Question

We have already noted two sources of systematic error in the use of this method: not enough sample to displace all the air from the flask and the finite vapor pressure of the unknown. Consider the following possible errors. For each one, describe how the results might be impacted. Please try to be specific (e.g. "The molar mass will be overestimated because...") a. The volume of your flask was found by weighing it, filled with water, at room temperature (20°C). At 100°C, because of the expansion of glass, the volume is slightly greater. How could this affect your results? b. We have assumed that the temperature of the vapor is the same as the temperature of the boiling water. What if this were not correct? c. What could be the impact if some the vapor leaks before all of it can be condensed? d. 2.1130 grams of an unknown organic compound was found to occupy 305 mL at 757 tort and 100°C.

Answer 1

We are to determine the molecular mass of a volatile liquid

We can determine the molecular mass of the liquid with the help of the ideal gas equation

PV=nRT,

n= no of moles = m/M

M=mRT/PV

M= molecular weight of gas

m= mass of condensate

V= volume of flask

R= gas constant.

T= temperature

We can determine the molecular mass of a volatile liquid by vaporizing the liquid in a flask by placing the flask in a hot water bath. Once the liquid vaporizes, the temperature of the gas is taken as the temperature of the hot water bath. As the experiment is conducted in atomospheric pressure, the pressure of the gas is the pressure of air in the laboratory ( most often atmospheric pressure).

The mass of the liquid that vaporized is found by cooling the flask which condenses the gas forming the liquid. By weighing the flask after the condensate has formed we find the mass of the liquid.

The volume of the flask is measured indirectly by filling the flask with water of a known temperature. The flask is then weighed. The density of water is known at various temperatures. From the known water densities and the mass of the flask, we can determine the volume of the flask

a. Now as we increase the temperature to 100°C, the glass flask expandes. Therefore the vaporized liquid will now occupy more volume than calculated.

As molecular weight is inversly proportional to the volume of the flask

M_measured= mRT/PV_measured

M_actual = mRT/PV_actual

Therefore

M_measured/M_actual = V_actual/V_measured

As V_actual > V_measured, M_measured > M_actual

Our calculated molecular mass is greater than the actual molecular mass of the liquid

b. We have assumed that the temperature of the vapor is the same as the temperature of the water in the water bath.

The molecular mass is directly proportional to the temperature. If heat losses are considered, the temperature of the bath > temperature of the gases. i,e, temperature of gas is less than the temperature of bath,

M_measured/M_actual = T_measured/T_actual

Now as T_acutal < T_measued.

M_measured > M_{actual .} We have overestimated the molecular mass of the liquid

c. If some of the vapor was to leak before it was condensed, then the mass of the condensate (m) has reduced.

as M=mRT/PV, if the mass of the condensate reduces, the molecular mass calculated reduces.

We end up with a lower molecular mass than the true molecular mass.

d. Find the molecular mass.

m= 2.1130 g

V=305 mL

P= 757 torr = 757 mm Hg

T= 100°C

We have

M=mRT/PV

M= [2.110g * 62.363 (mmHg.l/mol.K)*(100+273)K]/[757 mmHg * 305*10^-3L]

M= 212.88 g/mol