The moles can be defined as the mass of the substance with respect to molar mass. The moles of potassium nitrate is 1 mol.
<h3>How to calculate moles of a substance?</h3>
The moles of a compound can be calculated from:

The molarity can be defined as the moles of solute in a liter of solution.
The molarity can be expressed as:

The molarity of potassium nitrate solution is 2 M, and the volume is 500 mL.
The moles of potassium nitrate is given as:

The moles of potassium nitrate in 2 M, 500 mL solution are 1 mol.
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Electrons are either lost gained or shared in chemical reactions :)
Both trials of 1.2 g and 1.6 g will have the same mass percent of water because the ratio of the salt to the water of hydration is always constant for any hydrated salt.
<h3>Water of hydration</h3>
For every hydrated salt, the ratio of the salt to the water of hydration remains constant irrespective of the amount of salt taken for experimental analysis.
For example, assuming the mass percent of water in 10g of a hydrated salt is 40%, if 100g of the same salt is taken, the mass percent will remain 40%.
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Atoms of the same element but with different atomic masses are called isotopes. They possess a different atomic mass, due to the indifferent number of neutrons that the atoms of a given element have.