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Doctors Medicines MCQs Q&A

(i) What do the following symbols / formulae stand for: 

(a) 2O      (b) O2       (c) O3       (d) H2O 

(ii) Give the chemical formula of the following compounds: 

(a) Potassium carbonate (b) Calcium chloride 

(iii) Calculate the formula unit mass of Al2(SO4)3. 

(Given: Atomic mass of Al = 27 u, S = 32 u, O = 16 u) 

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2 Answers

(i) (a) 2O = 2 atoms of oxygen 

(b) O2 = Diatomic oxygen 

(c) O3 = Triatomic oxygen / ozone 

(d) H2O=Two atoms of hydrogen and one atom of oxygen construct one molecule of water (H2O) 

(ii) Potassium carbonate - K2CO3 

Calcium chloride - CaCl2 

(iii) Formula unit mass of any compound is the sum of atomic masses of all atoms in the formula of that compound. 

Formula unit mass of Al2(SO4)3 

Atomic mass of Al = 27 × 2 = 54 u 

Atomic mass of S = 32 × 3 = 96 u 

Atomic mass of O = 16 × 12 = 192 u 

Formula unit mass = 54 + 96 + 192 = 342 u. 

Hence, Formula unit mass of Al2(SO4)3 is 342.

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2O represent 2 separate atom while O2 represent 1 molecules of oxygen gas containing 2 atoms

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