1 amu is

Options

A. `1.66X10^-24` kg
B. `1.66X10^-27` kg
C. `frac{1}{NA}`
D. Both (2) & (3)

Answer

To solve the question "1 amu is", we need to understand what an atomic mass unit (amu) is and how it relates to mass in kilograms and grams.

### Step-by-Step Solution:

1. **Definition of AMU**:
- The atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom (C-12).
- A carbon-12 atom consists of 6 protons and 6 neutrons.

2. **Mass of Carbon-12 Atom**:
- The mass of a carbon-12 atom is approximately 1.994 × 10^-23 grams.

3. **Calculating 1 AMU**:
- To find the mass of 1 amu, we take the mass of the carbon-12 atom and divide it by 12:
\[ 1 \text{ amu} = \frac{1.994 \times 10^{-23} \text{ g}}{12} \]

4. **Performing the Calculation**:
- Performing the division:
\[ 1 \text{ amu} = 1.662 \times 10^{-24} \text{ g} \]

5. **Converting to Kilograms**:
- Since 1 gram = 0.001 kilograms, we can convert grams to kilograms:
\[ 1 \text{ amu} = 1.662 \times 10^{-24} \text{ g} = 1.662 \times 10^{-27} \text{ kg} \]

6. **Identifying Options**:
- Now we can identify the options provided:
- Option 1: \(1.662 \times 10^{-24} \text{ kg}\) (Incorrect)
- Option 2: \(1.662 \times 10^{-27} \text{ kg}\) (Correct)
- Option 3: \(1/N_A\) (Correct, where \(N_A\) is Avogadro's number)
- Option 4: Both option 2 and option 3 (Correct)

### Conclusion:
- The correct answer is that 1 amu is equal to \(1.662 \times 10^{-27} \text{ kg}\) and can also be represented as \(1/N_A\).