Answer:
1) It is the number of particles in one mole of substance. It is an arbitrary definition to denote an amount of substance, not by its weight but by its number of molecules. Why? because that is convenient when doing certain calculations involving molecule formulas.
2) 1 mole of substance contains exactly as many particles as there are atoms in 12 grams of carbon-12.
3) The unified atomic mass unit is 1/12 of the mass of 1 carbon-12 atom.
So you see all three concepts are linked together.
This is a incomplete question.The complete question is:
A chemist adds 180.0 ml of a 1.77 mol/L of sodium thiosulfate solution to a reaction flask. Calculate the mass in grams of sodium thiosulfate the chemist has added to the flask. Be sure your answer has the correct number of significant digits.
Answer: 50.4 g
Explanation:
To calculate the number of moles for given molarity, we use the equation:
.....(1)
Molarity of sodium thiosulfate solution = 1.77 M
Volume of sodium thiosulfate solution = 180.0 mL = 0.1800 L
Putting values in equation 1, we get:

Mass of sodium thiosulfate =
Thus 50.4 g of sodium thiosulfate the chemist has added to the flask.
Answer:
Ground state
the state with the smallest amount of energy.
Henry law:
P = KC
P = solvent vapour pressure
K = Henry constant
C = conc of solute
So, 0.2 = 773 C
so, C = 0.2 / 773 = 2.5 x 10^{-4} molal