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 %`

Answer

To solve the problem step by step, we need to determine how much copper remains unoxidized after the reaction with oxygen. Here’s how we can approach it:

### Step 1: Determine the initial mass of copper
The initial mass of copper (Cu) is given as 4.00 g.

### Step 2: Determine the final mass after oxidation
The final mass after the reaction is given as 4.90 g.

### Step 3: Calculate the mass gained during the reaction
The mass gained during the reaction can be calculated as:
\[ \text{Mass gained} = \text{Final mass} - \text{Initial mass} = 4.90 \, \text{g} - 4.00 \, \text{g} = 0.90 \, \text{g} \]

### Step 4: Determine the mass of copper that reacted
Let \( n \) be the mass of copper that reacted. The mass of copper remaining unreacted will then be:
\[ \text{Mass of unreacted copper} = 4.00 \, \text{g} - n \]

### Step 5: Write the reaction equation
The reaction of copper with oxygen to form copper(II) oxide (CuO) can be represented as:
\[ 2 \, \text{Cu} + \text{O}_2 \rightarrow 2 \, \text{CuO} \]
From the stoichiometry of the reaction, we know that:
- 2 moles of Cu (128 g) react with 1 mole of O2 (32 g) to produce 2 moles of CuO (160 g).

### Step 6: Relate the mass gain to the mass of copper that reacted
From the reaction, we can set up a proportion based on the mass gain:
\[ \text{Mass of CuO formed} = \text{Mass of O2 used} = \frac{160}{128} \times n \]
Since the mass gained is equal to the mass of oxygen that reacted, we have:
\[ 0.90 \, \text{g} = \frac{160}{128} \times n \]

### Step 7: Solve for \( n \)
Rearranging the equation gives:
\[ n = \frac{0.90 \times 128}{160} = 0.72 \, \text{g} \]

### Step 8: Calculate the mass of unreacted copper
Now, we can find the mass of unreacted copper:
\[ \text{Mass of unreacted copper} = 4.00 \, \text{g} - 0.72 \, \text{g} = 3.28 \, \text{g} \]

### Step 9: Calculate the percentage of unreacted copper
The percentage of copper that remains unreacted is calculated as:
\[ \text{Percentage of unreacted copper} = \left( \frac{3.28 \, \text{g}}{4.00 \, \text{g}} \right) \times 100 = 82.0\% \]

### Final Answer
Thus, the percentage of copper that remains unoxidized is approximately **82.0%**.

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