(a)(i) List the three types of particles present in atoms.
(ii) name the element which does not contain all the three particles in its atom. Mention the particle that is not present.
(b) Give the reason why:
(i) the relative atomic masses of some elements are not whole number;
(ii) relative atomic masses are used instead of the actual masses of atoms in grams;
(iii) metals are good conductors of electricity.
(c)(i) Name the type of bond present in the oxonium ion,
(ii) State one effect of the existence of intermolecular hydrogen bonding on the physical properties of ethanol.
(d)(i) Explain what is meant by water of crystallization.
(ii) When 5.0g of a compound Y was heated to constant mass, 1.8g of water vapour was given off. Determine the number of molecules of water of crystallization in one molecule of Y, given that the molar mass of its anhydrous form is 160g. [H = 1, 0 = 16]
(a)(i) Three types of particles in atoms
- Protons (positively charged, in the nucleus)
- Neutrons (uncharged, in the nucleus)
- Electrons (negatively charged, in the shells)
(a)(ii) The element is hydrogen (the ordinary isotope, \(^{1}_{1}H\)). The particle absent is the neutron (its atom has 1 proton and 1 electron only).
(b) Reasons
- (i) The relative atomic masses of some elements are not whole numbers because such elements exist as a mixture of isotopes; the relative atomic mass is the weighted average of the masses of the isotopes according to their relative abundances.
- (ii) Relative atomic masses are used instead of actual masses in grams because the actual mass of a single atom is extremely small (of the order of \(10^{-23}\) g) and inconvenient to handle, whereas relative values are simple comparative numbers.
- (iii) Metals are good conductors of electricity because they contain mobile (delocalised) free electrons that drift through the metallic lattice and carry charge.
(c)(i) The oxonium ion \(H_3O^+\) contains ordinary covalent bonds and one coordinate (dative) covalent bond (the oxygen lone pair is donated to the extra proton).
(c)(ii) Intermolecular hydrogen bonding raises the boiling point of ethanol (it boils much higher than expected for its molecular mass because extra energy is needed to break the hydrogen bonds). It also makes ethanol fully miscible with water.
(d)(i) Water of crystallisation is the fixed number of water molecules chemically combined with one formula unit of a salt in its crystal structure.
(d)(ii) Mass of anhydrous salt \(= 5.0 - 1.8 = 3.2\) g.
\[n(\text{anhydrous}) = \frac{3.2}{160} = 0.02\ \text{mol}\]\[n(H_2O) = \frac{1.8}{18} = 0.10\ \text{mol}\]\[\text{ratio } H_2O : \text{salt} = \frac{0.10}{0.02} = 5\]
Therefore there are 5 molecules of water of crystallisation in one molecule of Y.
(a)(i) Three types of particles in atoms
- Protons (positively charged, in the nucleus)
- Neutrons (uncharged, in the nucleus)
- Electrons (negatively charged, in the shells)
(a)(ii) The element is hydrogen (the ordinary isotope, \(^{1}_{1}H\)). The particle absent is the neutron (its atom has 1 proton and 1 electron only).
(b) Reasons
- (i) The relative atomic masses of some elements are not whole numbers because such elements exist as a mixture of isotopes; the relative atomic mass is the weighted average of the masses of the isotopes according to their relative abundances.
- (ii) Relative atomic masses are used instead of actual masses in grams because the actual mass of a single atom is extremely small (of the order of \(10^{-23}\) g) and inconvenient to handle, whereas relative values are simple comparative numbers.
- (iii) Metals are good conductors of electricity because they contain mobile (delocalised) free electrons that drift through the metallic lattice and carry charge.
(c)(i) The oxonium ion \(H_3O^+\) contains ordinary covalent bonds and one coordinate (dative) covalent bond (the oxygen lone pair is donated to the extra proton).
(c)(ii) Intermolecular hydrogen bonding raises the boiling point of ethanol (it boils much higher than expected for its molecular mass because extra energy is needed to break the hydrogen bonds). It also makes ethanol fully miscible with water.
(d)(i) Water of crystallisation is the fixed number of water molecules chemically combined with one formula unit of a salt in its crystal structure.
(d)(ii) Mass of anhydrous salt \(= 5.0 - 1.8 = 3.2\) g.
\[n(\text{anhydrous}) = \frac{3.2}{160} = 0.02\ \text{mol}\]\[n(H_2O) = \frac{1.8}{18} = 0.10\ \text{mol}\]\[\text{ratio } H_2O : \text{salt} = \frac{0.10}{0.02} = 5\]
Therefore there are 5 molecules of water of crystallisation in one molecule of Y.
