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Magnesium chloride is an important coagulant used in the preparation of tofu from soy milk. Its solubility in water at 200C is 54.6 g/100 g. At 800C, its solubility is 66.1 g/100 g. A mixture is made up of 16.2 g of magnesium chloride and 38.2 g of water at 200C.
(a) Is the mixture homogeneous? If it is, how many more grams of magnesium chloride are required to make a saturated solution? If the mixture is not homogeneous, how many grams of magnesium chloride are undissolved?
(b) How many more grams of magnesium chloride are needed to make a saturated solution at 800C?

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Answer:

(a) Homogeneous. 4.7 g of MgCl₂.

(b) 9.1 g

Explanation:

(a)

At 200°C, we can dissolve 54.6g of MgCl₂ in 100 g of water. The mass that we could dissolve in 38.2 g of water is:

[tex]38.2gWater.\frac{54.6gMgCl_{2}}{100gWater} =20.9g[/tex]

Since we can dissolve up to 20.9 g of MgCl₂ and we added only 16.2 g, the mixture is homogeneous and we could add 20.9 g -16.2 g = 4.7 g of solute to make it saturated.

(b)

At 800°C, we can dissolve 66.1 g of MgCl₂ in 100 g of water. The mass that we could dissolve in 38.2 g of water is:

[tex]38.2gWater.\frac{66.1gMgCl_{2}}{100gWater} =25.3g[/tex]

Since we can dissolve up to 25.3 g of MgCl₂ and we added only 16.2 g, we could add 25.3 g - 16.2 g = 9.1 g of solute to make it saturated.

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