2. SA PART I

IMPORTANT SHORT ANSWERS PART ONE

1. BASIC CONCEPTS 

AND EXPERIMENTAL TECHNIQUES IN CHEMISTRY

Write short answers of the following questions. 

1. Define empirical and molecular formula. How are they inter-related?

Ans. Empirical Formula
     The formula which shows the simplest whole number ratio between the atoms of a molecule is called empirical formula. For example, empirical formula of benzene and glucose is CH and CH₂O respectively. 

Molecular Formula
     The formula which shows the actual or total number of atoms in a molecule is called molecular formula. For example, molecular formula of benzene is C₆H₆.
Relationship

     Empirical formula can be converted into molecular formula by using the following relationship;

Molecular formula = n × empirical formula. 

2. Give examples of the molecules possessing same empirical and molecular formula. 

Ans. Some examples of the compounds having same empirical and molecular formula are; water (H₂O), methane (CH₄), carbon tetrachloride (CCl₄), amonia (NH₃) and boron trifluoride (BF₃). 

3. Differentiate between qualitative and quantitative analysis.

Ans. Qualitative Analysis
     A type of analysis in which a chemist is associated with the quality of a substance is called qualitative analysis. For example, qualitative analysis of glucose means it contains carbon, hydrogen and oxygen. 

Quantitative Analysis
     A type of analysis in which a chemist is associated with the quantity of each element present in a compound is called quantitative analysis. For example, if we say that glucose contains 50% carbon, 30% hydrogen and 20% oxygen, then it is called quantitative analysis. 

4. What is meant by Avogadro's number? Give its relation to mass of element. 

Ans. Avogadro's Number
     The number of particles present in one mole of a substance is called Avogadro's number. It is represented by NA and its numerical value is 6.022 × 10²³ particles. 

Relation to Mass of Element
     Avogadro's number is equal to the molar mass of the substance. 

2.016g of H₂ = 1 mole of H₂ = 6.022 × 10²³ particles = Avogadro's number

Number of particles are related to mass of the element as follow;

Number of particles = mass of the element/molar mass of the element ×⁢ 6.022 × 10²³

5. Define limiting reactant. How does a limiting reactant control the amount of product formed?
Ans. Limiting Reactant
     Limiting reactant is the reactant which disappears first from the reaction mixture thus stopping the reaction and further product is not formed. So we can say that limiting reactant controls the amount of product formed.
Example

H₂ + O₂   2H₂O

     When two moles of hydrogen (4g) are reacted with 2 moles of oxygen (64g), they produce two moles of water (36g). From the equation, it is clear that 2 moles of hydrogen (4g) react with one mole of oxygen (64g) while one mole of oxygen (32g) is left behind in the reaction mixture. As hydrogen disappears first from the reaction so hydrogen is the limiting reactant and it controls the amount of product formed.
6. How will you identify the limiting reactant?
Ans. Identification of Limiting Reactant
     Limiting reactant is determined through the following steps;
(i) Calculate the number of moles of given amount of reactants.
(ii) Calculate the number of moles of given amount of products.
(iii) Identify the limiting reactant which produces less number of moles of products.
7.