Tutor profile: Maxi T.

What is the relationship between intermolecular forces of attraction and vapor pressure of a liquid?

Vapor pressure is indicative of the tendency of a liquid to transition into the gaseous state. This means that liquid molecules are able to leave the bulk material and become gaseous particles. The ability of liquid molecules to transition into gaseous particles is limited by intermolecular forces of attraction (IMFAs). IMFAs describe how much molecules prefer to stick to each other. With higher IMFAs, molecules tend to stick closer together, thus making it harder for individual molecules to leave. In contrast, with lower IMFAs, molecules aren't as tightly associated. As such, with lower IMFAs, liquid molecules can more easily transition into the gaseous state, which means that vapor pressure of such a substance would be higher. In short, the strength of the IMFAs in a liquid is inversely proportional to the vapor pressure.

Why are noble gases inert?

Chemistry is primarily driven by the need for stability. Atoms/molecules react because they want to give/take electrons to reach a more stable state. One of the most stable states is when the valence shell is complete with 8 electrons, that's why halogens aim to gain 1 electron to add to their 7 valence electrons. Similarly, that's why alkali metals aim to lose their 1 valence electron, thus making their next inner shell, which happens to be complete, their new valence shell. Noble gases, as seen in their position on the periodic table, have a completely filled valence shell with 8 electrons. Because these gases are in a stable state, they have no need to participate in reactions (i.e., the exchange of electrons).

When calculating with significant figures, why does the sum or difference need to have the same number of decimal places as the number with the least number of decimal places?

Significant figures help express how exact a quantity is. For measurements, the last digit is an estimation, and the rest of the digits are exact. Similarly, you want the final answer to have exact numbers, with only the last digit being an estimation. When you add/subtract 2 or more numbers, for example 13.46+6.779, adding the exact digits to other exact digits gives you an exact answer. in this case, 13.4 and 6.77 are exact, while the last digits of both numbers are estimates. Adding exact digits to each other yields an exact answer, but adding an estimate to an exact digit yields an estimate. Applying this to the numbers, the adding the 0.06 from 13.46 to the 0.07 in 6.779, will yield an estimate. As such, the final sum of 20.239 has estimated digits of 0.03 and 0.09. Because we want to minimize the number of estimated values, we cut the number to only have 1 estimated value, which is essentially the least number of decimal places based on the original values.