You removed the flask with the liquid in it before all of the liquid vaporized, and weighed it at that point

You removed the flask with the liquid in it before all of the liquid vaporized, and weighed it at that point.

The correct answer and explanation is:

If you removed the flask before all of the liquid vaporized, then the measured mass will be too high, and as a result, the calculated molar mass will be inaccurate (too high).

✅ Correct Answer:

The molar mass will be too high because not all of the liquid vaporized.

💡 Explanation (300 words):

This scenario typically relates to an experiment used to determine the molar mass of a volatile liquid using the ideal gas law, often referred to as the Dumas method. In this method, a known amount of volatile liquid is placed in a pre-weighed flask, then heated until all the liquid vaporizes and displaces the air in the flask. The flask is then cooled and weighed again. From the increase in mass (which represents the mass of the vapor), and using the pressure, volume, and temperature, the molar mass is calculated.

However, if you remove the flask before all the liquid has vaporized, then not all of the liquid has turned into gas, which means:

• The total mass of vapor in the flask is less than it should be.
• But the mass you record will include both vapor and remaining liquid, making the measured mass of vapor artificially high.
• This leads to a miscalculation in molar mass.

Let’s say:

• You calculate moles using the ideal gas law:
  n=PVRTn = \frac{PV}{RT}
• Then molar mass MM is:
  M=mass of vapornM = \frac{\text{mass of vapor}}{n}

Since mass is too high and moles are calculated assuming full vaporization, you end up dividing by too small a number of moles, which inflates the calculated molar mass.

In conclusion, failing to vaporize all of the liquid results in an overestimation of the vapor’s mass while underestimating the moles (because some liquid never contributed to pressure/volume), which causes the molar mass to appear higher than it actually is.