A metal oxide has `40%` oxygen. The equivalent weight of the metal is:

Explanation

To find the equivalent weight of the metal in the metal oxide, we can follow these steps:

### Step 1: Determine the percentage of the metal
Given that the metal oxide has 40% oxygen, we can find the percentage of the metal by subtracting the percentage of oxygen from 100%.

\[ \text{Percentage of metal} = 100\% - \text{Percentage of oxygen} = 100\% - 40\% = 60\% \]

### Step 2: Assume a mass for the metal oxide
To simplify calculations, we can assume the mass of the metal oxide to be 100 grams. This assumption makes it easier to work with percentages.

\[ \text{Mass of metal oxide} = 100 \text{ grams} \]

### Step 3: Calculate the mass of the metal
Using the percentage of metal calculated in Step 1, we can find the mass of the metal in the assumed 100 grams of metal oxide.

\[ \text{Mass of metal} = \frac{60}{100} \times 100 \text{ grams} = 60 \text{ grams} \]

### Step 4: Calculate the mass of oxygen
Similarly, we can calculate the mass of oxygen in the metal oxide.

\[ \text{Mass of oxygen} = \frac{40}{100} \times 100 \text{ grams} = 40 \text{ grams} \]

### Step 5: Determine the equivalent weight of oxygen
The equivalent weight of oxygen is known to be 8 grams (since the molar mass of oxygen is 16 grams and it is diatomic).

\[ \text{Equivalent weight of oxygen} = 8 \text{ grams} \]

### Step 6: Calculate the equivalent weight of the metal
Now we can use the formula for equivalent weight of the metal:

\[ \text{Equivalent weight of metal} = \frac{\text{Mass of metal}}{\text{Mass of oxygen}} \times \text{Equivalent weight of oxygen} \]

Substituting the values we found:

\[ \text{Equivalent weight of metal} = \frac{60 \text{ grams}}{40 \text{ grams}} \times 8 \text{ grams} \]

\[ \text{Equivalent weight of metal} = 1.5 \times 8 = 12 \text{ grams} \]

### Final Answer:
The equivalent weight of the metal is **12 grams**.

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