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Phosphorus burns in oxygen according to the equation P\(_4\) + 5 0\(_2\) → P\(_4\)O\(_{10}\). How...

Chemistry
JAMB 1979

Phosphorus burns in oxygen according to the equation P\(_4\) + 5 0\(_2\) → P\(_4\)O\(_{10}\). How many litres of oxygen will be required at S.T.P for complete oxidation of 12.4g of phosphorus? [P = 31,O = 16 and molar volume of a gas at S.T.P = 22.4 litres]

  • A. 5. 20
  • B. 11. 20
  • C. 2. 24
  • D. 20. 20
  • E. 6. 20
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Correct Answer: Option B
Explanation

P\(_4\)  + 50\(_2\) → P\(_4\)O\(_{10}\)

for the complete combustion of 124 grammes of phosphorus we need

5 x 22.4 litres of 0\(_2\)

for 12.4 gms of P\(_4\)  we need

\(\frac{5 \times 22.4}{124} \times \frac{12.4}{1}\)

\(\frac{22.4}{2}\) = 11.2 litres

 

Alternatively, 

                            P\(_4\)    +    5 0\(_2\)  →  P\(_4\)O\(_{10}\)

                            1 mole  :   5 moles

 Mass of P\(_4\)  = 12.4

Molar mass of P\(_4\)  = (31 x 4)=124g/mol

                    n = \(\frac{mass}{Molar mass}\)

                       = \(\frac{12.4}{124}\)

                       = 0.1 mole

Then               0.1 mole P\(_4\)  will react with   0.5 mole 0\(_2\)

But                                                              n = \(\frac{Volume}{Molar Volume}\)

                                                                  0.5 = \(\frac{Volume}{22.4}\)

                                                                   V = 0.5 X 22.4

                                                                    V = 11.20 Litres


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