Question

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 (C

Answer 1

Answer:

ANSWER

Milli-moles of KI=50×0.04=2

Milli-equivalents of Na

2

S

2

O

3

reacted with 10 mL solution=20×2.5×10

−3

=50×10

−3

Milli-equivalent of I

−

left unreacted in 100 mL solution=50×10

−3

×

10

100

=0.5

Milli-moles of I

−

reacted with CuCO

3

=2−0.5=1.5

∵2Cu

2+

+5I

−

→2CuI+I

3

−

Milli-moles of Cu

2+

=

5

2

×1.5=0.6

Weight of CuCO

3

=0.6×10

−3

×123.5=0.0741

Weight % of CuO

3

=

0.1

0.0741

×100=74.1

Answer 2

Given:

The mass of pure copper = 4 gm

The final mass = 4.9 gm

To Find:

The percentage of copper that remains unoxidized.

Calculation:

- Let the mass of oxidised Cu be x gm

⇒ The mass of unoxidised Cu = (4 - x) gm

- The mass of CuO formed = {4.9 - (4 - x)} gm ....(i)

- The oxidation of copper is given as:

Cu + O → CuO

⇒ The mass of CuO produced by 63.5 gm of Cu = 79.5 gm

⇒ The mass of CuO produced by x gm of Cu = {(79.5/63.5) × x} gm ....(ii)

- From (i) and (ii), we have:

{4.9 - (4 - x)} = {(79.5/63.5) × x}

⇒ 0.9 + x = 1.252 x

⇒ 1.252 x - x = 0.9

⇒ 0.252 x = 0.9

⇒ x = 0.9 / 0.252

⇒ x = 3.57 gm

- So, the mass of unoxidised Cu = (4 - x) = 4 - 3.57 = 0.43 gm

- The percentage of unoxidised Cu = (0.43/4) × 100 = 10.75 %

- So, 10.75 % of copper remains unoxidized.