Practice Test 8 - MCAT Chemical and Physical Foundations of Biological Systems

20 questions. Answer them in any order, then submit to see your score and every explanation.

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1. A tuning fork produces a longitudinal sound wave in air. A student claims that because the wave travels to the right, the air molecules also steadily drift to the right over many cycles. Which statement best reflects the correct interpretation of particle motion in a longitudinal mechanical wave?
2. A membrane potential was measured in a neuron while extracellular K$^+$ concentration was varied. All other ions were held constant. Table: Extracellular [K$^+$] vs. membrane potential [K$^+$]$_{out}$ (mM): 2, 4, 8, 16 $V_m$ (mV): -95, -80, -65, -50 What does the data most strongly suggest about the effect of increasing extracellular K$^+$?
3. A lab measures dipole moments of chloromethane (CH$_3$Cl) and carbon tetrachloride (CCl$_4$) in the gas phase. Both contain polar C–Cl bonds, but only one has a net dipole. Based on VSEPR, which geometry at carbon in CCl$_4$ best explains its near-zero dipole moment?
4. A lab tests an unknown noble-gas sample using electron-impact mass spectrometry. The ion source primarily forms singly charged ions ($z=+1$). The spectrum shows two dominant peaks at $m/z = 20$ and $m/z = 22$ with an intensity ratio of approximately 9:1 (20:22). The principle used is that the mass spectrometer separates ions by mass-to-charge ratio ($m/z$), so for $z=+1$, $m/z$ approximates isotopic mass in amu. Which atom is most likely present in the sample? Constants: $1\ \text{amu} = 1.66\times 10^{-27}\ \text{kg}$; elementary charge $e = 1.60\times 10^{-19}\ \text{C}$.
5. A physiologist models oxygen binding to hemoglobin in a capillary bed. She notes that in metabolically active tissue, local pH decreases and temperature increases. Assume hemoglobin exhibits cooperative binding and that oxygen unloading depends on both binding affinity and the partial pressure gradient. Based on the described relationship between pH (Bohr effect) and binding equilibria, which outcome would most likely result in the active tissue?
6. A study investigated whether adding salt to water increases its boiling point. Two beakers (same volume) were heated on identical hot plates. One contained pure water; the other contained water with dissolved NaCl. The salted sample reached a slightly higher temperature at the onset of sustained boiling. Which conclusion is most appropriate based on the design?
7. A single axon is exposed to tetrodotoxin (TTX), which blocks voltage-gated Na$^+$ channels. The axon is then stimulated with the same electrode and intensity that previously elicited an action potential. Resting potential remains near normal because leak channels and the Na$^+$/K$^+$ ATPase are intact. Which observation is most consistent with the presence of TTX? Assume TTX does not block ligand-gated channels or voltage-gated K$^+$ channels.
8. A wave on a rope is described by increasing the driving frequency of the hand motion while keeping rope tension and linear mass density constant. The observed wave speed on the rope does not change. Which statement best describes the wave behavior in this scenario? Assume an ideal string where wave speed depends only on medium properties.
9. A researcher models blood flow through a narrowed artery as laminar flow through a cylindrical tube. Poiseuille's law is provided: $Q = \dfrac{\pi r^4 \Delta P}{8\eta L}$, where $Q$ is volumetric flow rate, $r$ is radius, $\Delta P$ is pressure drop, $\eta$ is viscosity, and $L$ is length. If plaque reduces the radius by 20% while $\Delta P$, $\eta$, and $L$ remain constant, what is the most consistent qualitative change in $Q$?
10. Two stereoisomers of 2,3-dibromobutane were prepared: Molecule X and Molecule Y. Both have formula C4H8Br2 and two stereocenters. Polarimetry shows Molecule X is optically inactive, while Molecule Y is optically active. Which statement best accounts for this result?
11. In a photoelectric experiment, the stopping potential is measured for several frequencies above threshold. The student mistakenly concludes that because light is a wave, increasing frequency should increase the number of emitted electrons per second at fixed intensity. Which statement best corrects this conclusion using the photoelectric model?
12. A student analyzes a galvanic cell at 25°C: $\text{Zn}(s)|\text{Zn}^{2+}(1.0\ \text{M})||\text{Cu}^{2+}(1.0\ \text{M})|\text{Cu}(s)$. Standard reduction potentials: $E^{\circ}(\text{Cu}^{2+}/\text{Cu})=+0.34\ \text{V}$, $E^{\circ}(\text{Zn}^{2+}/\text{Zn})=-0.76\ \text{V}$. Which prediction is most consistent with these values regarding electron flow and the sign of $E^{\circ}_{cell}$?
13. In a study of brown adipose tissue, investigators report that mitochondrial uncoupling increases oxygen consumption while producing more heat and less ATP per glucose. The concept is first law of thermodynamics in biological energy conversion. Which statement is most consistent?
14. A research group prototypes an otoscope that uses a small concave mirror to focus light into the ear canal while allowing the clinician to view the tympanic membrane through a central aperture. The mirror has focal length $f=5.0\,\text{cm}$. The tympanic membrane is approximately $d_o=3.0\,\text{cm}$ in front of the mirror. Using the mirror equation as the central concept, which statement best describes the image formation of the tympanic membrane produced by the mirror?
15. In an industrial bioreactor, a dissolved gas substrate (G) is consumed by a catalyst-coated surface: $G \rightarrow$ products. At high [G], surface sites become saturated. Initial rates were measured at constant temperature while increasing [G]. Data: [G] = 0.10 mM, rate = 1.0 (arb units) [G] = 0.20 mM, rate = 2.0 [G] = 0.40 mM, rate = 2.0 Based on the data, what is the order of the reaction with respect to G in the highest-concentration regime (0.20–0.40 mM)?
16. A researcher performed SDS-PAGE (pH 8.3) on two proteins D1 and D2 that differ in MW (D1: 15 kDa; D2: 150 kDa). At 120 V for 50 min, D1 migrated 6.5 cm and D2 migrated 1.8 cm. The researcher then doubled the voltage but kept run time the same; both proteins migrated farther, but the distance between their bands (in cm) decreased. Which factor most likely contributed to the decreased band spacing at higher voltage?
17. A horizontal pipe carries water ($\rho=1000\ \text{kg/m}^3$) at steady flow. At point 1, $A_1=2A_2$ and $v_1=0.90\ \text{m/s}$. Neglect viscosity. Based on Bernoulli's equation, which statement is most consistent about the static pressure difference $P_1-P_2$?
18. A weak acid drug, HA, is dissolved in water at 25°C to a formal concentration of 0.10 M. Its $K_a=1.0\times10^{-5}$. Without doing detailed calculations, which change would most likely occur following addition of a small amount of NaA (the conjugate base) while keeping volume constant?
19. A gas-phase tracer is released into a long tube at constant temperature. The experiment compares molecular speeds predicted by kinetic molecular theory for two isotopes of the same element, with molar masses $M_1$ and $M_2$ where $M_2 > M_1$. Which prediction is most consistent for the rms speeds ($v_{\text{rms}}$) at the same temperature? A. $v_{\text{rms,2}} > v_{\text{rms,1}}$ because heavier molecules carry more momentum B. $v_{\text{rms,2}} = v_{\text{rms,1}}$ because temperature fixes speed, not mass C. $v_{\text{rms,2}} < v_{\text{rms,1}}$ because rms speed scales as $\frac{1}{\sqrt{M}}$ D. $v_{\text{rms,2}} < v_{\text{rms,1}}$ because rms speed scales as $\frac{1}{M}$
20. A lab evaluated whether increasing the area of a parallel-plate capacitor increases capacitance. Hypothesis: capacitance increases with plate area for fixed separation and dielectric. Methods: Two capacitors were built with the same plate separation and air as dielectric. Condition 1: plate area $A$. Condition 2: plate area $2A$. Each capacitor was charged to the same voltage, then disconnected and its stored charge was measured with an electrometer. Results: The larger-area capacitor stored more charge at the same voltage. What conclusion can be drawn about the variable change?

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