formula.
Chemical symbols of elements in the chemical formula represent the elements present, and subscript numbers represent mole proportions of the proceeding elements. Note that no subscript number means a subscript of 1.
From a chemical point of view, an element contained in the substance is a fundamental question, and we represent the elemental composition by a chemical formula, such as H2O for water. This formula implies that the water molecules consist of 2 hydrogen, and 1 oxygen atoms. The formula H2O is also the molecular formula of water. For non-molecular substances such as table salt, we represent the composition with an empirical formula. Sodium chloride is represented by NaCl, meaning that sodium and chlorine ratio in sodium chloride is 1 to 1. Again, the subscript 1 is omitted. Since table salt is an ionic compound, the formula implies that numbers of Na+ ions, and Cl- ions are the same in the solid. The subscript numbers in an empirical formula should have no common divisor.


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

Molecular mass of H2 = 1+ 1 = 2u

Molecular mass of O2 = 16 + 16 = 32 u

Molecular mass of Cl2 = 35.5 + 35.5 = 71 u

Molecular mass of CO2 = 12 + 32 = 44u

Molecular mass of CH4 = 12 + 1 4 = 16 u

Molecular mass of C2H4 = 12 2 + 1 4 = 28 u

Molecular mass of C2H6 = 12 2 + 1 6 = 30 u

Molecular mass of NH3 = 14 + 13 = 17u

Molecular mass of CH3OH. = 12 + 13 + 16 + 1 = 32 u


Calculate the formula unit masses of ZnO, Na2O, K2CO3,
given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

Formula unit mass of ZnO = 65 + 16 = 81 u

Formula unit mass of Na2O = 232 + 16 = 62 u

Formula unit mass of K2CO3 = 39 2 + 12 1 + 16 3 = 138 u.


 If one mole of carbon atom weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?

1 mole of carbon atom = 6.022 10 23 atoms

Now 6.022 10 23 atoms of carbon weigh 12 grams
one atom of carbon weighs = = 1.99 1023 g.


0.24 g sample of compound of oxygen and boron was found to contain 0.096 g of boron and 0.144 g of oxygen on analysis . Calculate the percentage composition of the compound by weight. 

% of boron in the sample = 100 = 40%

% of oxygen in the sample = 100 = 60%

The sample of compound contains 40% boron and 60% oxygen by weight.


When 3.0 g of carbon is burnt in 8.00 g of 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? 

The mass of the carbon dioxide formed will be only 11 grams. The remaining oxygen is not used up. This indicates the law of definite proportions which says that in compounds, the combining elements are present in definite proportions by mass.


What are polyatomic ions? Give examples.

When two or more atoms combine together and behave like one entity with a net charge, then it is called polyatomic ion. For example oxygen atom and hydrogen atom combine to form hydroxide ion (OH-). One carbon atom and three oxygen atoms act as carbonate ion (CO3-2).


Write the chemical formulae of the following.

(a) Magnesium chloride

(b) Calcium oxide

(c) Copper nitrate

(d) Aluminium chloride

(e) Calcium carbonate.

The chemical formulae for the following are:

(a) Magnesium chloride – MgCl

(b) Calcium oxide -CaO

(c) Copper nitrate –Cu(NO3)

(d) Aluminium chloride –AlCl3

(e) Calcium carbonate. –CaCO3


 Give the names of the elements present in the following compounds.

(a) Quick lime

(b) Hydrogen bromide

(c) Baking powder

(d) Potassium sulphate.

The names of the elements present in in the following compounds are:

(a) Quick lime – calcium and oxygen

(b) Hydrogen bromide-Hydrogen and bromide

(c) Baking powder –Sodium, hydrogen , carbon and oxygen

(d) Potassium sulphate- Potassium, sulphur and oxygen.



Calculate the molar mass of the following substances.

(a) Ethyne, C2H2

(b) Sulphur molecule, S8

(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)

(d) Hydrochloric acid, HCl

(e) Nitric acid, HNO3

The molar mass of the following substances are:

(a) Ethyne, C2H2 – 2 12+ 12 = 26 u

(b) Sulphur molecule, S8 – 8 32 = 256 u

(c) Phosphorus molecule, P4 - 4 31 = 124 u

(d) Hydrochloric acid, HCl - 11 + 135.5 = 36.5 u

(e) Nitric acid, HNO3 - 11 + 114 + 316 = 63 u


What is the mass of—

(a) 1 mole of nitrogen atom

(b) 4 moles of aluminium atoms (Atomic mass of aluminium
= 27)?

 (c) 10 moles of sodium sulphite (Na2SO3)?

a) 1 mole of nitrogen atom =14 u = 14 gm

(b) 4 moles of aluminium atoms = 4 27 = 108 u = 108 gram

(c) 10 moles of sodium sulphite = 10 (126 gram) = 1260 grams


Convert into mole.

(a) 12 g of oxygen gas

(b) 20 g of water

(c) 22 g of carbon dioxide.

a)32 gram of oxygen gas    = 1 mole

12 grams of oxygen gas          = = 0.375 mole

b) 18 grams of water              = 1 mole

20 grams of water                  = = 1.1 mole

c) 44 grams of carbon dioxide  = 1 mole

22 grams of carbon dioxide      = 0.5 mole


What is the mass of:
(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?

1 mole of oxygen atoms = 16 grams

0.2 moles of oxygen atoms = 16 0.2 = 3.2 grams

b. 1 mole of water molecules = 18 grams

0.5 mole of water molecules = 18 0.5 = 9 grams


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

1 mole of S8 = 32 8 = 256 grams

1 mole of S8 = 6.023 1023 molecules

256 grams S8 has = 6.023 10

16 grams S 8 has = 16

= 3.76 1022 molecules.


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)

Aluminium oxide is made up of alminium ions and oxide ions.
First we have to find out the chemical formula of aluminium oxide.
from the chemical formula Al2O3 we can say mass of Al2O3 in grams.
Molecular mass of Al2O3 is equal to molecular mass of Al2O3 in grams.
Molecular mass of Al2O3 = (2* atomin mass of Al in grams + 3* atomic mass if O in grams)
=2* 27.0+3*16.0=102.0 grams
102.0 g of Al2O3 has 1 mole of Al2O3
0.051 g of Al2O3 ‎will have 1/102 * 0.051 = 0.0005 mole of Al2O3
1mole of Al2O3 has moles of Al atoms or ions
0.0005 moles of Al2O3 will have 0.001 moles of aluminium atoms or ions
1mole of aluminium ions contain 6.022*1023
0.001 mole of aluminium will contain 6.022*1023 * 0.001= 6.022*1020 ions of aluminium.

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