A sample of 15.8 g of potassium permanganate is heated at 513 K until it decomposes completely. What mass of oxygen gas is released? (Atomic masses: K = 39 u, Mn = 55 u, O = 16 u)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 1.6 g
When heated, potassium permanganate decomposes at 513 K into potassium manganate, manganese dioxide and oxygen. Two formula units of KMnO4 give only one molecule of O2:
\( \displaystyle \ce{2KMnO4} \xrightarrow{\text{513 K}} \ce{K2MnO4 + MnO2 + O2} \)
\( \displaystyle M(\ce{KMnO4}) = 39 + 55 + 4 \times 16 = 158\ \mathrm{g\ mol^{-1}} \)
\( \displaystyle n(\ce{KMnO4}) = \frac{15.8\ \mathrm{g}}{158\ \mathrm{g\ mol^{-1}}} = 0.100\ \mathrm{mol} \)
\( \displaystyle n(\ce{O2}) = \frac{0.100\ \mathrm{mol}}{2} = 0.050\ \mathrm{mol} \)
\( \displaystyle m(\ce{O2}) = 0.050\ \mathrm{mol} \times 32\ \mathrm{g\ mol^{-1}} = 1.6\ \mathrm{g} \)
Of the eight oxygen atoms in two KMnO4 units, six stay in K2MnO4 and MnO2; only two leave as O2.
NCERT (2026-27 edition):
4.4.1 Oxides and Oxoanions of Metals – “Potassium permanganate forms dark purple (almost black) crystals which are isostructural with those of KClO4. The salt is not very soluble in water (6.4 g/100 g of water at 293 K), but when heated it decomposes at 513 K.”
“2KMnO4 → K2MnO4 + MnO2 + O2”