Some Basic Concepts of Chemistry

Question 1

Law of Conservation of Mass

Answer 1

his law was put forward by Antonie Lavoisier. It states that matter can neither be created nor be destroyed. During a chemical reaction sum of the masses of reactants is equal to the sum of masses of products.

Question 2

Law of Definite Proportion

Answer 2

This law was given by Joseph Proust It states that a given compound always contains the same proportion of elements by weight irrespective of the source.

Question 3

Law of Multiple Proportions

Answer 3

This law was put forward by Dalton .
It states that if two elements
combine to form more than one
compound , the masses of one
element that combine with a
fixed mass of other element ,
are in the ratio of small whole numbers.

Question 4

Gay Lussac’s Law of Gaseous Volumes

Answer 4

It states that when gases combine in a chemical reaction they do so in a simple ratio by volume if all gases are at same temperature and pressure .

Question 5

Avogadro’s Law

Answer 5

It states that equal volumes of all gases at same temperature and pressure should contain equal number of molecules.

Question 6

Dalton’s Atomic Theory

Answer 6

1. Matter is made up of indivisible atoms.
2. All the atoms of a given element have identical properties including identical mass. Atoms of different elements differ in mass.
3. Compounds are formed when atoms of different elements combine in a fixed ratio.
4. Chemical reactions involve reorganization of atoms. These are neither created nor destroyed in chemical reactions.

Question 7

Atomic Mass Unit

Answer 7

It is defined as one twelfth the mass of one atom C-12 atom.

Question 8

Average Atomic Mass

Answer 8

When we use atomic masses of elements in calculations, we actually use average atomic masses of elements. Many naturally occurring elements exist as more than one isotope. When we take into account the existence of these isotopes and their relative abundance (percent occurrence), the average atomic mass of that element can be computed.

Question 9

Molecular Mass

Answer 9

Molecular mass is the sum of atomic masses of the elements present in a molecule.

Question 10

Formula Mass

Answer 10

Formula mass of an ionic compound is obtained by adding atomic masses of all atoms present in one formula unit of a compound.

Question 11

Mole Concept

Answer 11

One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g (or 0.012 kg) of the 12C isotope. This number has been calculated to be 6.022×1023. This number of entities is known as ‘Avogadro’ constant.

Question 12

Molar Mass

Answer 12

The mass of one mole of a substance in grams is called its molar mass.

The molar mass in grams is numerically equal to atomic/molecular/formula mass in u.

Question 13

Percentage Composition

Answer 13

The percentage of any element in a compound is the number of parts by weight of the element in 100 parts by weight of compounds.

Question 14

Mass % of an element

Answer 14

mass of that element x 100
________________________________
total mass of the compound

Question 15

Empirical Formula

Answer 15

An empirical formula represents the simplest whole number ratio of various atoms present in a compound.

Question 16

Molecular Formula

Answer 16

The molecular formula shows the exact number of different types of atoms present in a molecule of a compound.

Question 17

Relationship between MF and EF

Answer 17

MF = n × EF

Question 17

Relationship between MF and EF

Answer 17

MF = n × EF

Question 18

Limiting Reagent

Answer 18

The reactant which is completely consumed during a chemical reaction and limits the amount of product formed is called the limiting reagent. In other words, the compound that gets used up first.

Question 19

Excess Reagent

Answer 19

The reactant which is not completely consumed during a chemical reaction and does not limits the amount of product formed is called the excess reagent.

Question 20

Mass %

Answer 20

Mass of the solute x 100
___________________________
total mass of the solution

Question 21

Mole Fraction

Answer 21

It is the ratio of number of moles of a particular component to the total number of moles of the solution.

Question 22

Mole Fraction Formula

Answer 22

No. of moles of A
___________________
Total no. of moles

Question 23

Molarity

Answer 23

It is denoted by M. It is defined as the number of moles of the solute present in 1 litre of the solution. The molarity of a solution depends upon temperature because volume of a solution is temperature dependent.

Question 24

Molarity Formula

Answer 24

No. of moles of solute
_____________________________
Volume of solution in litres

Question 25

Molality

Answer 25

It is defined as the number of moles of solute present in 1 kg of solvent. It is denoted by m. The molality of a solution does not change with temperature since mass remains unaffected with temperature.

Question 26

Molality Formula

Answer 26

No. of moles of solute
________________________
mass of solvent in kg

Question 27

Homogenous Mixture

Answer 27

In this the components completely mix with each other and its composition is uniform throughout.
Example: Sugar solution , air.

Question 28

Heterogenous Mixture

Answer 28

In this, the composition is not uniform throughout and sometimes the different components can be observed.
Example :the mixtures of salt and sugar, grains and pulses along with some stone pieces

Question 29

Element

Answer 29

An element consists of only one type of particles. These particles may be atoms or molecules .
Eg -Sodium, copper, silver, hydrogen, oxygen etc.