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Saturday, 27 June 2015

Atom and molecules,

Atom and molecules, Law of constant proportions,Law of conservation of mass,
          
1. Law of conservation of mass : Mass can neither be created nor can it be destroyed in a chemical reaction.

2. Law of constant proportions : In a pure substance same elements are always present in a definite proportion by weight.

3. Atom : Smallest unit of an element, which may or may not exist independently, but always takes part in a chemical reaction.

4. Molecule : Smallest unit of an element or a compound which always exists independently and retains the complete physical and chemical properties of the element or the compound.

5. Atomicity of a molecule of an element : It is the number of atoms which constitute one molecule of an element.

6. Molecular formula : The symbolic representation of the kind and the actual number of atoms in one molecule of a pure substance, may be an element or a compound.

7. Atomic mass unit : The mass of 1/12 part of C-12 (isotope of carbon) is equivalent to one atomic mass unit.

8. Gram-atomic mass : The atomic mass of an element expressed in terms of grams.

9. Atomic mass : The number of times an atom of an element is heavier than 1/12 part of C-12 (isotope of carbon).

10. Molecular mass : The number of times a molecule of a pure substance is heavier than 1/12 part of C-12 (isotope of carbon).

11. Gram-molecular mass : The molecular mass of a pure substance expressed in grams.

12. Mole : A group of 6.022 × 1023 particles (atoms, molecules, ions, electrons, protons, neutrons,etc.) of a substance is called mole.

13. Mole number : It is a number which states, how many times one molecular mass comes in certain mass of a substance in grams.
 

 Very Short Answer Questions (1 Mark)


1. State the postulate of Dalton's Atomic Theory which can explain the law of definite proportions?

1. Find the ratio by mass of the combining elements in the compound – C2H5OH.

2. Give the formula of the compound formed by the elements calcium and fluorine.

3. What is the acid radical present in sodium peroxide?

4. Carbon and silicon have the same valency. What is the formula of sodium 
silicate?

5. What is the ratio by number of atoms in mercurous chloride?

6. Name the element whose Latin name is Stibium.

7. What is the valency of a sulphide ion?

8. How many atoms of oxygen are present in 50g of CaCO3?

9. Calculate the number of molecules of water present in 1 ml of water vapour at STP.

10. What is the unit of measurement of atomic radius?

11. Why is potassium denoted by the symbol K?

12. Define relative atomic mass of an element.

13. State the Law of Conservation of Mass.

14. A substance is made of only one kind of atom. Name the general term applied to the substance.

15. Define the term atom.

16. What is the formula of ferric nitrate?

17. Name the compound represented by the formula Ca3(PO4)2.

18. Name the acid radical present in NaClO3.

19. What is the concentration of hydrogen ions in 1 mol/dm3 of sulphuric acid?

 Short Answer Questions - I (2 Marks)


1. 5 g of calcium combine with 2 g of oxygen to form a compound. Find the molecular formula of the compound. Atomic mass of Ca = 40 u;O =16 u) 

2. (i) Name the body which approves the nomenclature of elements and compounds.
(ii) The symbol of sodium is written as Na and not as S. Give reason.
(iii) Name one element which form diatomic and one which form tetra atomic molecules.

3. (i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions. 

4. An element 'Z' forms the following compound when it reacts with hydrogen, 
chlorine, oxygen and phosphorus.
ZH3, ZCl3, Z2O3 and ZP
(a) What is the valency of element Z?
(b) Element 'Z' is metal or non-metal?

5. (a) Calculate the number of molecules in 8 g of O2.
(b) Calculate the number of moles in 52 grams of He (Helium).At. mass : O = 16 u He = 4 u

6. 2.8 g of nitrogen gas was allowed to react with 0.6 g of hydrogen gas to produce 3.4 g of ammonia. Show that these observations are in agreement with the law of Conservation of mass. State the law of conservation of mass.

7. State law of conservation of mass? If 12 g of carbon is burnt in the presence of 32 g of oxygen,how much carbon dioxide will be formed?

8. Potassium chlorate decomposes, on heating,to form potassium chloride and oxygen. When 24.5 g of potassium chlorate is decomposed completely, then 
14.9 g of potassium chloride is formed. Calculate the mass of oxygen formed. State the law of chemical combination which you have used in solving this problem.
9. (a) Write chemical formulae of sodium sulphate.
(b) Name one element each which forms diatomic and tetra atomic molecule. 

10. State the law of constant proportion. Magnesium and oxygen combine in the ratio of 3 : 2 by mass to form magnesium oxide. How much oxygen is required to react completely with 12 g of magnesium? 

11. (a) Write chemical formula of potassium sulphate.
(b) Calculate the molar mass of :
(i) Ethyne (C2H2) and
(ii) Phosphorus molecule (P4) (Atomic mass of C =12u, H =1u and P = 31u). 

12. Calculate mass of 3.011 × 1023 number of nitrogen atoms ( Atomic mass of N =14 u)?

13. (a) Write a chemical formula of a compound using zinc ion and phosphate ion.
(b) Calculate the ratio by mass of atoms present in a molecule of carbon dioxide. (Given C =12, O =16) 

14. Write the name of the compound repesented by the following formulae : 
(a) Al2(SO4)3 (b) CaCl2
(c) KNO3 (d) Mg3N2

15. Write the chemical formula of :
(a) Aluminium phosphate
(b) Sodium sulphide

16. Mention the two postulates of Dalton's Atomic Theory that explain :
(a) Law of Conservation of Mass b) Law of Constant Proportions.

17. (a) What is molar mass?
(b) Calculate the molar mass of carbon dioxide gas.

18. Write the chemical formula for :
(a) Zinc phosphate
(b) Lead carbonate

19. (a) The ratio of hydrogen and oxygen in water is 1:8 by mass, find out their ratio by number
of atoms, in one molecule of water.( At mass H=1 u ; O=16 u)
(b) Write the formulae of the following compounds :
(i) Ammonium sulphate
(ii) Magnesium chloride
(Given Ammonium = NH4
+ , Sulphate = SO4

Magnesium = Mg2+, chloride = Cl.)

20. (a) What is a Cation? Give one example.
(b) Name the compounds represented by the following formulae
(1) KNO3 (2) H2

21. Calculate number of moles in 34 g of NH3 (Given atomic mass of N = 14 u; H = 1 u).

22. Write chemical formula of :
(a) Sodium carbonate
(b) Ammonium chloride
23. (i) Define poly atomic ions and give an example.
(ii) Write the chemical formula of zinc nitrate. 

24. Write the names of compounds represented by the following formulae : 
(i) KNO3 (ii) Al2(SO4)3
(iii) MgCl2 (iv) (NH4)2SO4

25. (a) Write down the names of compounds represented by the following formulae.
(i) Ca(OH)2
(ii) K2SO4
(b) Give two examples of bivalent cations. 

26. Give one word for the following :
(i) Positively charged atom
(ii) A group of atoms carrying a charge.

27. (a) Define atomicity.
(b) Give an example for an element, mono atomic and a poly atomic molecule. 

28. Calculate the number of moles in 17g of H2O2. (Atomic weight of H=1, O=16u) 

29. Calculate the number of moles in 5.75 g of sodium 
(atomic mass of sodium = 23)

Other Important Questions

1. In photosynthesis, 6 molecules of carbon dioxide combine with an equal number of water molecules through a complex series of reactions to give a molecule of glucose having a molecular formula, C6H12O6. How many grams
of water would be required to produce 18 g of glucose? Compute the volume of water so consumed, assuming the density of water to be 1 g cm–3.

2. Raunak took 5 moles of carbon atoms in a container and Krish also took 5 moles of sodium atoms in another container of the same weight. 
(a) Whose container is heavier? 
(b) Whose container has more number of atoms?

3. Compute the number of ions present in 5.85 g of sodium chloride.

4. A gold sample contains 90% of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold?

5. What are ionic and molecular compounds? Give examples.

6. How many molecules are present in 1 ml of water?

7. What fraction of the mass of water is constituted by the neutrons?

8. Classify the following on the basis of their atomicity :
(i) chlorine (ii) phosphorus
(iii) helium (iv) ozone

III. Short Answer Questions - II (3 Marks)
Previous Years’ Questions

1. (i) State and explain the law of constant proportion taking an example of ammonia. (ii) Write the symbol of an element A with atomic number thirteen and mass number 27 respectively.
(iii) Give definition of ion in your own words. 
2. (i) Mention the formula of sodium nitride.
(ii) Calculate the formula unit mass of CaCO3
(Given, Ca=40 u, C=12 u and O=16 u)
(iii) Calculate the mass of 10 moles of carbon dioxide. 

3. (i) Define the term 'atomic mass unit'. How is it linked with relative atomic mass?
(ii) How do we know the presence of atoms if they do not exist independently for most of the elements?

4. (i) Define Avogadro's number. Why is it also known as Avogadro constant?
(ii) Calculate the molar mass of Na2O. (Given, Na = 23 u; O = 16 u)
(iii) Find the mass of 10 moles of carbon dioxide (Given, C = 12 u; O = 16 u) 

5. (a) Write the formula of the compounds (1) Calcium sulphate (2) Aluminium chloride
(b) The average atomic mass of a sample of an element X is 16.2 u. What is the % of isotopes 16 8X and 18 8X in the sample?

6. (a) What are poly atomic ions? 

7. Ravi prepared a solution of sodium chloride by mixing 5.85 g of salt in 1 litre of water. Find (a) Molar mass of sodium chloride (b) Number of moles of sodium chloride dissolved. (Atomic masses of sodium and chlorine are 23 u and 35.5 u respectively). (c) Concentration of the sodium chloride solution. 

8. Write the chemical formulae of the following.
(a) Potassium chloride
(b) Magnesium hydroxide
(c) Ammonium sulphate 

9. Calculate the following : [2011 (T-II)]
(i) The mass of 1.0505 × 1023 molecules of carbon dioxide (CO2)
(ii) The number of molecules of 0.25 moles of ammonia (NH3)
(iii) The formula unit mass of sodium sulphite (Na2SO3
(Atomic mass : Na = 23 u ; S = 32 u ; O=16u ; H =1u) (NA =6.022 × 1023 mol–1)

10. (a) Mention the postulate of Dalton's atomic Theory that explains the law of constant proportions. Explain the law by giving example of water.
(b) Write the chemical formulae for the following compounds :
(i) Calcium hydroxide Ca2+ OH
(ii) Ammonium sulphate NH4 + SO4 2–

11. (a) Define molar mass. What are its units?
(b) Write the names of compounds represented by the following formulas : (i) K2CO3 (ii) Na2SO4
(iii) Ca(HCO3)2 (iv) H2S
12. If the number of molecules in a given sample of sulphur dioxide (SO2) is 3.011 × 1023, calculate
the following : 
(i) the number of moles in the given sample.
(ii) mass of sulphur dioxide in the given sample.
(iii) number of oxygen atoms in the given sample (Atomic mass of S = 32u ; O=16u)

13. Name the two laws of chemicals combinations and write the statement of each one of them. 
14. (a) Calculate the formula unit mass of Na2CO3.
(b) What is the mass of one mol of oxygen atoms?
(c) Convert 12g of oxygen gas into mole.

15. Calculate the molar mass of the following compounds : 
(a) Lead sulphate
(b) Calcium phosphate [Given atomic masses of various elements Ca - 40, S - 32, O - 16, Pb - 207, P - 31]

16. (a) What do the following abbreviations stand for? (i) 2O (ii) 3O2
(b) Which amongst the following has more number of atoms 11.5 g of sodium or 15 g of calcium? How?
[Given atomic mass of Na = 23, Ca = 40]

17. (a) Define law of constant proportion. (b) Give one example of each.
(i) Positively charged ion.
(ii) A group of atoms carrying a charge. 

18. Calculate :
(a) (i) number of molecules in 90gm of H2O.
(ii) number of moles in 19gm of H2O2.
(b) Define the term mole.
19. Calculate : 
(1) number of molecules in 90g of H2?
(2) number of moles in 19g of H2O2?

20. Using cross method, write the chemical formula of copper chloride, calcium sulphate, sodium phosphate.
21. (a) State the law of Definite proportion with an example.
(b) Give any two limitations of Dalton's Atomic Theory. 

22. A gas jar contains 1.7 g of ammonia gas. Calculate the following :
(i) Molar mass of ammonia.
(ii) How many moles of ammonia are present in the gas jar?
(iii) How many molecules of ammonia are present in the sample? 

23. Calculate the number of moles of 4g of methane (CH4) and 11g of CO2, Which has more molecules? [Atomic mass : H = 1u, C = 12u, O = 16u] 
24. (a) How many atoms are present in (i) Pcl5 molecule and (ii) PO3– 4 ion? 
b) Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. 

25.Calculate the mass of oxygen gas that would be required to react with 3 g of hydrogen gas.
(c) Which postulate of Dalton's Atomic Theory is the basis of law of conservation of mass ?

26. What is the mass of 0.5 mole of NH3
Given atomic mass of N = 14u, atomic mass of H = 1u.

Topic related to polytechnic entrance exam




            Atoms and Molecule  ,          Atoms and Molecule,            Atoms and Molecule,            Atoms and Molecule,            Atoms and Molecule

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