Home / iGCSE Chemistry (0620) Theory (Core):10.3 Air quality and climate: Exam Style Questions Paper 3
This question is about phosphorus and its compounds.
(a) (i) Phosphorus is in Group V of the Periodic Table. Explain why phosphorus is placed in this group. [1]
(a) (ii) Two isotopes of phosphorus are shown in Fig. 3.1.
Complete Table 3.1 to show the number of protons, neutrons and electrons in one atom of these isotopes.
(iii) Phosphorus burns in oxygen to produce phosphorus(V) oxide.
Complete the symbol equation for this reaction.
$$\dots \mathrm{P} + \dots \mathrm{O}_2 \rightarrow \mathrm{P}_4\mathrm{O}_{10}$$
(iv) State whether phosphorus(V) oxide is an acidic oxide or a basic oxide.
Give a reason for your answer.
(b) (i) Phosphorus is one element in NPK fertilisers.
Draw a circle around one other element that is also in NPK fertilisers.
krypton nickel platinum potassium
(b) (ii) State why farmers use fertilisers on fields where crops are grown.
(c) A compound of phosphorus has the formula $\mathrm{Ca_3(PO_4)_2}$.
Complete Table 3.2 to calculate the relative formula mass of $\mathrm{Ca_3(PO_4)_2}$.
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 8.1 — Arrangement of elements (Part (a)(i))
• Topic 2.3 — Isotopes (Part (a)(ii))
• Topic 3.1 — Formulae (Part (a)(iii))
• Topic 7.2 — Oxides (Part (a)(iv))
• Topic 10.2 — Fertilisers (Parts (b)(i), (b)(ii))
• Topic 3.2 — Relative masses of atoms and molecules (Part (c))
(a)(i) Has 5 electrons in its outer shell.
Explanation: The group number for elements in Groups I to VII is determined by the number of valence electrons. A phosphorus atom has the configuration $2,8,5$, so it has 5 electrons in its outermost shell, placing it in Group V.
(a)(ii)
Explanation: The atomic number (bottom number) is 15, which defines the element and equals the number of protons. In a neutral atom, electrons = protons = 15. Neutrons = mass number (top) – atomic number: for $\frac{31}{15}\mathrm{P}$: $31-15 = 16$ neutrons; for $\frac{32}{15}\mathrm{P}$: $32-15 = 17$ neutrons.
(a)(iii) $4\mathrm{P} + 5\mathrm{O}_2 \rightarrow \mathrm{P}_4\mathrm{O}_{10}$
Explanation: The product side shows 4 phosphorus atoms and 10 oxygen atoms. To balance, 4 P atoms are needed on the left. To get 10 oxygen atoms, $5\,\mathrm{O}_2$ molecules are required, as each $\mathrm{O}_2$ molecule contains 2 atoms of oxygen.
(a)(iv) acidic because phosphorus is a non-metal.
Explanation: Metal oxides are basic (e.g., $\mathrm{CaO}$), while non-metal oxides are acidic. Phosphorus(V) oxide reacts with water to form phosphoric acid: $\mathrm{P}_4\mathrm{O}_{10} + 6\mathrm{H}_2\mathrm{O} \to 4\mathrm{H}_3\mathrm{PO}_4$.
(b)(i) potassium
Explanation: NPK fertilisers supply the three primary macronutrients for plants: Nitrogen (N), Phosphorus (P), and Potassium (K).
(b)(ii) to increase crop growth / to replace elements taken from the soil by previous crops.
(c)
Table 3.2: calcium atoms = $3$, so $3 \times 40 = 120$; phosphorus atoms = $2$, so $2 \times 31 = 62$; oxygen atoms = $8$, so $8 \times 16 = 128$.
relative formula mass = $120 + 62 + 128 = 310$