easy chem lab

2010-10-04 8:25 pm

1.Some physical properties of phenacetin are shown in the table below:

Compound :phenacetin
Molecular weight :179.2
melting point :135
solubility in H2O@5°C(g/100mL):0.076
solubility in H2O@5°C(g/100mL):1.2

What is the minimum volume of boiling water needed to dissolve 0.2g of phenacetin?Using 75mL of water to effort a recrystallization, what mass of phenacetin remains dissolved? What is the maximum possible recovery of phenacetin after recrystallization (again ,suing 75mL of water)?

2.Suppose that the observed melting point range of a solid was initially 150-160°C.After recrystallization, the melting point was determined to be 145-146°C.How might you account for this observation?

更新1:

For #1 is suppose to be :solubility in H2O@5°C(g/100mL):0.076 solubility in H2O@100°C(g/100mL):1.2 thank you 己式庚辛 pointing that mistake out.

回答 (1)

己式庚辛

2010-10-04 9:14 pm

✔ 最佳答案

1.
i guess it's "solubility in H2O@100°C(g/100mL):1.2 " , right?
volume required = 100 x (0.2/1.2) = 16.67 ml.

mass remained dissolved = 0.076 x (75/100) = 0.057 g
max. recovery = mass recovered / total mass = (1-0.057)/0.2 = 71.5%

2.
the melting point range of original sample is wider than that of recrystallized sample, showing that the original sample is impure.
the melting point of impure sample is higher than that of pure sample, suggesting that the impure sample is contaminated by high-boiling substances.

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