11. even when something is always found in nature as a whole number, its average may not work out to be a…

11.

Explanation:

Step1: Gather data

You need to count the number of pets each student in the class has. Let the number of students be $n$, and the number of pets of each student be $x_1,x_2,\cdots,x_n$.

Step2: Calculate the sum

Find the sum of the number of pets $S = x_1 + x_2+\cdots+x_n$.

Step3: Calculate the average

The average number of pets $\bar{x}=\frac{S}{n}$. Since we don't have the actual data of the number of pets per - student in the class, we can't give a specific numerical answer. But the formula for calculating it is $\bar{x}=\frac{\sum_{i = 1}^{n}x_i}{n}$.

Answer:

We need to collect data on the number of pets per student in the class and use the formula $\bar{x}=\frac{\sum_{i=1}^{n}x_i}{n}$ to calculate, where $n$ is the number of students and $x_i$ is the number of pets of the $i$-th student.

12.

Brief Explanations:

The mass number of an atom is the sum of protons and neutrons, and it is always a whole - number. The average atomic mass is a weighted average of the masses of different isotopes of an element.

Answer:

No. The mass number of a single atom represents the sum of protons and neutrons in the nucleus and is always a whole number. The average atomic mass of 35.45 u for chlorine is a weighted - average of the masses of its isotopes (such as chlorine - 35 and chlorine - 37), not the mass number of a single atom.

13.

Brief Explanations:

The average atomic mass is a weighted average of the masses of isotopes. The isotope with a mass closest to the average atomic mass is likely to be the most abundant.

Answer:

Potassium - 39. Since the average atomic mass of potassium is 39.1 u, which is closest to the mass number of potassium - 39 among the three isotopes (potassium - 39, potassium - 40, and potassium - 41), potassium - 39 is likely to be the most commonly found isotope in nature.