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Practice each question, then open the answer or worked solution.
Raunak took 5 moles of carbon atoms in a container and Krish also took 5 moles of sodium atoms in another container of same weight.
(a) Whose container is heavier?
(b) Whose container has more number of atoms?
a) Mass of sodium atoms carried by Krish = (5 ×23) g = 115 g
The atomic weight of Na = 23
While the mass of carbon atom carried by Raunak = (5 ×12) g = 60g
b) Both container has an equal number of atoms
Fill in the missing data in the Table 3.1
| Species property | H2O | CO2 | Na atom | MgCl2 |
| No of Moles | 2 | – | – | 0.5 |
| No of particles | – | 3.011×1023 | – | 0 |
| Mass | 36g | – | 115g | 0 |
| Species property | H2O | CO2 | Na atom | MgCl2 |
| No of Moles | 2 | 0.5 | 5 | 0.5 |
| No of particles | 12.044×1023 | 3.011×1023 | 3.011×1023 | 3.011×1023 |
| Mass | 36g | 22g | 115g | 47.5g |
The visible universe is estimated to contain 1022 stars. How many moles are present in the visible universe?
Number of moles of stars = 1022 / 6.023 × 1023
Number of moles of stars = 0.0166
What is the SI prefix for each of the following multiples and submultiples of a unit?
(a) 103
(b) 10–1
(c) 10–2
(d) 10–6
(e) 10–9
(f) 10–12
a) 103 = 1000= kilo
(b) 10–1 =1/10= 0.1= deci
(c) 10–2 =1/100 = 0.01= centi
(d) 10–6 = 0.000 001= micro
(e) 10–9 =0.000 000 001 = nano
(f) 10–12 =0.000 000 000 001 = pico
Express each of the following in kilograms
(a) 5.84×10-3 mg
(b) 58.34 g
(c) 0.584g
(d) 5.873×10-21g
(a) 5.84 × 10–3 mg = 5.84 ×10–9 kg
(b) 58.34 g =5.834 ×10–2 kg
(c) 0.584g =5.84 ×10–4 kg
(d) 5.873×10-21g=5.873 ×10–24 kg
Compute the difference in masses of 103 moles each of magnesium atoms and magnesium ions. (Mass of an electron = 9.1×10–31 kg)
Mg2+ ion and Mg atom differ by two electrons.
103 moles of Mg2+ and Mg atoms would differ by
103 × 2 moles of electrons
Mass of 2 ×103 moles of electrons = 2×103 × 6.023 ×1023 × 9.1 ×10–31 kg
2×6.022 × 9.1×10–5kg
109.6004 ×10–5 kg
1.096 × 10–3kg
Which has more number of atoms? 100g of N2 or 100 g of NH3
No. of moles of atoms = weight / atomic weight.
For N2
100 gms of N2 = 100/2 x 14 moles = 100/28 moles
Number of molecules = 100 / 28 x 6.022 x 10²³
Molar mass of N2 = 2 x molar mass of monoatomic N
Molar mass of N2 = 2 x 14.0067 = 28 moles.
Number of molecules = 100/28 x 6.022 x 10²³
No. of atoms = 2 x 100/28 x 6.022 x 10²³ = 43.01 x 10²³
For NH3
100 gm of NH3 = 100/17 moles
Number of molecules = 100/17 x 6.022 x 10²³ molecules
No. of atoms in NH3 = (1 + 3) = 4 x 100/17 x 6.022 x 10²³ =
141.69 x 10²³ atoms.
Therefore, NH3 has more atoms than N2.
Compute the number of ions present in 5.85 g of sodium chloride.
58.5 g NaCl contains 6.023 x 1023 molecules
therefore 58.5 g NaCl contains 12.046 x 1023 ions.
Hence, 5.85 g NaCl contains 5.85 x 12.046 x 1023/ 58.5
= 1.2046 x 1023 ions
A gold sample of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold ?
one gram of gold sample will contain 90/100 = 0.9g of gold
number of moles of gold = mass of gold / atomic mass of gold
= 0.9/19.7 = 0.0046
one mole of gold contains NA atoms = 6.022 × 1023
Therefore, 0.0046 mole of gold will contain = 0.0046 × 6.022
= 2.77 × 1021
What are ionic and molecular compounds? Give examples.
While forming some compounds, atoms gain or lose electrons, and form electrically charged particles called ions. Compounds that are formed by the attraction of cations and anions are called as ionic compounds.
Ex : 2Na + Cl2 → 2Na+ Cl- → 2NaCl (sodium chloride- common salt.)
Sodium is a group 1 metal, thus forms a +1 charged cation. Chlorine is a non-metal, and has the ability to form a -1 charged anion.
Compounds formed due to bonding of uncharged ions are called as molecular compounds and the bonding between them is called covalent bonding. Molecular compounds are formed by sharing of electrons between the two atoms and the elements are held together by covalent bonds.
Ex: 2C + O2 → 2CO ( Carbon monoxide)