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Tutor profile: Emily C.

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Emily C.
UCLA Biochemistry Senior
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Questions

Subject: Organic Chemistry

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Question:

Suppose you run a TLC plate with a silica-coated stationary phase and a mobile phase of ethyl acetate. You are comparing two samples: butanoic acid, butanal, and butane. Order these molecules from lowest expected Rf value to highest.

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Emily C.
Answer:

Rf (retention factor) = distance traveled by the compound divided by the distance traveled by the solvent. This TLC plate has a polar stationary phase and a nonpolar mobile phase. We expect the most polar molecule to associate strongly with the polar stationary phase, thus its Rf value will be the lowest. The most nonpolar molecule will associate strongly with the nonpolar mobile phase, so it will travel the furthest and the Rf value will be the highest. Thus the expected order from lowest to highest Rf value is: Butanoic acid < butanal < butane

Subject: Chemistry

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Question:

406 moles of a monatomic ideal gas undergoes an isothermal process from standard temperature and pressure (300K and 1atm) to 5atm. Calculate the final state, heat, work, and internal energy change for this process.

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Emily C.
Answer:

Initial Volume can be calculated using the ideal gas law: V i= (406mol) (0.0821 L*atm/mol*K) (300k) / 1atm Vi= 10,000L Final volume can also be calculated using ideal gas law. We know that in an isothermal process, temperature will not change. Vf = (406mol) (0.0821 L*atm/mol*K) (300k) / 5atm Vf=2,000L First law of thermodynamics states: dq = du - dw (when w is work done by the system) heat (U) is related to temperature by dU = 3/2 nRdT if dT = 0 then dU = 0 Thus dq = -dw = nRTln(Vf/Vi) = (406)(8.314J/mol*K)(300K)ln(2000/10000) = -1.63*10^6 J

Subject: Biochemistry

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Question:

Scientists have discovered a mutant form of Protein X that results in a surface-exposed threonine residue to be replaced with a tryptophan residue. What is the predicted effect of this mutation? Is the mutant form expected to be more or less soluble?

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Emily C.
Answer:

A threonine to tryptophan mutation would result in a switch from a hydrophilic (polar) residue to a hydrophobic (nonpolar) residue. As a result, protein folding will likely be disrupted, as solvent-exposed hydrophobic residues are entropically disfavored. Thus, the mutant form will be less soluble than the non-mutated form.

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