Here's why:

* Atomic weight refers to the average mass of an atom of a specific element.

* Brass is a mixture of copper (Cu) and zinc (Zn), with sometimes other elements added in small amounts.

Therefore, brass doesn't have a single atomic weight. Instead, you can calculate the average atomic weight of a specific brass alloy based on its composition:

1. Determine the composition of the brass alloy: You need to know the percentage by weight of each element present in the alloy (e.g., 60% copper, 40% zinc).

2. Look up the atomic weights of the elements: The atomic weight of copper is 63.546 amu, and the atomic weight of zinc is 65.38 amu.

3. Calculate the weighted average: Multiply the atomic weight of each element by its percentage in the alloy, then add the results together.

Example: For a brass alloy with 60% copper and 40% zinc:

Average atomic weight = (0.60 * 63.546 amu) + (0.40 * 65.38 amu) = 64.38 amu (approximately)

Remember, this calculated average atomic weight is specific to the given brass alloy. Different brass alloys with varying compositions will have different average atomic weights.