Symbol,Formula

SYMBOLS,FORMULAS AND EQUATIONS

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1. Who was the first scientist to use the symbols for elements? Ans: Dalton was the first scientist to use the symbols for elements, such as circle (O) for an oxygen atom. 2. Explain How Berzelius suggested the modern symbols of elements? Ans: Berzelius suggested that the first letter of the name of the element in capital should represent a particular element as “O” for oxygen , “H” for hydrogen and so on. 3. What is meant by the symbol of an element? Explain with example. OR What is the significance of the symbol of an element. Ans: A symbol of an element is not only an abbreviation. It also denotes one atom of that element as well as the mass of an atom. Example: The symbole “ Na” represents (a) The element sodium (b) an atom of sodium (c) the mass of an atom of sodium 4. What do you mean by a chemical formula? Give example. Ans: A chemical formula is a symbolic representation of the composition of the molecule. Example:

H 2 O , NaCl etc

5. What is mean by the chemical formula

O2

?

Ans: It shows that the molecule contains two atoms of oxygen in combination. 6. What is meant by the valency of an element? Ans: The combining capacity of an atom of an element to form a molecule is called valency. 7. What do you mean by ions ? Ans: An atom or group of atoms having positive or negative charge is known as ions. Prepared by : Ph.Gokul Sharma (8974434474) M.Sc (Physics , 1st Class ) , B.Ed ( 1st Class)

SYMBOLS,FORMULAS AND EQUATIONS

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8. What are cations and anions?

Ans: The positively charged ions are called cations and negatively charged ions are called anions. Example: The compound sodium chloride consists of positively charged sodium ions (Na+) and Negatively charged chloride (Cl-) ions 9. What is the difference between a cation and an anion? Give examples. Ans: The positively charged ions are called cations and negatively charged ions are called anions. Example: The compound sodium chloride consists of positively charged sodium ions (Na+) and Negatively charged chloride (Cl-) ions

10.

What is meant by polyatomic ion?

Ans: A group of atoms carrying a charge is known as a polyatomic ion. Example: 11.

NO−¿ 3 ¿

Write the rules for writing chemical formulae?

Ans: The general rules for writing chemical formulae are as follows: (a) The valencies of the elements (atoms) or the changes of the ions must balance. (b)When a compound consists of a metal and a non-metal the name or symbol of the metal is written first. (c) In compound formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio. 12. An element ‘X’ forms an oxide with formula X2O3 .State the valency of X. Ans: 13.

Valency of X is 3 and the valency of O is 2. What is molecular mass of a substance ?

Ans: The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. Prepared by : Ph.Gokul Sharma (8974434474) M.Sc (Physics , 1st Class ) , B.Ed ( 1st Class)

What is formula unit?

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SYMBOLS,FORMULAS AND EQUATIONS

Ans: The formula unit mass is the sum of the atomic masses of all atoms in a formula unit of a compound. 15. What is the difference between molecular mass and formula unit? Ans: The only difference between the formula unit mass and molecular mass is that we use the word formula unit for those substances whose constituent particles are ions. 16.

What do you mean by Gram Molecular Mass?

Ans: The molecular mass of a substance expressed in grams is called the gram molecular mass of the substance. Example: The molecular mass of water (H2O) is 18u. The gram molecular mass of water is 18g. 17.

What is meant by Avogadro’s number?

Ans: The number 18.

6.022× 1023

is called Avogadro’s number.

What do you mean by a mole?

Ans: A mole of any substance contains the number of elementary particles equal to Avogadro’s constant. 19. What is mole so significant? Or what is the advantage of mole? Ans: Besides being related to a number, the mass of 1 mole of a particular substance is also fixed. 20.

Explain the difference between 2 N and N2 .

Ans: 2 N mean two atoms of Nitrogen and N2 means a molecule of nitrogen. 21. Write the relationships between mole,Avogadro number and mass. Ans: Prepared by : Ph.Gokul Sharma (8974434474) M.Sc (Physics , 1st Class ) , B.Ed ( 1st Class)

SYMBOLS,FORMULAS AND EQUATIONS

2. 1 mole of water 3. 1 mole of any particle 4. 1 mole of molecules

22.

Mass 12 g Carbon

6.022× 1023

18 g water

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Avogadro number 23 6.022× 10 atoms of C

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Mole 1. 1 mole carbon atoms

of H2O 6.022× 1023

molecules number

of that particle. 6.022× 1023

number of

molecules

Relative mass of those particles in grams Molecular mass in grams

Write the information conveys by a chemical formula .

Ans: (a) Qualitatively , it represents:

(b)

23. CO2

(i) The name of the compound. (ii) The elements present in the compound. Quantitatively , it represents: (i) One molecule of the substance. (ii) The number of atoms of various elements which constitute one molecule. (iii) The molecular mass of the substance. (iv) The number of parts by mass of the elements present in the molecule. (v) One mole of the substance. What information is conveyed by the chemical formula

Ans: (a) Qualitatively , it represents: (iii) (iv) (c)

Carbon dioxide. Carbon dioxide is made up of carbon and oxygen elements. Quantitatively , it represents: (vi) One molecule of carbon dioxide. (vii) One molecule of carbon dioxide consist of one atom of carbon and two atoms of oxygen. (viii) The molecular mass of carbon dioxide is 12+16 x 2 = 44u. (ix) 44 parts by mass of carbon dioxide contains 12 parts by , mass of carbon and 32 parts by mass of oxygen.

Prepared by : Ph.Gokul Sharma (8974434474) M.Sc (Physics , 1st Class ) , B.Ed ( 1st Class)

SYMBOLS,FORMULAS AND EQUATIONS

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One mole of carbon dioxide i.e. 6.022 x 1023 molecules of CO2. 24. What is meant by chemical equation? Explain with examples. (x)

Ans: A chemical equation is the systematic representation of an actual chemical reaction with the help of symbols and formulas of respective reactants and products For example, the reaction of zinc with dilute hydrochloric acid to give zinc chloride and hydrogen is written as : Zn + 2HCl → ZnCl2 + H2 25.

Is it necessary to obtain a balanced chemical equation?

Why? Ans. According to the law of conservation of mass the total mass of reactants and products remains the same during a chemical reaction. Hence, the number of atoms of various elements on both the side are made equal i. e. equation is balanced. 26. What information does a balanced chemical equation convey? Ans: A chemical equation gives the following two types of information: 1. Qualitative Information: A chemical equation provides the following qualitative information about the reaction. It tells us the , (i) Names of the reactants which take part in the reaction (ii) Names of the products formed in the reaction. 2. Quantitative information: A chemical equation gives the following quantitative information. It tells us about , (i) The numbers of molecules or atoms of reactants and products taking part in the reaction. (ii) The number of moles of each substance involved in the reaction. (iii) The mass of each substance involved in the reaction. (iv) Mass-mass, mass-volume, volume-volume relationship between the reactants and products. 27. Write the information conveyed by the following chemical equation : Prepared by : Ph.Gokul Sharma (8974434474) M.Sc (Physics , 1st Class ) , B.Ed ( 1st Class)

Ans:

We have Mg+2 HCl → Mg Cl 2 +H 2

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Mg+2 HCl → Mg Cl 2 +H 2

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SYMBOLS,FORMULAS AND EQUATIONS

24 2 ( 1+ 35.5 )=73 24+ (2 ×35.5 )=95 2 (a) Qualitatively , (i) It shows that magnesium reacts with hydrogen chloride to produce magnesium chloride and hydrogen. (b) Quantitavely , (i) Two molecules of hydrogen chloride react with on atom of magnesium to produce one molecule of magnesium chloride and one molecule of hydrogen. (ii) 73 parts mass of hydrogen chloride react with 24 parts by mass of magnesium to produce 95 parts by mass of magnesium chloride and 2 parts by mass of hydrogen. (iii) Two moles of hydrogen chloride react with one mole of magnesium to produce one mole of magnesium chloride and one of hydrogen.

Prepared by : Ph.Gokul Sharma (8974434474) M.Sc (Physics , 1st Class ) , B.Ed ( 1st Class)