Atoms Molecules
In Text Question Answer
Question 1: In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.
sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water
Answer: Total Mass of Reactants = 5.3 + 6 = 11.3 g
Total Mass of Products = 2.2 + 0.9 + 8.2 = 11.3 g
Here; Total mass of reactants = Total mass of products
Hence, these observations are in agreement with the law of conservation of mass.
Question 2: Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Answer: `H/O = 1/8`
Or, `3/O = 1/8`
Or, `O = 3 xx 8 = 24` g
Hence, 24 g of oxygen is required to completely react with 3 g of hydrogen.
Question 3: Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Answer: The relative number of and kinds of atoms are constant in a given compound.
Question 4: Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Answer: Atoms combine in the ratio of small whole numbers to form compounds.
Question 5: Define the atomic mass unit.
Answer: One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12.
Question 6: Why is it not possible to see an atom with naked eyes?
Answer: Atoms are very small in size. Hence, it is not possible to see an atom with naked eye.
Question 7: Write down the formulae of
(a) sodium oxide
Answer: Na2O
(b) aluminium chloride
Answer: AlCl3
(c) sodium suphide
Answer: Na2S
(d) magnesium hydroxide
Answer: Mg(OH)2
Question 8: Write down the names of compounds represented by the following formulae:
(a) Al2(SO4)3
Answer: Aluminium sulphate
(b) CaCl2
Answer: Calcium Chloride
(c) K2SO4
Answer: Potassium sulphate
(d) KNO3
Answer: Potassium nitrate
(e) CaCO3
Answer: Calcium carbonate
Question 9: What is meant by the term chemical formula?
Answer: The symbolic representation of a compound is called chemical formula.
Question 10: How many atoms are present in a
(a) H2S molecule and
(b) PO43– ion?
Answer: (a) 3, (b) 5
Question 11: Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.
Answer: Molecular mass of H2:
`= 2 xx 1 + 1 xx 16 = 18` u
Molecular mass of O2:
`= 2 xx 16 = 32` u
Molecular mass of Cl2:
`= 2 xx 35.5 = 71` u
Molecular mass of CO2:
`= 1 xx 12 + 2 xx 16 = 44` u
Molecular mass of CH4:
`= 1 xx 12 + 4 xx 1 = 16` u
Molecular mass of C2H6:
`=2 xx 12 + 6 xx 1 = 30` u
Molecular mass of C2H4:
`=2 xx 12 + 4 xx 1 = 28` u
Molecular mass of NH3:
`=1 xx 14 + 3 xx 1 = 17` u
Molecular mass of CH3OH:
`= 1 xx 12 + 3 xx 1 + 1 xx 16 + 1 xx 1 = 32` u
Question 12: Calculate the formula unit masses of ZnO, Na2O, K2CO3, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.
Answer: Formula unit mass of ZnO:
`= 1 xx 65 + 1 xx 16 = 81` u
Formula unit mass of Na2O:
`=2 xx 23 + 1 xx 16 = 62` u
Formula unit mass of K2CO3:
`=2 xx 39 + 1 xx 12 + 3 xx 16 = 78 + 12 + 48 = 138` u
Question 13: If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Answer: Number of atoms in one mole of carbon atom = 6.022 x 1023
Weight of 1 mole carbon atoms = 12 g
So, weight of 1 atom of carbon `= 12/(6.022 xx 10^23) = 1.9926 xx 10^-23` g
Question 14: Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?
Answer: Atomic mass of sodium = 23 u
So, 23 g of sodium contains 1 mole of sodium atoms
Hence, 100 g of sodium contains 100/23 mole of sodium atoms
Atomic mass of iron = 56 u
So, 56 g of iron contains 1 mole of iron atoms
Hence, 100 g of iron contains 100/56 mole of sodium atoms
Here; 100/23 is greater than 100/56 because numerator is same but denominator is smaller in case of sodium.
So, 100 g of sodium contains more atoms than 100 g of iron.