Chapter 03 Atoms and Molecules Exercise
Exercises
1. A $0.24 g$ sample of compound of oxygen and boron was found by analysis to contain $0.096 g$ of boron and $0.144 g$ of oxygen. Calculate the percentage composition of the compound by weight.
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Answer
Mass of the compound $=0.24 g$
Mass of boron $=0.096 g$
Mass of oxygen $=0.144 g$
$\text{Percentage of boron } = \frac{\text{Mass of boron}}{\text{Mass of compound}}\times 100 = \frac{0.096 g}{0.240 g }\times 100 = 40 \%$
$\text{Percentage of oxygen } = \frac{\text{Mass of oxygen}}{\text{Mass of compound}}\times 100 = \frac{0.144 g}{0.240 g }\times 100 = 60 \%$
Alternative method
Percentage of oxygen $=100 -$ percentage of boron
$ =100-40=60 \% $
2. When $3.0 g$ of carbon is burnt in $8.00 g$ oxygen, $11.00 g$ of carbon dioxide is produced. What mass of carbon dioxide will be formed when $3.00 g$ of carbon is burnt in $50.00 g$ of oxygen? Which law of chemical combination will govern your answer?
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Answer
First we find the proportion of mass of carbon and oxygen in carbon dioxide.
In $CO 2, C: O=12: 32$ or $3: 8$
In other words, we can say that
$12.00 g$ carbon reacts with oxygen $=32.00 g$
$3.00 g$ carbon will react with oxygen $=32 \times 3=8 g$
Therefore, $3.00 g$ of carbon will always react with $8.00 g$ of oxygen to form CO2 $(11 g)$, even if large amount $(50.00 g)$ of oxygen is present. This answer will be governed by ’the law of constant proportions'.
3. What are polyatomic ions? Give examples.
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Answer
The group of atoms which carry a fixed charge (either positive or negative) on them and behave as ions are called polyatomic ions.
Example
(i) Carbonate ion
(ii) Sulphate ion
(iii) Ammonium ion
(iv) Phosphate ion
4. Write the chemical formulae of the following.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
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Answer
(i) Formula $=MgCl_2$ (Magnesium chloride)
(ii) Formula $=CaO$ (Calcium oxide)
(iii) Formula $=Cu(NO_3)_2$ (Copper nitrate)
(iv) Formula $=AICl_3$ (Aluminium Chloride)
(v) Formula $=CaCO_3$ (Calcium carbonate)
5. Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
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Answer
(a) Quick lime- Calcium oxide - $CaO$
Elements-Calcium, oxygen.
(b) Hydrogen bromide- $HBr$
Elements- Hydrogen, bromine.
(c) Baking powder-Sodium hydrogen carbonate - $NaHCO_3$
Elements- Sodium, hydrogen, carbon, oxygen.
(d) Potassium sulphate- $K_2 SO_4$
Elements- Potassium, sulphur, oxygen.
6. Calculate the molar mass of the following substances.
(a) Ethyne, $C_2 H_2$
(b) Sulphur molecule, $S_8$
(c) Phosphorus molecule, $P_4$ (Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, $HCl$
(e) Nitric acid, $HNO_3$
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Answer
(a) Molar mass of $C_2 H_2$
$=(2 \times$ Atomic mass of $C)+(2 \times$ Atomic mass of $H)$
$=(2 \times 12)+(2 \times 1)$
$=26 u$
(b) Molar mass of $S_8$
$=(8 \times$ Atomic mass of $S)$
$=8 \times 32=256 u$
(c) Molar mass of $P_4$
$=4 \times$ Atomic mass of $P$
$=4 \times 31=124 u$
(d) Molar mass of $HCl$
$=$ Atomic mass of hydrogen + Atomic mass of $Cl$
$=1+35.5=36.5 u$
(e) Molar mass of $HNO_3$
$=$ Atomic mass of $H+$ Atomic mass of $N+(3 \times \text{ Atomic mass of } 0)$
$=1+14+(3 \times 16)=15+48=63 u$
