\nPlants in a garden make use of 22.4 liters of carbon-dioxide at STP. The numbers of molecules of carbon-dioxide utilized by the plant is
\n(a) 6.023 x 1023<\/sup>
\n(b) 22.4 molecules
\n(c) 6.023 x 1024<\/sup>
\n(d) 11.2 moles of molecules
\nAnswer:
\n(a) 6.023 x 1023<\/sup><\/p>\n
Question 3.
\nThe atomic mass of deuterium isotope of hydrogen is 2. The atomic mass of oxygen is 16. Then the molecular mass of heavy water formed is
\n(a) 16
\n(b) 18
\n(c) 20
\n(d) 24
\nAnswer:
\n(c) 20<\/p>\n
Question 4.
\nSome elements can have different valencies. It can happen due to the change in
\n(a) of electrons that participate in a reaction.
\n(b) Neutron number
\n(c) Proton number
\n(d) atomic mass number
\nAnswer:
\n(a) of electrons that participate in a reaction.<\/p>\n
II. Give scientific reason for the following:<\/p>\n
Question 1. Question 2. III. Answer the following:<\/p>\n Question 1. 1. Atomic mass: The mass of an atom of a chemical element is expressed in atomic mass units. It is approximately equivalent to the total number of protons and neutrons in the atom (the mass number) or to the average number allowing for the relative abundance of different isotopes.<\/p>\n 2. Relative atomic mass: The ratio of the average mass of one atom of an element to one-twelfth of the mass of an atom of carbon-12 is the relative atomic mass.<\/p>\n 3. Gram molecular mass: A mass in grams numerically equal to the molecular weight of a substance or the sum of all the atomic masses in its molecular formula.<\/p>\n 4. Mole: Mole is the amount of pure substance containing the same number of chemical units as there are atoms in exactly 12 grams of carbon-12 (ie 6.023 x 1023)<\/p>\n 5. Valency of elements: Valley of elements is a measure of its combining power with other atoms when it forms chemical compounds or molecules.<\/p>\n Question 2. Question 3. Question 4. I. Fill in the blanks with suitable words:<\/p>\n Question 1. Question 2: Question 3. Question 4. II. Answer the following :<\/p>\n Question 1. III. Match the following:<\/p>\n Answer:<\/p>\n IV. Answer the following:<\/p>\n Question 1. Question 2. Question 3. Question 4. Question 5. Question 6. Question 7. Question 8. Question 9. Question 10. Question 11. Question 12. Question 13. Question 14. Question 15. Question 16.
\nThe valency of noble gases is zero.
\nAnswer:
\nNoble gases are 18th <\/sup>group elements which have either completed shell of electrons or 8 electrons in the outermost shell. They do not have tendency to react with other elements to form compounds.<\/p>\n
\nIn the atmosphere, Oxygen is available as O2 <\/sub>but not as O.
\nAnswer:
\nOxygen is written as O2<\/sub> when there are two oxygen atoms in the molecule pure oxygen doesn\u2019t generally exist as individual atoms, two oxygen atoms bond together to form an oxygen molecule. This is because oxygen is highly reactive disconnected atoms have a very strong tendency to form bonds. If there\u2019s nothing else available they will bond to each other.<\/p>\n
\nDefine :
\n1) Atomic mass
\n2) Relative atomic mass
\n3) Gram molecular mass
\n4) Mole
\n5) Valency of an element
\nAnswer:<\/p>\n
\nCalculate the percentage composition of Carbon and Oxygen in \u00a0CO2 <\/sub>, (given atomic masses : Carbon = 12 and Oxygen = 16).
\nAnswer:
\nMass of carbon dioxide [CO2<\/sub>]
\n= 1 (C) + 2(0)
\n= 1(12) + 2(16)
\n= 12 + 32 = 44
\n% mass of carbon = \\(\\frac { 12 }{ 44 } \\) \u00d7 100 = 21.3%
\n% mass of oxygen = \\(\\frac { 32 }{ 44 }\\) \u00d7 100 = 72.7%<\/p>\n
\nA student has a piece of copper sulfate (CuSO4<\/sub>) crystal. Explain the method of calculating the number of CuSO4 \u00a0<\/sub>molecules in that crystal. (Given atomic masses : Copper = 40, Sulfur = 32 and Oxygen = 16)
\nAnswer:
\nMolecular formula of copper sulfate = CuSO4 \u00a0 \u00a0<\/sub>
\nGram molecular mass of CuSO4 <\/sub>= 40+32 + (16 \u00d7 4) = 72 + 64 = 136 grams
\nGram molecular mass of CuSO4 <\/sub>= 1 mole = 6.023 \u00d7 1023 <\/sup>molecules
\nIf the students has 136 grams of CuSO4\u00a0 <\/sub>then the number of molecules in that crystal will be 6.023 \u00d7 1023<\/sup><\/p>\n
\nCaC03 \\(\\xrightarrow { heat }\\) CaO + CO2 <\/sub>\u2191
\nFrom the above equation, calculate the amount of carbon dioxide in grams liberated by heating 25 g of calcium carbonate.
\nAnswer:
\nCaCO3<\/sub>\\(\\xrightarrow { heat }\\) CaO + CO2 \u2191
\n<\/sub> (40 \u00d7 1)+(1 \u00d7 12) + (16 \u00d7 3) \u2192 (40 + 1) + (16 \u00d7 1) + (12 \u00d7 1) + (16 \u00d7 2) 40 + 12+48 \u2192 (40 + 16) + (12 + 32)
\n100 gms of CaCO3 <\/sub>\u2192 56 gm of CaO + 44 gms of CO2 <\/sub>
\nWhen 100 gms of CaCO3 <\/sub> is heated 44 gms of CO2 <\/sub> is liberated.
\nTherefore when 25 gms of CaCO3 <\/sub>is heated the amount of CO2 <\/sub> liberated = 11 gms<\/p>\nKSEEB Class 8 Science Chapter 4 Additional Questions & Answers<\/h3>\n
\nModem standard for measuring atomic mass is \\(\\frac { 1 }{ 12 } \\) mass of ………… isotope
\nAnswer:
\n6<\/sub>C12<\/sup><\/p>\n
\nThe average atomic mass of chlorine is …………
\nAnswer:
\n35<\/p>\n
\nThe molecular mass of water is …………
\nAnswer:
\n18<\/p>\n
\nGram molecular volume of a gas at STP ………….. liter.
\nAnswer:
\n22.4<\/p>\n
\nA student writes \u201cpreparation of CO2 <\/sub>in the laboratory in his book. From the point of view of chemistry, what is the mistake made in writing?
\nAnswer:
\nFrom the point of view of the mole concept, he\/she should have written how many moles of CO2 <\/sub> he\/she is going to prepare.<\/p>\n![\"KSEEB](\"https:\/\/live.staticflickr.com\/65535\/48101975386_fe7e39696e_o.png\")
<\/p>\n\n
\nDefine atomic mass unit (a.m.u)
\nAnswer:
\none-twelth of mass of the atom 6<\/sub>C12 <\/sup>isotope is taken as a standard to compare the mass of other atoms and molecules this unit is called atomic mass unit.<\/p>\n
\nHow much is one a.m.u?
\nAnswer:
\nOne a.m.u = 1.66 x 10-24 <\/sup>g
\n= 1.66 x 10-27 <\/sup>kg
\n\\(\\frac { 1 }{ 12 } \\) of the mass of one atom of carbon
\n6C12<\/sup> is one a. m. u. (Atomic Mass Unit)
\ni.e. 1.66 \u00d7 10-24 <\/sup>g<\/p>\n
\nWhat is meant by relative atomic mass of an element?
\nAnswer:
\nThe relative atomic mass of an element is the ratio of the mass an atom of the element to
\none-twelfth of the mass of an atom of carbon 6 c
\n![\"KSEEB](\"https:\/\/live.staticflickr.com\/65535\/48102119202_6144a5171f_o.png\")
<\/p>\n
\nWhat is meant by the relative molecular mass of a substance?
\nAnswer:
\nThe relative molecular mass of a substance is the ratio of the mass of a molecule of
\nthe substance to \\(\\frac { 1 }{ 12 } \\) the mass of an atom of Carbon 6<\/sub>C12 <\/sup>isotope.<\/p>\n
\nWrite the relative atomic masses of some elements.<\/p>\n\n\n
\n Element<\/td>\n Symbol<\/td>\n Relative mass<\/td>\n<\/tr>\n \n Aluminum<\/td>\n Al<\/td>\n 26.98<\/td>\n<\/tr>\n \n Calcium<\/td>\n Ca<\/td>\n 40.08<\/td>\n<\/tr>\n \n Carbon<\/td>\n C<\/td>\n 12.01<\/td>\n<\/tr>\n \n Chlorine<\/td>\n Cl<\/td>\n 35.45<\/td>\n<\/tr>\n \n Hydrogen<\/td>\n H<\/td>\n 1.008<\/td>\n<\/tr>\n \n Magnesium<\/td>\n Mg<\/td>\n 24.31<\/td>\n<\/tr>\n \n Nitrogen<\/td>\n N<\/td>\n 14.01<\/td>\n<\/tr>\n \n Oxygen<\/td>\n 0<\/td>\n 15.99<\/td>\n<\/tr>\n \n Silver<\/td>\n Ag<\/td>\n 107.57<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
\nCalculate the relative molecular mass of carbon dioxide.
\nAnswer:
\nMolecular formula of carbon dioxide =CO2 <\/sub>
\nAtomic mass of Carbon = 12
\nAtomic mass of Oxygen = 16
\n1 \u00d7 atomic mass of carbon + 2 \u00d7 atomic mass of oxygen.
\n= (1 \u00d7 12) +(2 \u00d7 16) = 44
\nHence relative molecular mass of carbon dioxide = 44.<\/p>\n
\nWhat is meant by gram atomic mass? Give example.
\nAns. Gram atomic mass of an element is its relative atomic mass expressed in grams
\nEx :
\n(1)The gram atomic mass of a hydrogen atom is 1. 008 g.
\n(2) The gram atomic mass of an oxygen atom is 16 g.<\/p>\n
\nWhat is meant by gram molecular mass ? Give an example.
\nAnswer:
\nGram molecular mass of a substance is its relative molecular mass expressed in gram\u2019s
\nEx :
\n1. The gram molecular mass of hydrogen is 2 g
\n2. The gram molecular mass of chlorine is 71 g.<\/p>\n
\nWrite the molecular mass of some substances.
\nAnswer:<\/p>\n![\"KSEEB](\"https:\/\/live.staticflickr.com\/65535\/48102121958_621d98ebdd_o.png\")
<\/p>\n
\nWhat is meant by a mole? Explain.
\nAnswer:
\nA \u2018mole\u2019 represents a collection of 6.023 \u00d7 1023 <\/sup>atoms or molecules. A \u201cmole\u201d represents the number of atoms or molecules in 22.4 liters of any gas at standard temperature and pressure.
\nThe volume 22.4 liters is also called gram molecular volume. A \u2018mole\u2019 is equal to 6.023 \u00d7 1023 <\/sup> atoms or molecules. This is also called Avogadro’s number and is represented by \u2018N\u2019.<\/p>\n
\nMention the signification of mole.
\nAnswer:
\nSignification mole
\n(1) mole concept gives us a method of calculating the number of atoms present in a given mass of a substance.
\n(2) It helps in the calculation of the ratio of reactions consumed and products formed quantitatively.<\/p>\n
\nTo prepare one gram of hydrogen molecules using hydro caloric acid how many grams of zinc should be taken?
\nAnswer:
\nZn + 2HCl \u2192 ZnCl2<\/sub>+H2<\/sub> \u2191
\nGram atomic mass of zinc is = 65 g and Gram molecular mass of Hydrogen = 2 g From the above equation we can understand that 65 g of zinc gives 2 g of hydrogen molecules. To prepare 1 g of hydrogen molecules we need 32.5 g of zinc.<\/p>\n
\nWhat are inert gases or noble gases? Give example.
\nAnswer:
\nSome elements do not have the tendency to react with other elements to form compounds. They are called inert gases or noble gases.
\nEx: Helium, Neon, Argon.
\n[Zero valency elements] or [Inner gases]<\/p>\n
\nWhat is meant by valency? Give an example.
\nAnswer:
\nValency can be considered as the Combining capacity of an element.
\nEx:<\/p>\n\n\n
\n Elements<\/td>\n Symbol<\/td>\n<\/tr>\n \n Oxygen<\/td>\n 2<\/td>\n<\/tr>\n \n Sodium<\/td>\n 1<\/td>\n<\/tr>\n \n Silicon<\/td>\n 2<\/td>\n<\/tr>\n \n Nitrogen<\/td>\n 5<\/td>\n<\/tr>\n \n Sulfur<\/td>\n 2<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
\nWhat is meant by variable valency?
\nAnswer:
\nThe same element may exhibit different valencies in different compounds and this property is called as variable valency.
\nEx: Iron has 2 valencies.
\nFerric has 3 valencies It is because the number of electrons from the iron atoms which participate in the chemical reaction will be either 2 or 3 depending upon conditions.<\/p>\n
\nWrite the common features of the Noble gases?
\nAnswer:<\/p>\n