Atoms And Molecules Chapter 3 Class 9 Science Worksheets

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Students should practice questions given in Atoms And Molecules Chapter 3 Class 9 Science Worksheets. These worksheets for Class 9 Science have a good collection of important questions and answers which are expected to come in your class tests and examinations. You should learn these solved worksheet questions for Science Class 9 as it will help you to understand all topics and give you more marks.

Class 9 Science Worksheets Chapter 3 Atoms And Molecules

Please refer to below questions and answers for Atoms And Molecules Chapter 3 Class 9 Science Worksheets. Prepared by expert teachers for Standard 9 Science

Question. Write the value of charge of electron.

Question. Who stated the Law of Constant Proportion

Question. What is meant by the term chemical formula?

Question.Name the instrument which produces image of the surface of element that shows atoms

Question. Write the symbols of tungsten and iron.

Question.Give the derivation source of symbol of sodium (Na).

Question. What is the atomicity of argon?

Question. What is the latest short form of atomic mass unit?

Question. Name any two monatomic atoms.

Question. Give difference between 2H and H₂.

Question.Give two examples of triatomic molecules.

Question. Define valency.

Question. Which organisation approves the names of elements all over the world?

Question. Who introduced the word ‘Mole’?

Question. What is Avogadro Constant?

Question. How does an atom exist?

Question. Name the element which is used as the reference for atomic mass.

Question. Give Latin name of Silver.

Question. What is the symbol of the element of molybdenum?

Question. Give an example in each of the following cases :
(i) a divalent anion
(ii) a trivalent cation
(iii) a mono-valent anion.

Question. What is ion?

Question. What do you mean by symbols of elements?

Question. Who was the first scientist to give the concept of formation of compounds?

Question. Give the symbol of copper, silver, gold, oxygen, zinc.

Question. What is the difference between an atom and molecule?

Question. Name the two laws of chemical combination.

Question. Name two elements which have same atomic number.

Question. An element has 8 electrons in its valence shell. What is its general name?

Question. What is meant by a chemical formula? Give examples.
Answer. A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element, e.g., chemical formula of ammonia is NH₃, water is H₂O and carbon dioxide is CO₂.

Question. Calculate the ratio of the numbers of atoms for magnesium sulphide.
Answer.
Atomic mass of Mg 3/24 = 1/8
Atomic mass of S 4/32 = 1/8
Ratio of atoms in MgS  =1/8 : 1/8  or 1 : 1

Question. Why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas? Explain.
Answer. The number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas because hydrogen molecule is diatomic, i.e., a molecule of hydrogen consists of two atoms of hydrogen, whereas helium is monatomic.

Question. Give the chemical name, chemical formulae for the following : Washing soda, blue vitriol, baking soda, green vitriol, oil of vitriol, soda ash, marble chips,lime water.
Answer.

Question. Give one example each of (i) Monovalent cation, (ii) Bivalent cation, (iii) Monovalent anion, (iv) Bivalent anion.
Answer.
(i) K⁺ or Na⁺
(ii) Mg⁺² or Ca⁺²
(iii) F^– or Cl^–
(iv) O^2– or S^2–

Question. What is Avogadro number?
Answer. The number 6.022 × 10²³ is referred to as Avogadro number and is denoted by symbol NA.

Question. What are ionic compounds?
Answer. Ionic compounds are charged particles. Such compounds form by joining or losing or sharing the electron. For example : Sodium chloride is an ionic compound. Its constituent particles are positively charged sodium ion (Na+) and negatively charged chloride ion (Cl–).

Question. State two examples in each case and write their chemical formulae :
(a) Molecules having same kind of atoms only.
(b) Molecules having two different kinds of atoms.
(c) Molecules having three different kinds of atoms.
Answer.
(a) F₂, Cl₂, P₄, S₈
(b) Ammonia (NH₃), Sulphur dioxide (SO₂), Carbon disulphide (CS₂).
(c) Calcium sulphate (CaSO₄), Sodium nitrate (NaNO₃).

Question. What is the difference between hydrogen chloride and nitrogen molecule formation?
Answer. Hydrogen chloride is molecular compound and formed by the union of different kinds of atoms while nitrogen is diatomic molecule and formed by union of two atoms of same kinds.

Question. If the valency of an element X is 3, then write the chemical formula of its oxide.
Answer.

Question. Write the meaning of these formulae :
(i) 2O,
(ii) O₂,
(iii) O₃
Answer.
(i) 2O = Two atoms of oxygen
(ii) O₂ = One molecule of oxygen
(iii) O₃ = One molecule of ozone

Question. Write the name of the compounds : NaBr, Al₂O₃, CaCO₃.
Answer.
NaBr = Sodium bromide
Al₂O₃ = Aluminium oxide
C_aCO₃ = Calcium carbonate

Question. Out of these Na⁺, K⁺, Al³⁺ and O^2–, which is isoelectronic?
Answer.
Iso-electronic means species having same number of electrons.

Na⁺, K⁺, Al³⁺ and O^2– have 10 electrons each Hence, they are iso-electronic.

Question. Find the number of atoms in the 0.5 mole of C atom.
Answer.
0.5 mole of C atom :
Number of atoms in 1 mole of C atom
= 6.022 × 10²³ atoms
Number of atoms in 0.5 mole of C atom
= 6.022 × 10²³ × 0.5
= 3.011 × 10²³ atoms

Question. Name the scientists whose experimentation established laws of chemical combination. Name the laws also.
Answer.Antoine Laurent Lavoisier and Joseph L. Proust experimented and established two laws of chemical combination.
These laws are :
(i) Law of conservation of mass,
(ii) Law of constant proportions.

Question. Write the postulate given by the Indian philosopher Maharishi Kanad.
Answer. Indian philosopher Maharishi Kanad postulated if we divide matter we will get smaller and smaller particles.He said that a time will come when we come across smallest particles beyond which further division will not be possible.

Question. Sunita calculated molecular mass of S8 molecule and reported it as 64u from the given atomic mass of 32u. But teacher considered her answer as wrong. What is the correct molecular mass of S8? Calculate the number of moles of S8 in 25.6 g of the sample.
Answer.
Molecular mass of S8 = 32 × 8 = 256u
Moles of S₈ in 25.6 g sample = 25.6/25 6 = 0.1 mole

Question. Glucose has the molecular formula C₆H1₂O₆. Calculate
(a) Its molecular mass.
(b) The number of atoms in one molecule of glucose.
(c) The number of gram molecule in 18 g of glucose.
Answer.
(a) Molecular mass of C₆H1₂O₆
= (6 × 12u) + (12 × 1u) + (6 × 16u)
= 72u + 12u + 96u = 180u

(b) The number of atoms in one molecule of C₆H1₂O₆
= 6 atoms of C + 12 atoms of H
+ 6 atoms of O
= 6 + 12 + 6 = 24 atoms

(c) Number of gram molecules

18/180=0.1

Question. What is the mass of (i) 2.5 moles of CO₂ and (ii) 1 mole of water?
Answer.
(i) 1 mole of CO₂ = Molecular mass expressed in grams
= 1 × 44 g
2.5 moles of CO₂ = 2.5 × 44 = 110 g
(ii) Mass of the substance = Moles of substance
× Molecular mass in grams
Mass of water = 1 × 18 g = 18 g

Question. Calculate the number of H₂O molecules in one drop of water having a mass of 0.05 g.
Answer.
Number of moles of H₂O in 0.05 g of water

= 1.673 × 10²³ molecules

Question. What is the mass percentage of different elements in calcium carbonate? (Atomic mass : Ca = 40, O = 16)
Answer.
Molecular mass of CaCO₃ = At. Mass of Ca + At.
Mass of C + 3 × At. Mass of O
= 40 + 12 + 3 × 16 = 100
Mass percentage of Ca

Question. What is the use of mole concept?
Answer. Applications of mole concept :
(i) We can calculate the number of basic particles from the number of moles as the number of moles of a substance is directly proportional to the number of elementary particles.
(ii) One mole of gas occupies 22.4 litres at 273K.
(iii) One mole of any gas occupies the same volume at same pressure and temperature.
(iv) One mole is equal to 6.022 × 10²³ atoms. So, we can calculate the absolute masses of atoms and molecules.

Question. Define the term gram atom. How is it related to mole and Avogadro number?
Answer.
The atomic mass of an element expressed in grams is called gram atomic mass. One gram atom of any element contains 6.022 × 1023 atoms of the element. It is equal to one mole of atoms.
One gram atomic mass = 6.022 × 10²³ atoms = 1 mole

Question. Give symbol and valency of : Potassium, Barium,Aluminium, Calcium, Cobalt, Fluorine, Lead, Zinc,Iodine, Sulphide.
Answer.

Question. Ca₂P₂O₇ is the formula of calcium pyrophosphate. Write the formula for ferric pyrophosphate.
Answer.
Valency of calcium is +2. Ca₂P₂O₇ has two calcium atoms. So, calcium have total of +4 charges. Thus,pyrophosphate has a valency of –4. Since ferric ion has a valency of +3, the formula of ferric pyrophosphate is Fe4(P₂O₇)₃.

Question. The mass of any single atom X is 3.05 × 10^–22 g. What is its atomic weight? Name the possible element.
Answer.
1 mole = atomic mass
= 6.022 × 10²³ atoms
Now, mass of one atom of X = 3.05 × 10–22 g

Mass of 6.022 × 10²³ atoms of X
= 3.05 × 10–22 × 6.022 × 10²³ g
= 183.7 g
This element could be tungsten.

Question. Write formula for the following :
(a) Zinc sulphate,
(b) Methane,
(c) Ammonium carbonate.
Answer.
(a) Zinc sulphate
Thus, Zn₂(SO₄)₂ and finally = ZnSO₄

(b) Methane
Thus, finally = CH₄

(c) Ammonium carbonate
Thus, finally = (NH₄)₂CO₃

Question. 50 g of 10% lead nitrate is mixed with 50 g of 10% sodium chloride in a closed vessel. It was found after reaction that 6.83 g of lead chloride was precipitated.
Besides, the reaction mixture contained 90 g water and sodium nitrate. Calculate the amount of sodium nitrate formed.
Answer.
50 g of 10% lead nitrate = 5 g lead nitrate + 45 g water
50 g of 10% sodium chloride = 5 g sodium chloride + 45 g water
Total content before reaction = 5 + 5 + 90 = 100
Total content after reaction = 90 g
Amount of precipitate = 6.83 g
According to law of conservation,
Total mass of reaction mixture = 100 g
Amount of sodium nitrate = 100 – 90 – 6.83 = 3.17 g

Question. Explain the law of multiple proportions.
Answer.
According to law of multiple proportions, when two elements combine to make one or more compounds then the ratio of weights of these element remain in fixed ratio to one another. For example : Hydrogen and oxygen combine to form water (H₂O) and hydrogen peroxide (H₂O₂) under different condition. 2 grams of hydrogen combines with 16 grams of oxygen in case of water while 2 grams of hydrogen combines with 32 grams of oxygen to form hydrogen peroxide. Now, the weights of oxygen combine with a fixed weight of hydrogen in water and hydrogen peroxide respectively are 16 and 32 which are in simple ratio of 16: 32 or 1 : 2.

Question. Explain the form of atoms in a solid.
Answer. A solid element is a cluster of atoms. The property of solid does not depend on a single atom but on cluster of atoms. For example : Diamond and graphite though both are composed of carbon atoms but due to different arrangements of carbon atoms in these.They are different in physical and chemical properties.

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