Because a solubility is calculated for saturated solutions we can write:
36.3 g NaCl ---100g water
x g NaCl ---- 300.0 g water
x=36.3*300/100=108.9 g NaCl
Answer:

Explanation:
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In this case, according to the given information it turns out possible for us to realize that one mole of the given compound, Mg(ClO₄)₂, has one mole of Mg, two moles of Cl and eight moles of O; thus, we proceed as follows:

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Mass of X₂O₇ = 54,9g
2x + 33,6g = 54,9g
2x = 54,9g - 33,6g
2x = 21,3g | :2
x = 10,65g/mol