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Tutor profile: John K.

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John K.
Stanford Engineering PhD Student
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Questions

Subject: Chemistry

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Question:

Consider the following reaction: H2(g) + I2(g) --> (reversibly) 2 HI (g). If, at 448C, the equilibrium constant is 50.5, and the initial concentrations are 2.0e-2 mol of HI, 1e-2 mol of H2 and 3e-2 mol of I2 in a 2.00 L container, determine which way the reaction will proceed.

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John K.
Answer:

First, determine the initial concentrations in molarity by dividing all the initial moles by the volume of the container: [HI] = 1.0e-2 M [H2] = 5.0e-3 M [I2] = 3.0e-2 M Now, determine Q, the reaction quotient: Q = [HI]^2 / ([H2] * [I2]) = 1.3 Compare Q to K. Since Q < K, and the reaction will proceed so as to establish equilibrium, the concentration of HI must increase. Therefore, the reaction will proceed to the right (products).

Subject: Basic Chemistry

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Question:

Describe and explain the periodic trend(s) related to atomic radii.

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John K.
Answer:

Atomic radii tend to increase as you move from top to bottom within a column (group). This is because the principle quantum number n increases; as you add layers to the valence electrons, the electrons have a greater probability of being farther from the nucleus, causing the atom to increase in size.

Subject: Algebra

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Question:

Six years ago, Nick was three times as old as Erin. Two years later, Nick was twice as old as Erin. How old is Erin now?

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John K.
Answer:

Let x be Nick's current age and let y be Erin's current age. Then, six years ago, Nick's age was x-6, and Erin's age was y-6. At that time, Nick was three times as old as Erin, so we can set up the first of two equations: x - 6 = 3(y-6). Distributing the 3 to each term on the left hand side, and rearranging to group all variables on the left hand side, we have: x-2y = -12. Continuing, we know that two years later, Nick was twice as old as Erin. This would be four years ago. We set up an analogous equation: x-4 = 2(y-4), from which we get x-2y = -4. Now, we have two equations and two unknowns. We can solve the system via the elimination technique to find that y = 8. Erin is currently 8 years old.

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