MCAT TEST 4 review questions and 100% correct answers 2025

When a gas atom (or molecule) is ionized, in which direction are the electrons accelerated? - ANSWER Toward the cylinder Being negatively-charged, electrons would be accelerated toward the cylinder, since the cylinder is at a higher potential than the wire. [Note that the cylinder is connected to the (+) terminal of the battery, while the wire is connected to the (-) terminal.] Which of the following changes would decrease the sensitivity of the Geiger counter? - ANSWER Using a gas with a higher ionization energy The Geiger counter emits clicks when radiation from decaying radioactive nuclei ionizes the gas in the tube and initiates a cascade of emitted electrons. If the gas had a high ionization energy, then decaying radioactive nuclei which emit radiation of low energy would be unable to ionize the gas, so the decay would go undetected (no click). Therefore, using ages with a higher ionization energy would decrease the sensitivity of the Geiger counter. All forms of nuclear radiation - ANSWER alpha, beta, and gamma—are ionizing to some extent, so can cause risk If the ionizing gas were evacuated from the Geiger counter tube while it was in operation, which of the following would occur? - ANSWER The charge on the cylindrical shell would decrease. according to the passage, the cylindrical shell and wire act as a capacitor. The gas in between the two therefore acts as a dielectric. Removal of that gas (replacing it with vacuum) reduces the dielectric constant to 1 between the wire and the cylinder, thus reducing capacitance. If capacitance decreases while the high voltage battery remains connected (as it does while the Geiger counter is in operation, thereby maintaining the voltage between the cylindrical shell and the wire, the charge must also decrease according to Q = CV. Why is the copper(I) catalyst not necessary to link the fluorophore bearing the cyclooctyne to the stem peptide, even though it is required for other alkynes? - ANSWER The Ea of the reaction is low due to the ring strain from the cyclooctyne The activation energy of a reaction can be lowered by stabilizing the transition state of the rate-determining step, or by increasing the relative energy of a substrate that is involved in the kinetics of the reaction (less stable molecules are more reactive). The cyclooctyne ring is highly strained due to the linear geometry of the alkyne. Therefore this substrate is highly reactive and will readily react with an azide without the need for a rate increase from the copper(I) catalyst It has two total stereocenters and three stereoisomers, because one of its - ANSWER stereoisomers is meso compound