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A student dissolved 1.50 g contaminated benzoic acid in 100mL hot water to purify it. After cooling down the solution very slowly recrystallization did not start, scratching the glass and putting the solution in ice bath for a day didn’t help either and still no crystals. (benzoic acid solubility in water is 2.9 g/L at 25°C and 68 g/L at 95°C). What statement is correct?

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sebassandin

Answer:

Explanation:

Hello,

Supposing that the hot water was at 95 °C, considering a volume of 100 mL (0.1 L) the maximum dissolved amount is:

[tex]m_{dissolved}=\frac{0.1L*68g}{1L}=6.8g[/tex]

Now, considering that the 1.50 g were completely dissolved at 95°C, we must know whether that mass could be crystallized after the cooling down (25 °C), thus:

[tex]m_{dissolved}=\frac{0.1L*2.9g}{1L}=0.29g[/tex]

By considering that result, there should be an ideal undissolved amount of benzoic acid, for that reason and taking into account that it didn't crystallize, the only option is that the benzoic acid had a low purity, so less than 0.29 g of pure benzoic acid were present to do not present the crystallization.

Best regards.

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