Question

Using the given formula of aluminum oxide, Al2O3, refer to the periodic table and predict the empirical formula for the oxide, B?O?

Answer 1

aluminium is present in period 3 and group 13 ,so atomic number is 13, so the valence electron is 3

electronic configuration is 1s²2s²2p⁶3s²3p¹ so it will exhibit valency of +3 after the loss of 3 valence electron

oxygen is present in period 2 and group 16(group 6) ,so atomic number is 6, so the valence electron is 6

electronic configuration is 1s²2s²2p⁴ so it will exhibit valency of -2  after the gain of 2 in valence shell to attain stability

boron is present in same group of aluminium i.e. group 13(groupIII) ,so atomic number is 5, so the valence electron is 3

electronic configuration is 1s²2s²2p¹ so it will exhibit valency of +3 after the loss of 3 valence electron to attain stability

therefore the chemical formula between boron and oxygen would be B₂O₃ [charges interchange their position]

ii) The balanced equation between chromium metal and sulphur is

2Cr +3S$\rightarrow$ Cr₂S₃

thus we see 2 moles of chromium reacts with 3 moles of sulphur to produce 1 mole of chromium sulphide

1 mole of any substance = molecular mass of that substance

from literature molecular mass of chromium sulphide is 200 g

atomic mass of sulphur is 32 gm

moles of chromium sulphide =mass of product/molecular mass of chromium sulphide

moles of chromium sulphide=0.963g/200g = 4.815 *10^-3 mole

1mole of chromium sulphide produced by 3 moles of S

4.815 *10^-3 mole of chromium sulphide produced by 3 *4.815 *10^-3moles of S

4.815 *10^-3 mole of chromium sulphide produced by 0.0144 moles of S

1mole of S =32 g

0.0144 moles of sulphur = 0.0144 *32 g =0.4622 g of S

0.4622 g of S has reacted to produce 0.963 g of product

ii) theemperical formula of chromium sulphide is Cr₂S_{3 because}

sulphur is present in period 3 and group 16(group 6) ,so atomic number is 16, so the valence electron is 6

electronic configuration is 1s²2s²2p⁶3s²3p⁴so it will exhibit valency of -2  after the gain of 2 in valence shell to attain stability

chromium is present in period 4 , atomic number is 24,

electronic configuration is1s²2s²2p⁶3s²3p⁶4s²3d⁶ so it will exhibit valency of +3 after the loss  of 3 in valence shell to attain stability as the d orbital will be half filled 1s²2s²2p⁶3s²3p⁶3d⁵

Cr⁺³:S^2- =Cr₂S₃ [charges interchange their position]