Posted by Sandy on .
I got think I got the following questions almost figured, but I just want someone to make sure that I am correct.
1) The whole experiment involves using the ideal gas law to predict the molar mass of an unknown liquid. The formula is molar mass of liquid = gR(0.0821)T(temperature in Kelvins) divided by Pressure(atm) multiplied by Volume (liters).
A student did not dry the erlenmeyer flask before weighing it. As a result of this error, would the molar mass be higher or lower than the correct value?
I said that the molar mass would be higher due to there is a greater difference in when one has to subtract the mass of the flask from the mass of the flask with the condensed liquid, there would be a greater difference and the mass of the substance would be calculated to be higher.
2) A student added 0.1mL of unknown liquid rather than 6mL to a 125mL flask. What effect would this action have on the calculated molar mass?
I said that there would be no difference because the volume of the flask is what the volume of the gas will be equal to.
I don't know exactly how the experiment was conducted but I think 1 is right. For 2, however, I think the results must be either too high or too low (but see the note below). If too little liquid is placed in the flask that means that vapor cannot fill the flask so your second weighing weighs all of the vapor produced and some air also. That means the mass of the vapor is too small which will lead to low results. Note: That answer assumes that 0.1 mL doesn't provide enough sample to fill the flask with vapor. If it did the result would not be changed.