Atoms And Molecules
Class: 9 | Chapter: 3
Extra Questions & Answer
1. Name two scientists who established the laws of chemical combination?Answer: Antony L. Lavoisier and Joseph L. Proust.
2.Give an example of a triatomic molecule of an element.
Answer: It is ozone, O3 made by three atoms of oxygen element.
3. What is atomicity?
Answer: Atomicity : It is defined as number of atoms in a molecule.
4. Write the atomicity of following molecules : (a) Sulphur (b) phosphorus
Answer: (a) Sulphur molecule exists as S8 So, atomicity is 8.
(b) Phosphorus molecule exists as P4 So, atomicity is 4.
5. What is an ion? Give one example.
Answer: Ion : Atom or group of atoms containing any electrical charge. It may be positive or negative. If it is positively charged, it is called cation and if it is negatively charged, it is called anion. Examples : Na+, NH4+, Cl- etc.
6. Give one word for the following :
a. A group of atoms carrying a charge
b. Positively charged ion
Answer: (a) polyatomic ion
(b) cation
7. The atomic number of three elements A, B and C are 9, 10 and 13 respectively. Which of them will form a cation?
Answer: Generally metals and the element which is having 1,2 or 3 ele trons in the outermost shell form cations easily. Here A with atomic number 9 is Fluorine, it is a non - metal. B with atomic number 10 , is a noble gas and C with atomic number 13 is a metal with electronic configuration 2,8,3 it will form cation easily.
8. What is wrong in saying 'One mole of oxygen'?
Answer: By saying 'One mole of Oxygen',it is not clear that given 1 mole is for oxygen atoms or oxygen molecules.So, it is incomplete information for any conclusion.
9. An element A is tetravalent and another element B is divalent. What will be the formula of compound made by these two elements.
Answer: Element A has valency 4 and B has valency 2 So, after cross-multiplication of their valencies and simplification, the formula will be AB2.
10. How many times heavier is one atom of carbon than one atom of oxygen?
Answer: Mass of one atom of carbon = 12/ 6.022× 1023 = 1.994 × 10-23 g Mass of one atom of oxygen = 16 / 6.022 × 1023 = 2.65 × 10-23g So, it is clear that one atom of carbon is almost 0.75 times of mass of one stom of oxygen.
11. Give an example to show law of conservation of mass applies to physical changes also.
Answer: Law of conservation is mass says that Mass can't be created or destroyed during a change. It can only change its form. For example, in physical changes like melting of ice or boiling of water the mass of water taken remains same after change of state. Therefore, this is in agreement of law of conservation of mass.
12. Differentiate between the actual mass of a molecule and gram molecular mass.
Answer: Mass of a molecule is mass of a single molecule and gram molecular mass is the mass of 1 mole of molecules.
13. Calculate the formula mass of sodium carbonate ( Na2CO3. 10H2O) .
Answer: As we know, Atomic masses of Na - 23u, C - 12u , O - 16u , H - 1u So, according to the formula ( Na2CO3. 10 H2O) = 2 × 23 + 12 ×1 + 3 × 16 + 10 ( 2 × 1 + 16 ×1) => 286 u
14. Calculate the mass of one atom of hydrogen atom
Answer: We know that Atomic mass of Hydrogen = 1 u = Mass of 1 mole of Hydrogen atoms So, Mass of 1 atom of Hydrogen = 1 / 6.022 × 1023 = 1.67 × 10-24 g
15. Does the solubility of a substance change with temperature? Explain with the help of an example.
Answer: Solubility of a solid in liquid generally increases with increase in temperature. For example if we add salt or sugar in water then by heating the mixture, solubility of salt or sugar in water will increase . However, Solubility of a gas in liquid decreases with increase in temperarure for example the solubility of oxygen gas in water is more at low temperatures hence aquatic animals feel more comfortable in winters.
16. Calculate the molecular mass of the following :
(a) H2CO3
(b) C2H5OH
(c) MgSO4
Answer: (a) . Atomic masses of H - 1 u, C - 12 u, O - 16 u So, by the formula 2×1 + 1× 12 + 3 × 16 = 62 u
(b) Atomic masses of C - 12 u, H - 1 u, O - 16 u So, by the formula 2 × 12 + 6 ×1 + 16 ×1= 46 u
(c) Atomic masses of Mg - 24 u, S - 32 u, O - 16 u So, by the formula 24 + 32 + 4 × 16 = 120 u .
17. Ram took 5 moles of carbon atoms in container and Krishna also took 5 moles of sodium atoms in another container of same weight. (a) Whose container is heavier?
(b)Whose container had more number of atoms?
Answer: (a) 5 mole of Carbon atoms has mass in grams = 5 × 12 => 60g And, 5 moles of Sodium atoms has mass in grams = 5 × 23 => 105 g Now, it is clear that krishna's container is heavier.
(b) 5 moles means 5 × 6.022 ×1023 atoms, it does not depend upon type of atoms. It just depend upon quantity of particles. Therefore both container have same number of atoms.
18.Which of the following are tri-atomic and tetra-atomic molecules? CH3Cl, CaCl2, NH3, PCl3, P2O5, H2O, C2H5OH
Answer: (i) Tri-atomic molecules ( 3 atoms) are CaCl2, H2O. (ii) Tetra-atomic molecules ( 4 atoms) are NH3, PCl3
19. Write the cations and anions present (if any) in the following compounds:
(a) CH3COONa
(b) NaCl
(c) H2
(d) NH4NO3
Answer: (a) Na+ and CH3COO- (b) Na+ and Cl- (c) H2— It is a covalent compound (d) NH4+ and NO3-
20. Give two significance of mole.
Answer: 1. It is an important measurement unit for small particles like atoms and molecules.
2. It represents one gram formula mass of the given substance
21. Calculate the number of moles present in:
(i) 3.011 × 1023 number of oxygen atoms
(it) 60 g of calcium
[Given that atomic mass of Ca = 40 u, Avogadro No. = 6.022 × 1023]
Answer: (i) We know that, No. Of moles = Given no. of particles / Avogadro No. => 3.011× 10 23 / 6.022 × 10 23= 0.5 moles
(ii) Here, No. Of moles = Given mass / Atomic mass=> 60 / 40 = 1.5 moles
22. What is Avogadro’s constant ?
Answer: Avogadro 's constant = 6.022 × 1023
23. Name the isotope of an element which is used in the treatment of cancer.
Answer: Cobalt - 60 is used in treatment of cancer.
24. What do you understand by an octet of electrons in the valence shell ?
Answer: Octet of electrons relates to 8 electrons in the valence shell or outermost shell of inert gases. When there are 8 electrons in valence shell then element has no reactivity or very low reactivity in some cases.
25. Who gave the laws of chemical combination ?
Answer: In 1789, Antoine Lavoisier gave the laws of chemical combination.
26. Two elements X and Y combine to form XY in the ratio 1:35.5 by mass. What mass of Y will combine with 2 grams of X ?
Answer: By this statement, 1 gram of X combine with 35.5 grams of Y So, 2 grams of X combine with = 2 × 35.5 => 71 grams of Y
27. How many moles are present in 4.6 g of Na ? (Atomic mass of Na = 23).
Answer: Given mass of Na = 4.6 g We know that, No. of moles = Given mass / Atomic mass => 4.6 / 23 = 0.2 moles
28.Name two trivalent elements having positive valency.
Answer: Chromium (Cr3+), Aluminium (Al3+)
29. An element M has valency 2. Write the formulae of its hydroxide and nitride.
Answer: M has valency 2, hydrocide ion (OH-) has valency 1 . So, after cross multiplication the formula will be M(OH)2. Nitride (N3-) has valency 3 . Therefore with M it's formula will be M3N2.
30. The valency of potassium is +1. Write the molecular formulae for potassium dichromate and potassium permanganate.
Answer: Potassium dichromate : Potassium (K) has valency 1 and dichromate ion ( Cr2O7)2- has valency 2 . So, it's formula will be K2Cr2O7. Permanganate ion( MnO4)- has valency 1 . So, Potassium permanganate has formula KMnO4.
31. How many molecules are present in 9 g of H2O ?
Answer: Molecular mass of H2O = 2 × 1+ 1 × 16 = 18 No. Of moles = 9 /18 => 0.5 moles No. Of molecules = 0.5 × 6.022 ×1023 = 3.011 × 1023 molecules.
32. What do you understand by valance electrons?
Answer: The electrons in outermost shell of an atom are known as valance electrons. They are responsible for physical and chemical nature of that element.
33. Define valency and give the valency for the following elements : Magnesium, Carbon, Aluminium
Answer: Valency is combining capacity of an element. It decides chemical properties like types of bonds the elements forms, with what kind of elements it forms bond etc. Trick : (if an elements has 1/2/3/4 electrons in last shell it is equal to its valency) Magnesium = 2,8,2 [ In last shell it has 2 electrons so, valency is 2] Carbon = 2,4 [ In last shell carbon has 4 electrons and that is equal to its valency] Aluminium = 2,8,3 [ In last shell, it has 3 electrons so, Valency is 3]
34. Calculate the formula unit mass of NaCl and CaCl2. Given Atomic masses : Na - 23,Cl - 35.5 , Ca - 40
Answer: Formula unit mass of NaCl = 23 × 1 + 35.5 × 1 = 58.5 Formula unit mass of CaCl2 = 40 × 1 + 35.5 × 2 = 111
35. Give the atomicity of the following compounds :
(a) Ca(OH)2
(b) Cu2O
(c) MgCl2
(d) H2SO4
Answer: (a) 1 Ca + 2 OH = 3
(b) 2 Cu + 1 O = 3
(c) 1 Mg + 2 Cl = 3
(d) 2 H + 1 S + 4 O = 7
36. Explain the difference between 2O, O2 and O3?
Answer: 2O is the two oxygen atoms, O2 is the molecular form of oxygen in which two oxygen atoms are bonded together and O3 represents ozone molecule in which three oxygen atoms are there.
37. How do atoms exist?
Answer: Atoms are very small and reactive and therefore they react to form different molecules to get stability. But some atoms are inert and have fully filled electronic connfiguration.
38. Find the number of atoms in 0.5 mole of C atom.
Answer: we know, No. Of moles = Given no. Of particles / Avagadro No. = 0.5 = Given no. Of particles / 6.022 × 10 23 => 3.011 × 1023atoms.
39. Find the mass of 1.5 mole of CO2 molecule.
Answer: Molecular mass of CO2 = 12 × 1 + 16 × 2 = > 44 No. Of moles = Given mass / Molecular mass => 1.5 × 44 = 66
40. What is Law of conservation of mass and Law of constant proportions?
Answer: Law of conservation of mass : Mass is neither created nor destroyed in a chemical change. Law of constant proportions : Every pure substance always contains the same elements combined in the same proportions by mass.