CAIE IGCSE Chemistry 6.4.9 Redox using oxidation numbers: (a)

Practise assigning oxidation numbers in elements, ions and compounds, then using changes to identify oxidation or reduction.

Exam points

apply rules for uncombined elements, monatomic ions, neutral compounds and ions

[Maximum number: 2]

Ethanol is manufactured by two methods:
method 1 fermentation of aqueous glucose
method 2 catalytic addition of steam to an alkene.

The catalyst in method 2 is phosphoric acid, $\mathrm{H}_{3} \mathrm{PO}_{4}$ . Dilute phosphoric acid is a weak acid which contains phosphate ions, $\mathrm{PO}_{4}{ }^{3-}$ .

Determine the oxidation number of phosphorus in the $\mathrm{PO}_{4}{ }^{3-}$ ion.

Show your working.
oxidation number =

$\text { M1 } 4 \times-2 \text { or }-8$

M2 $\mathrm{P}+(4 \times-2)=-3 \therefore \mathrm{P}=+5 \quad$ (1)

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6.4.9—Redox using oxidation numbers: (a) question 1