Chapter 7 The p Block Elements Solutions
Question - 11 : - What is the basicity of H3PO4?
Answer - 11 : -
H3PO4
Since there are three OH groupspresent in H3PO4, its basicity is three i.e., it is a tribasicacid.
Question - 12 : - What happens when H3PO3 is heated?
Answer - 12 : -
H3PO3, on heating,undergoes disproportionation reaction to form PH3 and H3PO4. The oxidation numbers of P in H3PO3, PH3, and H3PO4 are +3, −3, and +5 respectively. As theoxidation number of the same element is decreasing and increasing during aparticular reaction, the reaction is a disproportionation reaction.
Question - 13 : - List the important sources ofsulphur.
Answer - 13 : -
Sulphur mainly exists in combinedform in the earth’s crust primarily as sulphates [gypsum (CaSO4.2H2O), Epsom salt (MgSO4.7H2O), baryte (BaSO4)] and sulphides [(galena (PbS), zinc blends(ZnS), copper pyrites (CuFeS2)].
Question - 14 : - Write the order of thermalstability of the hydrides of Group 16 elements.
Answer - 14 : -
The thermal stability of hydridesdecreases on moving down the group. This is due to a decrease in the bonddissociation enthalpy (H−E) ofhydrides on moving down the group.
Therefore,
Question - 15 : - Why is H2O a liquid and H2S a gas?
Answer - 15 : -
H2O has oxygen as the central atom. Oxygen hassmaller size and higher electronegativity as compared to sulphur. Therefore,there is extensive hydrogen bonding in H2O, which is absent in H2S. Molecules of H2S are held together only by weak van der Waal’sforces of attraction.
Hence, H2Oexists as a liquid while H2Sas a gas.
Question - 16 : - Which of the following does notreact with oxygen directly?
Zn, Ti,Pt, Fe
Answer - 16 : -
Pt is a noble metal and does notreact very easily. All other elements, Zn, Ti, Fe, are quite reactive. Hence, oxygen does not react with platinum (Pt) directly.
Question - 17 : - Complete the following reactions:
Answer - 17 : -
(i) C_2H_4 + O_2 →
(ii) 4Al + 3O_2 →
Answer
i 
ii 
Question - 18 : - Why does O3 act as a powerful oxidising agent?
Answer - 18 : -
Ozone is not a very stablecompound under normal conditions and decomposes readily on heating to give amolecule of oxygen and nascent oxygen. Nascent oxygen, being a free radical, is very reactive.
Therefore, ozone acts as apowerful oxidising agent.
Question - 19 : - How is O3 estimated quantitatively?
Answer - 19 : -
Quantitatively, ozone can beestimated with the help of potassium iodide. When ozone is made to react withpotassium iodide solution buffered with a borate buffer (pH 9.2), iodine isliberated. This liberated iodine can be titrated against a standard solution ofsodium thiosulphate using starch as an indicator. The reactions involved in theprocess are given below.
Question - 20 : - What happens when sulphur dioxideis passed through an aqueous solution of Fe(III) salt?
Answer - 20 : -
SO2 acts as a reducing agent when passedthrough an aqueous solution containing Fe(III) salt. It reduces Fe(III) toFe(II) i.e., ferric ions to ferrous ions.
