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Answers
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Answer:
yggiyfjjyfuufffjhdgbbc is the most important thing in life and is not a good choice for a person to do it again 6or the class is a natural and well definedAtomic mass & Molecular mass
1) (a) Define atomic mass unit. (CBSE 2012)
(b) Distinguish between molecular mass and molar mass.
2) What do you understand from the statement “relative atomic mass of sulphur is 32”?
3) (a) Define molecular mass. Calculate the relative molecular mass of sodium nitrate.
(Atomic mass of Na →23, N→14, O→16)
(b) Find the relative molecular mass of : (i) CaCl2 and (ii) HNO3 .
(Given Ca = 40 u ; Cl =35.5 u ; H = 1 u ; N = 14 u ; O = 16 (CBSE 2012)
4) Calculate the molar mass of the following compounds : (CBSE 2011)
(a) Lead sulphate (b) Calcium phosphate
[Given atomic masses of various elements Ca - 40, S - 32, O - 16, Pb - 207, P - 31]
5) Calculate the molecular mass of the following:
(a) potassium dichromate ( K2Cr2O7)
(b) methane (CH4),
(c) ammonia(NH3)
(d) potassium permanganate (KMnO4)
(e) Glucose (C6H12O6).
(Atomic masses : K = 39.0u, O = 16.0u, Cr = 52.0, Mn = 55.0u.)
6) 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) (CBSE 2011)
Chapter -3 Atoms and Molecules
Assignment-3, Based on Mole Concept (CBSE 2012)
1.(i) Define mole
(ii) How is it related to Avogadro constant, relative mass and molecular mass?
(iii) Define Avogadro's number. Why is it also known as Avogadro constant? (CBSE 2011)
(iv) How many atoms are there in one gram of hydrogen ?
(v) What is the number of molecules in 0.25 moles of oxygen ? Avogadro’s no. 6.022 X 10-23
Explanation:
ATOMS AND MOLECULES
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.
Ch. -3 Atoms and Molecules
ASSIGNMENT -1
1. Which of the following correctly represents 360 g of water?
(i) 2 moles of H2O
(ii) 20 moles of water
(iii) 6.022 × 1023 molecules of water
(iv) 1.2044×1025 molecules of water
(a) (i) (b) (i) and (iv)
(c) (ii) and (iii) (d) (ii) and (iv)
2. Which of the following statements is not true about an atom?
(a) Atoms are not able to exist independently
(b) Atoms are the basic units from which molecules and ions are formedn(c) Atoms are always neutral in nature
(d) Atoms aggregate in large numbers to form the matter that we can see,
feel or touch
3. The chemical symbol for nitrogen gas is
(a) Ni (b) N2 (c) N+
(d) N
4. The chemical symbol for sodium is
(a) So (b) Sd (c) NA (d) Na
5. Which of the following would weigh the highest?
(a) 0.2 mole of sucrose (C12 H22 O11) (b) 2 moles of CO2
(c) 2 moles of CaCO3 (d) 10 moles of H2O
6. Which of the following has maximum number of atoms?
(a) 18g of H2O (b) 18g of O2
(c) 18g of CO2 (d) 18g of CH4
7. Which of the following contains maximum number of molecules?
(a) 1g CO2 (b) 1g N2
(c) 1g H2 (d) 1g CH2
8. 3.42 g of sucrose are dissolved in 18g of water in a beaker. The number of
oxygen atoms in the solution are
(a) 6.68 × 1023 (b) 6.09 × 1023
(c) 6.022 × 1023 (d) 6.022 × 1023
9. A change in the physical state can be brought about
(a) only when energy is given to the system
(b) only when energy is taken out from the system
(c) when energy is either given to, or taken out from the system
(d) without any energy change
Ch. -3 Atoms and Molecules
ASSIGNMENT -2
Laws of Chemical Combination & Dalton’s Atomic Theory
1) If 6.8g of sodium carbonate are added to 15.5g of acetic acid solution, the residue is
found to weigh 9.6g. What is the mass of carbon dioxide released in the reaction?