Mole ConceptHard
Question
A sample of pure Cu (4.00 g) heated in a stream of oxygen for some time, gains in weight with the formation of black oxide of copper (CuO). The final mass is 4.90 g. What percent of copper remains unoxidized?
(Cu = 64)
Options
A.90%
B.10%
C.20%
D.80%
Solution
Cu → CuO
64 g 80 g
$\therefore xg \frac{80}{64} \times x = 1.25x\text{ g}$
Increase in mass, 1.25x – x = 4.90 – 4.00 ⇒ x = 3.6
∴ Percentage of copper unoxidized $= \frac{4 - 3.6}{4} \times 100 = 10\%$
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