November 22, 2013

MCAT Question of the Day Solution! Thermodynamics

Our question Thursday was from Thermodynamics in General Chemistry. The question presents you with the combustion reaction of methane and asks you to calculate the energy of a carbon-hydrogen bond. To begin this problem, you must use Hess's Law, which says that the heat of reaction is equal to the sum of the heats of formation of the products minus the sum of the heats of formation for the reactants. The heat of reaction is -802 kJ/mol, and the heats of formation of CO2, H2O, and Oare -393 kJ/mol, -242 kJ/mol, and 0 kJ/mol (the heat of formation of a compound in its elemental state is always 0), respectively. Now you want to solve for the heat of formation for CH4 -802 kJ/mol = -393kJ/mol + 2(-242 kJmol) - x where x is the heat of formation of methane. Solving for x, we get -75 kJ/mol for the heat of formation of methane. Since there are four equal carbon-hydrogen bonds in methane, the energy of one bond is -75 kJ/mol divided by 4, or about -19 kJ/mol. Match to answer choice B!   More question to come next week! [cf]skyword_tracking_tag[/cf] more
November 21, 2013

Dr. Josh Question of the Week! – Electrostatics

Our question this week comes from electrostatics in Physics. While electrostatics is a commonly feared sub-topic in Physics, remember it is going to reward the student who is able to UNDERSTAND both the equation to use and the concept behind it! Let's take a closer look at today's correct answer... The direction of the magnetic field is determined using the right hand rule: Remember to point your thumb in the direction of qv, your fingers point in the direction of the field, and the palm points in the direction of the force. In this question, since the negatively-charged electron is going to the right, point your thumb to the right, fingers point out of the page/ screen, and your palm is going to face upward. MCAT TRICK: Remember this is a NEGATIVELY charged electron so the force will direct in the opposite direction. This means the force on the electron due to the magnetic field will point up!  We can now eliminate answer choices B and D. This is CLASSIC MCAT. It is testing two concepts here. Not only do you have to know how to do the right hand rule you also need to recall the equation F = qvBsinθ, where θ is the angle between v and B. In this case the two are perpendicular, and so the sine term is sin90 = 1. The absolute value of the electron's charge, given in the question stem, is 1.6 x 10-19 C, v = 105 m/s, and B = 5 T. Multiply this together and you will get F = 8 x 10-14 N. Match to answer choice A!   Are you looking for more MCAT practice? Come join us in a FREE MCAT Sample Class!  Reserve your spot now. We look forward to seeing you soon!

Upcoming dates:

Sunday, November 24th 4:00 pm EST

Sunday, December 8th 3:00 pm EST

    TAKEAWAY: The MCAT is going to reward the student who remembers not only the right equation but UNDERSTAND how and where to use it. Great work this week guys! Happy studies! [cf]skyword_tracking_tag[/cf] more

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