Compound A is recrystallized from n-propanol. Given are the following solubilities (in 100 mL of solvent): 160 g/100 °C, 100 g/80 °C, 60 g/60 °C, 20 g/20 °C, 15 g/10 °C and 10 g/0 °C. How many milliliters of solvent are needed to dissolve 80.0 g of compound A during the recrystallization?

Question

800 mL
134 mL
400 mL
50 mL
80 mL

Answer 1

At what temperature do you wish to start? That will be dictated partially by the melting point of compound A as well as the boiling point of n-propanol. For example, (but you don't want to use this one), if we use the 100 C data,

100 mL x (80/160) = 50 mL. However, you don't want to use this one because n-propanol boils < 100 C. In fact, if you will look at the data, every possible answer is there using the formula,
100 mL x (80/table value) = ??
Therefore,
100 mL x 80/160 = 50 mL
100 mL x 80/100 = 80
100 mL x 80/60 = 133
100 mL x 80/20 = 400
etc.
Look at your notes and in the lab preparation material, as well as the melting point of compound A to make your choice.