Pheromones are compounds secreted by the females of many insect species to attract males. one of these compounds contains 80.78 percent c, 13.56 percent h, and 5.66 percent o. a solution of 1.00 g of this pheromone in 8.50 g of benzene freezes at 3.37°c. what is the molecular formula of the compound?

Answer : The molecular formula of a compound is,

Solution :

If percentage are given then we are taking total mass is 100 grams.

So, the mass of each element is equal to the percentage given.

Mass of C = 80.78 g

Mass of H = 13.56 g

Mass of O = 5.66 g

Molar mass of C = 12 g/mole

Molar mass of H = 1 g/mole

Molar mass of O = 16 g/mole

Step 1 : convert given masses into moles.

Moles of C =

Moles of H =

Moles of O =

Step 2 : For the mole ratio, divide each value of moles by the smallest number of moles calculated.

For C =

For H =

For O =

The ratio of C : H : O = 19 : 38 : 1

The mole ratio of the element is represented by subscripts in empirical formula.

The Empirical formula =

The empirical formula weight = 19(12) + 38(1) + 1(16) = 282 gram/eq

Now we have to calculate the molar mass of pheromone.

Formula used :  

where,

= change in freezing point

= freezing point of solution =

= freezing point of benzene =

i = Van't Hoff factor = 1  (for non-electrolyte)

= freezing point constant for benzene =

m = molality

Now put all the given values in this formula, we get

Now we have to calculate the molecular formula of the compound.

Formula used :

Molecular formula =

Therefore, the molecular of the compound is,

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