3. Will the weight of a kg of iron in air and the weight of a kg of cotton in air be equal? Why?

A kg of cotton is heavier.

Explanation:

Because in air, cotton gets upthrust that's why weighing machine gives less weight than its actually have.

for eg: If cotton has 800g weight, the machine shows a kg(1kg) when measure in air due to upthrust.

I hope this will be helpful for you.

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 =