if a lab group did not keep the flask submerged 1. Ifa lab group did not keep th

Question

if a lab group did not keep the flask submerged

1. Ifa lab group did not keep the flask submerged in boiling water for a full ten minutes, and some unknown never vaporized, how would that affect the calculated molar mass? In other words, would it make the calculated value too high or too low? Explain why. 2. Whi le performing this experiment, ten lab groups have ten different unknown volatile liquids with a range of molar masses. If all lab groups have exactly the same size of flask (255 mL), and all groups took their unknown vapor to the same temperature (100.000) and pressure (760.0 mmHg, how would the number of molecules within the different flasks compare? Would it vary with molar mass? How? Explain. A student has determined that an unknown liquid is either 1-chlorobutane or t- butyl chloride. What other physical property(s) could be used to determine the identity of the unknown?

Explanation / Answer

The molecular mass of volatile liquid (here) using ideal gas law is obtained in following two steps:

1. The volatile liquid kept in a flask of known volume (V) is heated by placing in boiling water bath (temperature, T =1000C). The pressure (p) of the vapor/gas in the flask in measure.

The number of moles (n) of the sample is calculated using, pV = nRT, because p, V and T are known.

2. Determination of molecular mass. The flask is allowed to cool and the vapor attains the liquid phase. The weight of the sample is calculated as follow:

Weigh of sample = (weight of flask + sample) – weight of flask

Thus, mass of the sample is known; say it be X gram.

Molecular mass = weight in gram / number of moles

                        = X gram/ n moles = (X/n) gram per mol ---------- equation 1

# If flask is not submerged: If flask is NOT submerged in boiling water bath, the pressure generated in the flask will be very low due to little or no increase in temperature (p is directly proportional to temperature). Thus, low pressure in the flask gives a lower number of moles.

If number of moles is lower than the actual, then putting the value of ‘n’ in equation 1 gives a higher value of molecular mass of the sample.

Thus, the value of calculated value of molecular mass will be very high.      

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