Question

1.89g of an organic compound A was dissolved per 85 cm3 of water. The boiling point under 1 atmospheric pressure of this solution is increased to 100C . what is molecular mass of compund A

Answer 1

The molar heat of vaporization, ΔHvap, sometimes called the molar enthalpy of vaporization, tells you how much energy is needed in order to boil 1 mole of a given substance at its boiling point.

In water's case, a molar heat of vaporization of 40.66 kJ mol−1 means that you need to supply 40.66 kJ of heat in order to boil 1 mole of water at its normal boiling point, i.e. at 100∘C.

ΔHvap=40.66 kJ.mol−1

You need 40.66 kJ of heat to boil1 mole of water at its normal boiling point.

Now, the first thing to do here is to convert the mass of water to moles by using its molar mass

2.87g⋅1 mole H2O18.015g=0.1593 moles H2O

You can now use the molar heat of vaporization as a conversion factor to determine how much heat would be needed to boil 0.1593 moles of water at its boiling point

0.1593moles H2O⋅40.66 kJ1mole H2O=6.48 kJ−−−−−−

The answer is rounded to three sig figs, the number of sig figs you have for the mass of the sample.