Basic Concepts of Chemistry for Class 11th

 

Question 1.
Calculate the molecular mass of the following :
(i) H₂o
(ii) CO₂
(iii) CH₄
Solution:
(i) Molecular mass of H₂O :2 × 1 + 1 × 16 = 18u
(ii) Molecular mass of CO₂ :1 × 12 + 2× 16 = 44 u
(iii) Molecular mass of CH₄ : 12 + 4 × 1 = 16 u

Question 2.
Calculate the mass percent of different elements present in sodium sulphate (Na₂SO₄).
Solution:
Molecular mass of Na₂SO₄ = 2 × Atomic mass of Na + Atomic mass of S + 4 × Atomic mass of O
= 2 × 23 + 32 + 4 × 16 = 46 + 32 + 64 = 142 u.

Question 3.
Determine the empirical formula of an oxide of iron which has 69.9% iron and 30.1% oxygen by mass.
Solution:

Hence, the empirical formula is Fe₂CO₃

Question 4.
Calculate the amount of carbon dioxide that could be produced when
(i) 1 mole of carbon is burnt in air.
(ii) 1 mole of carbon is burnt in 16 g of dioxygen.
(iii) 2 moles of carbon are burnt in 16 g of dioxygen.
Solution:
(i)

Hence, 1 mole of C produces 44 g of CO₂
(ii)

Hence, O₂ is the limiting reagent.
∵ 32 g O₂ reacts with C to produce 44 g of CO₂
∵ 16 g O₂ reacts with C to produce 

(iii)

∵ 64 g O₂ reacts with C to produce 88 g of CO₂
∵ 16 g O₂ reacts with C to produce = 

Question 5.
Calculate the mass of sodium acetate (CH3COONa) required to make 500 mL of 0.375 molar aqueous solution. Molar mass of sodium acetate is 82.0245 g mol^-1.
Solution:

Question 6.
Calculate the concentration of nitric acid in moles per litre in a sample which has a density 1-41 g mL-1 and the mass per cent of nitric acid in it being 69%.
Solution:
Mass percent 69 means that 69 g of HNO₃ are dissolved in 100 g of the solution.

Question 7.
How much copper can be obtained from 100 g of copper sulphate (CuS0₄) ?
Solution:
Molar mass of CuSO₄ = 63.5 + 32 + 4 × 16 =63.5 + 32 + 64 = 159.5 amu or u
159.5 g of CuSO₄ contains copper = 63.5 g
100 g of CuSO₄ contains copper =  = 39.81 g

Question 8.
Determine the molecular formula of an oxide of iron in which the mass per cent of iron and oxygen are 69.9 and 30.1 respectively.
Solution.
For empirical formula, Fe₂O₃.
Molecular mass of Fe₂O₃ x 1 = 2 x 56 + 3 x 16 = 112 + 48 = 160
Molecular formula = u (Empirical formula)
∴ 
∴ Molecular formula = (Fe₂O₃) x 1 = (Fe₂O₃)

Question 9.
Calculate the average atomic mass of chlorine using the following data.

Solution.

Question 10.
In three moles of ethane (C₂H₆), calculate the following:
(i) Number of moles of carbon atoms.
(ii) Number of moles of hydrogen atoms.
(iii) Number of molecules of ethane.
Solution.
(i) 1 mole of C₂H₆ contains 2 moles of carbon atoms.
Number of moles of carbon atoms in 3 moles of C₂H₆ = 3 x 2 = 6
(ii) 1 mole of C₂H₆ contains 6 moles of hydrogen atoms.
Number of moles of hydrogen atoms in 3 moles of C₂H₆ = 3 x 6 = 18
(iii) 1 mole of C₂H₆ = 6.022 x 10²³ molecules Number of molecules in 3 moles of C₂H₆
= 3 x 6.022 x 10²³ = 1.807 x 10²⁴ molecules

Question 11.
What is the concentration of sugar (C₁₂H₂₂O₁₁) in mol L ^-1 if its 20 g are dissolved in enough water to make a final volume up to 2 L?
Solution.
The molecular mass of sugar ((C₁₂H₂₂O₁₁) = 12 x 12 + 1 x 22 + 11 x 16 = 144 + 22 + 176 = 342

Question 12.
If the density of methanol is 0.793 kg L^-1 what is its volume needed for making 2.5 L of its 0.25 M solution?
Solution.
Moles of methanol present in 2.5 L of 0.25 M solution

Question 13.
Pressure is determined as force per unit area of the surface. The SI unit of pressure, pascal is 1 Pa = 1 Nm^-2. If mass of air at sea level is 1034 g cm^-2, calculate the pressure in pascal.
Solution.
Mass of air at sea level = 1034 g cm^-2
Acceleration due to gravity, g = 9.8 m s^-2

Question 14.
What is the S.I. unit of mass? How is it defined?
Solution.
The S I. unit of mass is kilogram. The amount of matter present in a substance is called mass. The unit of mass kilogram is defined as being equal to the mass of international prototype of the kilogram.

Question 15.
Match the following prefixes with their multiplies
Prefixes Multiples
(i) micro 10⁶
(ii) deca 10⁹
(iii) mega 10^-6
(iv) giga 10^-15
(v) femto 10
Solution.
(i) micro → 10^-6
(ii) deca → 10
(iii) mega → 10⁶
(iv) giga → 10⁹
(v) femto → 10^-15

Question 16.
What do you mean by significant figures?
Solution.
Significant figures : The significant figures in a number are all the certain digits plus one doubtful digit, e.g., 2005 has four significant figures.

Question 17.
A sample of drinking water was found to be severely contaminated with chloroform, CHCI₃, supposed to be carcinogenic in nature. The level of contamination was 15 ppm (by mass).
(i) Express this in percent by mass.
(ii) Determine the molality of chloroform in the water sample.
Solution.
(i) 15 ppm means 15 parts in million (10⁶) parts

Question 18.
Express the following in the scientific notation:
(i) 0.0048
(ii) 234,000
(iii) 8008
(iv) 500.0
(v) 6.0012
Solution.
(i) 4.8 x 10⁶
(ii) 2.34 x 10⁵
(iii) 8.008 x 10³
(iv) 5.00 x 10²
(v) 6.0012 x 10⁰ or 6.0012