Question

3(a)(i) Define the term Avogadro's number.
(ii) If 2.30g of an oxide of nitrogen, x, contains 3.01×10²² molecules, calculate the molar mass of x.
(iii) Deduce the formula of x. [NA=6.02×10²³, N=14.0, O=16.0]
(b)(i) Describe briefly what happens when each of the following substances are added to water: (I) CCl4; (II) SiCl4
(ii) Explain briefly why the reactions in 3(b)(i)(I) and 3(b)(i)(II) are different.

Accepted Answer

(a)(i) Avogadro's number is defined as the number of particles (atoms, molecules, ions etc.) present in one mole of a substance. It is also the number of particles present in 12.0g of carbon-12. (Avogadro's number = 6.02×10²³ mol⁻¹.)

(ii) Number of moles of x = 3.01×10²²/6.02×10²³ = 0.05 mol.
Molar mass = mass/moles = 2.30/0.05 = 46 g/mol.

(iii) Let the formula of the oxide be NxOy.
mm(NxOy) = 14x + 16y = 46.
By inspection: x=1, y=2: 14(1)+16(2)=14+32=46. ✓
So the formula of the oxide is NO2.

(b)(i)(I) CCl4 added to water: No visible reaction. Two immiscible layers of liquid are formed; CCl4 forms a layer below the water (denser, non-polar solvent, does not dissolve in the polar water).
(II) SiCl4 added to water: There is a visible reaction; hydrolysis reaction with water occurs to give a steamy fume (giving off HCl gas), and the solution becomes hot (denser due to dissolved products).

(ii) This is because the central atom in CCl4 (carbon) does not have a vacant d-orbital to accept a lone pair of electrons from water to undergo hydrolysis. CCl4 is thus non-polar and cannot be dissolved by water, a polar solvent. In SiCl4, the central atom Si has a vacant d-orbital that can accept a lone pair of electrons from water; thus, SiCl4 is said to be polar and is hydrolysed by water.

WAEC Chemistry 2022 Theory — Question 4

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