CHEM Lab Final EXAM 2025 | ACTUAL REAL EXAM ACCURATE QUESTIONS AND ANSWERS WITH RATIONALES | VERIFIED AND LATEST UPDATED |GUARANTEED PASS

The molar absorptivity of a compound at 500 nm wavelength is 252 M⁻¹cm⁻¹. Suppose one prepares a solution by dissolving 0.00140 moles of a solute in enough water to make a 500.0 mL solution. What would be the absorbance in a 3.00 mm pathlength cell? A. 0.10584 B. 0.21168 C. 0.15876 D. 0.31542 Correct answer: B. 0.21168 Rationale: Absorbance = ε × l × c ε = 252 M⁻¹cm⁻¹, l = 0.3 cm (since 3 mm = 0.3 cm), c = 0.00140 mol / 0.500 L = 0.0028 M Absorbance = 252 × 0.3 × 0.0028 = 0.21168 To run a spectrophotometry experiment, begin by ____ the spectrophotometer and preparing the samples. Be sure to select the correct ____, then run a measurement on the ___. Follow up by running measurements on ____ solutions. Once data is collected, turn off the instrument, clean the area, and discard the samples. A. heating, solvent, sample, blank B. powering, filter, blank, unknown C. warming up, wavelength, blank, sample D. cleaning, temperature, sample, test Correct answer: C. warming up, wavelength, blank, sample Rationale: Proper spectrophotometry procedure starts with warming up the instrument, selecting the correct wavelength, calibrating with a blank, and measuring the samples. Identify the first step in preparing a spectrophotometer for use. A. Calibrate the instrument with the blank B. Turn on the light source and the spectrophotometer C. Insert the sample D. Choose the wavelength Correct answer: B. Turn on the light source and the spectrophotometer Rationale: Powering on the device and the light source is the first step in using a spectrophotometer to allow proper warm-up and function. Identify the color absorbed by a solution that appears the color given. Blue = ? Red = ? Orange = ? Green = ? A. Blue=Red, Red=Orange, Orange=Green, Green=Blue B. Blue=Orange, Red=Green, Orange=Blue, Green=Red C. Blue=Yellow, Red=Blue, Orange=Red, Green=Orange D. Blue=Green, Red=Blue, Orange=Yellow, Green=Purple Correct answer: B. Blue=Orange, Red=Green, Orange=Blue, Green=Red Rationale: The color a substance appears is the complement of the color it absorbs. Complementary color pairs: Blue–Orange, Red–Green. What type of plot can be used to determine λmax of a solution? A. Concentration vs. Absorbance B. Wavelength vs. Transmittance C. Absorbance vs. Wavelength D. Molarity vs. Volume Correct answer: C. Absorbance vs. Wavelength Rationale: Plotting absorbance versus wavelength allows identification of the wavelength at which the substance absorbs maximally (λmax). Match each formula to its name. CuCl₂ Cu₂O₃ Cu₂O CaO CaCl₂ A. CuCl₂ = Copper(I) chloride, Cu₂O₃ = Copper(III) oxide, Cu₂O = Copper(II) oxide, CaO = Calcium oxide, CaCl₂ = Calcium chloride B. CuCl₂ = Copper(II) chloride, Cu₂O₃ = Copper(III) oxide, Cu₂O = Copper(I) oxide, CaO = Calcium oxide, CaCl₂ = Calcium chloride C. CuCl₂ = Copper(I) chloride, Cu₂O₃ = Copper(II) oxide, Cu₂O = Copper(III) oxide, CaO = Calcium carbonate, CaCl₂ = Calcium chloride D. CuCl₂ = Copper(II) chloride, Cu₂O₃ = Copper(I) oxide, Cu₂O = Copper(III) oxide, CaO = Calcium chloride, CaCl₂ = Calcium oxide Correct answer: B. CuCl₂ = Copper(II) chloride, Cu₂O₃ = Copper(III) oxide, Cu₂O = Copper(I) oxide, CaO = Calcium oxide, CaCl₂ = Calcium chloride Rationale: Use oxidation states and naming conventions of ionic compounds with transition metals and alkaline earth metals. What is the correct formula for manganese(II) fluoride? A. MnF₃ B. Mn₂F C. MnF₂ D. MnF Correct answer: C. MnF₂ Rationale: Manganese(II) means Mn²⁺. Fluoride is F⁻. Two F⁻ ions are needed to balance one Mn²⁺ ion: MnF₂. In the reaction of magnesium metal with hydrochloric acid, how do you determine when the magnesium has reacted completely? A. When the reaction stops bubbling and no metal remains B. When the solution changes color C. When the temperature starts dropping D. When the pH equals 7 Correct answer: A. When the reaction stops bubbling and no metal remains Rationale: The reaction produces hydrogen gas, so bubbling stops once all the magnesium is consumed. A balloon is inflated outside on a cool morning. As the temperature rises in the afternoon, the volume of the balloon will ____. As it cools down again in the evening, the volume will ____. A. decrease; increase B. increase; decrease C. remain constant; remain constant D. expand; explode Correct answer: B. increase; decrease Rationale: According to Charles’s Law, volume increases with temperature and decreases when temperature drops. A sample of gas is subject to changes. What is the effect? The pressure and temperature are both decreased by 50% → The pressure and volume are increased → The pressure is constant, temperature is decreased → Volume and temperature constant, gas is released → A. It will increase; it will increase; it will increase; it will stay the same B. It will decrease; it will increase; it will decrease; it will decrease C. It will stay the same; it will increase; it will decrease; it will decrease D. It will increase; it will decrease; it will increase; it will stay the same Correct answer: C. It will stay the same; it will increase; it will decrease; it will decrease Rationale: • Decreasing both P and T proportionally → no volume change. • P and V ↑ → T must ↑ (Gay-Lussac). • T ↓ at constant P → V ↓ (Charles’s Law). • Letting gas out at constant V and T → fewer particles = ↓ pressure. The H₂ produced in a reaction is collected over water in a eudiometer. If the total pressure is 760.0 torr and the vapor pressure of water is 23.8 torr, what is the pressure of the H₂ gas? A. 736.2 torr B. 783.8 torr C. 760.0 torr D. 23.8 torr Correct answer: A. 736.2 torr Rationale: PH₂ = Ptotal − PH₂O = 760.0 − 23.8 = 736.2 torr Suppose an air bubble is trapped in the eudiometer before starting the experiment. After the experiment, the resulting value of R would be: A. Too small B. Unchanged C. Zero D. Too large Correct answer: D. Too large Rationale: The trapped air inflates the measured volume, making the calculated gas constant (R) artificially high. In the Ideal Gas Law lab, how is the temperature of the hydrogen gas determined? A. By using a gas thermometer B. Measuring room air temperature C. Measuring temperature of the water bath D. Guessing based on volume Correct answer: C. Measuring temperature of the water bath Rationale: Gas temperature is assumed equal to the water bath temperature after equilibrium. The relationship between pressure and volume of a gas is ____ so that as volume decreases, pressure ____. A. direct; increases B. linear; decreases C. inverse; increases D. exponential; decreases Correct answer: C. inverse; increases Rationale: Boyle’s Law states pressure and volume are inversely related. The temperature of a room is 19.1°C. What is this in Kelvin? A. 256.1 K B. 292.25 K C. 273.15 K D. 312.6 K Correct answer: B. 292.25 K Rationale: Kelvin = Celsius + 273.15 → 19.1 + 273.15 = 292.25 K What is the value of the gas constant, R, in units of L·atm/mol·K? A. 0.080 B. 0.00821 C. 8.31 D. 0.0821 Correct answer: D. 0.0821 Rationale: R = 0.0821 L·atm/mol·K is the standard value used in the ideal gas law equation