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Illustrations

Example-1
 
Write the empirical formula for the following molecules:
(Recall that to write the empirical formula, the subscripts in the molecular formula must be converted into the smallest possible whole numbers)

 
Example
Acetylene (C2H2), used in welding torches, is ________.
  1. CH2
  2. CH4
  3. C4H8
  4. CH
Solution
Ans. D
 
There are two carbon atoms and two hydrogen atoms in acetylene.
 
Dividing the subscripts by 2, we obtain the empirical formula CH.
 
 
Example
​Glucose (C6H12O6), a substance known as blood sugar, is ________.
  1. CH2O
  2. C2HO2
  3. CHO
  4. C2H2O2
Solution
Ans. A
 
In glucose there are 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms.
 
Dividing the subscripts by 6 we obtain the empirical formula CH2O.
 
 
Example
​Nitrous oxide that is used as an anaesthetic (‘laughing gas’) and an aerosol propellant for whipped creams is________.
  1. N2O2
  2. NO2
  3. N2O
  4. NO
Solution
Ans. C
 
As the subscript in N2O is already the smallest possible whole number, the empirical formula for nitrous oxide is the same as its molecular formula.
 
  
Example-2
The formula of magnesium nitride containing Mg2+ and N3- ions is ________.
  1. MgN3
  2. Mg3N2
  3. Mg2N3
  4. Mg3N
Solution
Ans. B
 
To satisfy electrical neutrality, the following relationship should hold:
(+2) x + (-3)y = 0

Solving, we obtain x/y = 3/2.
 
Substituting x = 3 and y = 2, we write
 

Check: The subscripts are reduced to the smallest whole number ratio of the atoms because the chemical formula of an ionic compound is usually its empirical formula.
 
Molecular mass of water = 18
 
Gram molecular mass of water = 18 g
 
 
Strategy: Our guide for writing formulas for ionic compounds is electrical neutrality, i.e. the total charge on the cation(s) must be equal to the total charge on the anion(s). Because the charges on Mg2+ and N3– ions are not equal, we know the formula cannot be MgN. Instead we write the formula as MgxNy, where x and y are the subscripts to be determined.
 
Example-3
What is the relative mass formula of hydrogen gas? [Relative atomic mass: H = 1.]
  1. 4
  2. 1
  3. 2
  4. 3
Solution
Ans. C
 
The formula for hydrogen gas is H2.
 
Each molecule contains 2 hydrogen atoms.
 
The relative mass formula of hydrogen gas is Mr(H2) = 2 × Ar(H) = 2 × 1 = 2.
 
 
Example-4
What percentage of the mass of ammonium nitrate is nitrogen? [Ammonium nitrate formula: NH4NO3. Relative atomic masses: H = 1, O = 16, N = 14.]
  1. 65
  2. 5
  3. 60
  4. 35
Solution
Ans. D
 
Relative molecular mass = (2 × 14) + (4 × 1) + (3 × 16)
 
= 28 + 4 + 48 = 80
 
Mass of nitrogen in the formula = 28
 
Mass of nitrogen as a fraction of the total mass Description: 28514.png
 
Percentage of nitrogen Description: 28505.png × 100 = 35%
 
 
Example-5
What percentage of the mass of magnesium sulphate heptahydrate is water? [Magnesium sulphate heptahydrate formula: MgSO4 · 7H2O. Relative atomic masses: H = 1, O = 16, S = 32, Mg = 24.]
  1. 50.0
  2. 51.21
  3. 53.46
  4. 47.30
Solution
Ans. B
 
Relative molecular mass = 24 + 32 + (4 × 16) + (7 × 18) = 246
 
Mass of water in the formula = 18 × 7 = 126
 
Mass of water as a fraction of the total Description: 28497.png
 
Mass of water as percentage of total mass Description: 28488.png × 100 = 51.21
 
 
Example-6
A signature written in carbon pencil weighs 1 mg. Determine the number of carbon atoms present in the signature.
  1. 50.2 × 1023
  2. 5.02 × 1020
  3. 0.502 × 1019 
  4. 5.02 × 1019
Solution
Ans. D
 
Amount of carbon present in the signature = 1 mg = 0.001 g
 
Number of gram atoms of carbon in 0.01 g Description: 28481.png
 
Number of carbon atoms in 0.001/12 g atoms of Description: 28472.png
 
= (5.02 × 1019) = 0.502 × 1020 = (5.02 × 1019)
 
 
Example-7
The number of moles of sulphur dioxide in 32 g of SO2 gas is _________. [Atomic masses: S = 32, O = 16.]
  1. 1
  2. 2
  3. 0.5
  4. 3.2
Solution
Ans. C
 
Molecular mass of SO2 is 32 × 1 + 16 × 2 = 32 + 32 = 64
 
64 g of SO2 = 1 mol of SO2
 
 32 g of SO2 = Description: 28463.png = 0.5 mol

 
 
Example-8
The mass of 3 mol of methane is _________. [Atomic masses: C = 12, H = 1.]
  1. 16 g
  2. 48 g
  3. 24 g
  4. 32 g
Solution
Ans. B
 
Molecular formula of methane is CH4
 
Molecular mass of methane = (1 × 12) + (4 × 1) = 16
 
Mass of 1 mol of CH4 = 16 g
 
Mass of 3 mol of CH4 = 48 g
 
 
Example-9
The percentage of Mg in MgSO4 is _________. [Atomic masses: Mg = 24, S = 32, O = 16.]
  1. 20%
  2. 40%
  3. 24%
  4. 32%
Solution
Ans. A
 
Molecular mass = (1 × 24) + (1 × 32) + (4 × 16)
 
= 24 + 32 + 64
 
= 120
 
Percentage of Mg Description: 28456.png
 
 
Example-10
What is the percentage yield if on heating 50 g of limestone (CaCO3), 21 g of lime (CaO) was obtained?
  1. 25%
  2. 50%
  3. 75%
  4. 93%
Solution
Ans. C
 
Calcium carbonate Description: 32766.jpg Calcium oxide + Carbon dioxide
 
 
Mole ratio = Description: 28449.png
 
No. of moles CaO = 1 × No. of moles CaCO3
 
No. of moles CaO = Description: 28441.png
 
No. of moles CaO = Description: 28430.png = 0.5
 
Theoretically, 0.5 mol or (0.5 × 56) = 28 g of CaO is produced when 50 g of CaCO3 is heated.
 
Percentage yield Description: 28422.png
 
Description: 28412.png
 

 





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