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Thyroxine, an important hormone that controls the rate of metabolism in the body, can be isolated from the thyroid gland. If 0.455 g of thyroxine is dissolved in 10.0 g of benzene, the freezing point of the solution could be measured as 5.144°C. Pure benzene freezes at 5.444°C and has a value for the molal freezing point depression constant of K f of 5.12°C/m. What is the approximate molar mass of thyroxine?

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Answer:

The molar mass of thyroxine is 18,2 g/m

Explanation:

To solve this we must apply the colligative property of cryoscopic descent whose formula is:

T° sv pure - T° sl = Kf x molality

So let's replace our values

5,444°C - 5,144°C = 5,12 °C/m x molality

0,3°C = 5,12 °C/m x molality

0,3°C / 5,12 m/°C = molality

2,5 m

Now you should know that molality is moles from solute in 1kg of solvent but you dont have 1kg you have just 10 g. Let's convert for the rule of three

10 g = 0'010 kg

If 1 kg has 2,5 moles of thyrosine, how many for 0,010 kg

moles = (0,010kg * 2,5 moles)/1kg

moles = 0,025

Now we have the moles of thyrosine in our solution and we were informed that we dissolved 0,455 g so let's get the molar mass (mass of compound for one mole)

moles = mass/ molar mass

molar mass = mass/moles

molar mass = 0,455g/0,025 moles

molar mass = 18,2 g/m

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