Question

liquid a is known to have a lower vapor pressure and lower surface tension than liquid b. use these facts to predict the result of each experiment in the table below, if you can. experiment predicted outcome 10.0 ml of liquid a are put in one sealed 5 l flask, and 10.0 ml of liquid b are put in another sealed 5 l flask. the pressure in each flask is then slowly lowered with a vacuum pump. eventually both liquids boil, a first and then b. eventually both liquids boil, b first and then a. neither liquid will boil its impossible to predict whether eitherliquid boils without more information. a 250 ml beaker is submerged in a large container of liquid a and turned upside down, and another 250 ml beaker is submerged and turned upside down in a large container of liquid b. hydrogen (h₂) gas is bubbled into each upside - down beaker until 100 ml of liquid has been displaced. the masses m_a and m_b of hydrogen inside each beaker is then chemically determined. note for advanced students: you may assume the solubility of h₂ in either liquid is very low. m_a will be greater than m_b m_a will be less than m_b m_a will be equal to m_b its impossible to predict whether m_a or m_b will be greater without more information.

Explanation:

Step1: Analyze boiling point

Vapor pressure is inversely related to boiling point. Liquid A has lower vapor pressure, so it has higher boiling point. When pressure is lowered, liquid with lower boiling point (Liquid B) will boil first.

Step2: Analyze mass of displaced gas

Surface tension is related to how “tightly” molecules are held. Lower surface tension (Liquid A) means molecules are less tightly held. When gas (H₂) displaces liquid, with lower surface - tension liquid (A), gas can displace liquid more “easily” in terms of volume. But since volume displaced (100 mL) is same for both. Mass \(m=
ho V\). Lower surface - tension liquid (A) has lower density (because less intermolecular forces, more spread out in some sense for liquid - gas interface in this context). So \(m_A\) (mass of H₂ in A) is less than \(m_B\) (mass of H₂ in B) as density of liquid A is lower (assuming ideal gas behavior and same conditions for gas collection, \(V = 100\space mL\) for both, \(m=
ho_{gas}V\), but also considering liquid - gas interaction. Lower surface - tension liquid (A) allows gas to have more “space” in a sense, but more accurately, using the concept that lower surface - tension liquid has weaker intermolecular forces. For a given volume of displaced liquid (gas volume), the liquid with lower surface tension (A) has lower density (because molecules are less closely packed in the liquid phase near the gas - liquid interface). Since \(m=
ho V\) (for the gas, assuming gas is ideal and same T and P for both experiments, but also considering liquid - gas interaction. The liquid with lower surface tension (A) has a lower “barrier” for gas to enter. But more precisely, using the relation between surface tension and intermolecular forces. Lower surface tension (A) implies weaker intermolecular forces. So liquid A is less “dense” (in terms of how closely molecules are held) near the gas - liquid interface. So when 100 mL of gas displaces liquid, the mass of gas (assuming ideal gas, same T and P) is related to the “ease” of gas entering. But actually, using the formula \(m=
ho V\). The liquid with lower surface tension (A) has a lower density (because of weaker intermolecular forces). So for the same volume of displaced liquid (gas volume \(V = 100\space mL\)), \(m_A=
ho_A V\) and \(m_B=
ho_B V\). Since \(
ho_A<
ho_B\) (because of lower surface tension for A, weaker intermolecular forces), \(m_A < m_B\)

Answer:

• For the first experiment: Eventually both liquids boil, B first and then A.
• For the second experiment: \(m_A\) will be less than \(m_B\)