You volunteer for chalkboard cleaning duty and you get to work with the erasers. As you rub the dust off of the chalkboard, white chalk dust enters the air and makes you cough. As you clean the chalkboard with the eraser, are you causing a physical change or a chemical change? Explain your answer.

A physical change

Explanation:

You are wiping chalk particles from the board to the eraser which is just changing the location of the chalk particles, a physical change.

you're not changing the chalk into a new substance or causing a chemical reaction, which would be a chemical change.

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =