Calculation: 2(H) = 2 x 1 = 2 . 1(O) = 1 x 16 = 16
∴ Gram molecular mass of H2O= 18g
Mole is defined as the amount of substance that contains as many specified elementary particles as the number of atoms in 12g of carbon-12 isotope.
One mole is also defined as the amount of substance which contains Avogadro number (6.023 x 10^23) of particles.
For eg. one mole of oxygen atoms represents 6.023 x 10^23 atoms of oxygen and 5 moles of oxygen atoms contain 5 x 6 . 0 2 3 x 10 ^23 a t oms of oxygen .
Q. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide. [Hint : The mass of an ion is same as that of an atom of the same element.
Sol: 1 mole of Al2O3 contains aluminium ions = 2 × 6.022 × 10^23 = 12.044 × 10^23
Now, 102 g of Al2O3 has number of aluminium ions = 12.044 × 10^23
0.051 g of Al2O3 has number of aluminium ions = (12.044 ×1023 × 0.051)/102= 6.022 × 10^20 ions
Q. How many atoms of oxygen are present in 50g of CaCO3? [Ca = 40, C = 12, O = 16]
Sol: 100 g of CaCO3 contains = 3 × 6.022 × 10^23 atoms of oxygen
50 g CaCO3 will contain = (3 × 6.022 × 10^23 × 50)/100 = 9.033 × 10^23 atoms of oxygen
Q. What is the concentration of hydrogen ions in 1 mol/dm3 of sulphuric acid?
Ans. H2SO4 → 2H+ + SO42–
1 mole of sulphuric acid (H2SO4) contains = 2 g of hydrogen ions.
∴ Concentration of hydrogen ions in 1 mole of sulphuric acid = 2 g/dm3 of hydrogen ions.
Q. What happens to an element 'Z' if its atom gains three electrons?
Ans. It forms Z3– ion. Z + 3e– → Z3–
Q. Calculate the mass of 1 molecule of oxygen. [Given atomic mass of hydrogen = 1 u and oxygen 16 u]
Ans. 6.022 × 10^23 atoms of oxygen weigh = 16 g
1 atom of oxygen will weigh = 16/6.022 ×10^23g = 2.66 × ^10–23 g
Q: (a) Calculate the number of molecules in 8 g of O2.
(b) Calculate the number of moles in 52 grams of He (Helium).
Sol: (a)32 g of O2 contains 6.022 ×10^23 molecules
8 g of O2 contains 6.022 ×10^23 molecules x(8/32)= 1.5055 × 10^23 molecules
(b) Molecular mass of He = 4 g,
4 g of He = 1 mole
52 g of He =(1/4) × 52 = 13 mole
Q. 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?
Ans. According to the law of constant proportion "In a pure substance same elements are always present in a definite proportion by weight".
3 g of magnesium reacts with oxygen = 2 g
12 g of magnesium reacts with oxygen = 12 × (2/3) = 8 g
Sol: Atomic mass of NH3 = 14 + 3 × 1 = 17
17 g of NH3 atoms = 1 mole
34 gm of NH3 atoms = 34x(1/17) = 2 mole
Q. Calculate the number of moles of the following :
(a) 84 g of nitrogen atom (b) 8.066 × 10^23 number of nitrogen atom (given atomic mass of N = 14)
Ans. (a) 14 g of nitrogen atoms = 1 mole
84 g of nitrogen atom = [1/14] 84 = 6 moles
(b) 6.022 × 10^23 of nitrogen atoms = 1 mole
∴ 8.066 × 10^23 nitrogen atoms = (1/6.022 × 10^23) x 8.066 × 10^23 = 1.339 moles
Q. A sample of vitamin C is known to contain 2.58 × 1024 oxygen atoms. How many moles of oxygen atoms are present in the sample?
Sol: 6.022 × 10^23 of oxygen atoms = 1 mole
Number of oxygen atoms in the sample of vitamin C = 2.58 × 10^24 /6.022 × 10^23 = 4.28 mol in the sample
Q. What is the mass of 0.5 mole of NH3 ? Given Atomic mass of N = 14u, Atomic mass of H = 1u.
Sol: 17 x 0.5 = 8.5g
Q. Calculate the number of particles in 31 g of P4 molecules. Atomic mass of P=31u.
Sol: 1.5x10^23
Q Find the number of moles in 87g of K2SO4 (Atomic mass of K=39u, S=32u, O=16u)
Sol: 0.5 mole
Q. Calculate the number of moles for the following.
(i) 36g of He (At. Mass of He =4 u) (ii) 12.044x10^23 molecules of H2O (At. mass of He=1 u, O= 16 u)
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