Surface tension is a phenomenon observed in fluids, and is defined as the resistance of a fluid to outside force (or a resistance to increased surface area). Surface tension is measured in force per unit length.

The SI unit for force is Newtons and the SI unit for length is meters; therefore, the surface tension is usually measured as .

Surface tension can also be measured in energy per unit area. Energy is measured in Joules and area is measured in square meters; therefore, the units for surface tension can also be .

We can see that these units are equivalent by looking at the conversion of Joules and Newtons:

Consider a beaker containing water. The molecules at the surface of water interact with fewer molecules than the molecules at the center of the beaker. The surface molecules have no water molecules above them, whereas the molecules in the center of the solution interact with molecules in all directions. Since the surface molecules have fewer interactions, the interactions between these molecules are much stronger than the interactions of a center molecule; fewer interactions have opposing counterparts, resulting in stronger forces.

Recall that surface tension can be defined as the resistance to increasing surface area. This means that the surface area is minimized due to surface tension. Of all the geometrical shapes, spheres have the lowest surface area to volume ratio; therefore, surface tension forces water droplets to take spherical shapes. Surface tension occurs due to the intermolecular forces between liquid molecules. These intermolecular forces between molecules are cohesive, preventing the spreading of water molecules. 

It is harder to move an object that is floating on water than an object submerged in water. This occurs because the interactions between the molecules at the surface are stronger than the interactions below the surface. Moving an object involves moving the water molecules around it. Since it is harder to move molecules at the surface (stronger interactions), it is harder to move an object at the surface. An object floating on water will have more resistance to motion.

Surface area is directly related to the number of molecules at the surface. An increase in surface area will lead to an increase in the number of molecules at the surface. Since surface tension is the ability of a fluid to resist an increase in surface area, the surface area decreases due to increased surface tension. This means that the amount of molecules at the surface also decreases due to surface tension.

One mole of gas has a volume of 22.4L at STP.

Since there are two moles of gas present, the volume is doubled.

2(22.4) = 44.8L

The fact that the gas is diatomic is irrelevant.

Choice 3 is correct. The question forces the respondent to recall that one mole of an ideal gas occupies 22.4 liters at standard temperature (zero Celsius) and pressure (1 atmosphere). Helium balloons rise because they are less dense than air by a factor of 29 g/mole – 4 g/mole = 25 g/mole. Since the sensor has a mass of 100 g, it will require about 4 molar volumes of He to lift it.

Charles's Law states that . To solve for the final volume, we simply plug in our given values to this equation.

This law applies only for isobaric (constant pressure) changes.

At constant pressure, , where the temperatures are measured in Kelvin (absolute temperature).

First, convert the given temperature (C) to Kelvin (K).

K = C + 273 = 30 + 273 = 303K

Plug the temperature and volumes into the above equation and solve for the final temperature.

Convert this value back to Celsius.

C = K - 273 = 606 - 273 = 333^oC

Using the ideal gas law equation we can find that P= nRT/V. We then plug in the given values.

Solving for P gives us 12.4atm.

Note: 30^oC must be converted into Kelvin by adding 273K

Using the ideal gas law, , we can solve for pressure.

Using the equation  and solving for P you get, .

Recall that hydrogen forms a diatomic molecule when in gas form. This should always be an assumption when working with hydrogen gas on the MCAT. When we convert 5g to moles, we must use a conversion factor of 2g/mol.

Temperature must be converted to Kelvin. You must have this conversion memorized for the MCAT.

Now we can solve for P.