Conservation of mass can be checked in an experiment . There are three steps to do it in a best way:
1. Weigh all the equipment and materials required in the experiment before the experiment.
2. Avoid spillage and evaporation during the experiment.
3. Weigh all the equipment and materials after the experiment.
If the mass is conserved then weight from step 1 is equal to weight from step 3.
Answer: 
Explanation:
Depression in freezing point is given by:
= Depression in freezing point
i= vant hoff factor = 1 (for non electrolyte like urea)
= freezing point constant = 
m= molality
Weight of solvent (X)= 950 g = 0.95 kg
Molar mass of non electrolyte (urea) = 60.06 g/mol
Mass of non electrolyte (urea) added = ?
Thus urea was dissolved.
I am assuming you are talking about Neon. The rate of diffusion is directly proportional to the molar mass of the gas. Since neon has a molar mass of 20.18 grams, the gas must have a lower molar mass and must be a gas at 273 Kelvin. There are several elements that fulfill this criteria: Hydrogen, Helium, Oxygen, Nitrogen, and Fluorine.
