How would you prepare a 1 L solution of 3 M MgO?

 175.35g

Explanation:  A 3 M solution has 3 moles of solute per litre.

The mass of one mole of NaCl equals the MW of NaCl MW =  35.45 + 23 =58.45 g/mol

The mass of 3 moles is 58.45 g/mol ×3 mol=175.35 g NaCl  or  200 g rounded to one sigfig.

if the temperature drops and becomes cool

then the atmosphere can be freezing as the water vapor in the air is at maximum (100%) and can be severely cold

also can be snowing lightly as the moisture in the air will start to solidify

and btw this is not chemistry but