chem lab EXAM NEW VERSION LATEST UPDATE WITH ACCURATE ANSWERS GUARANTEED PASS BEST STUDYING MATERIAL

What type of information can we derive from the melting point of a compound? A. Molecular weight B. Solubility in water C. Purity and identity D. Boiling point Rationale: Melting point helps determine the purity (sharp or broad range) and identity (comparison with known values) of a compound. What effect do you anticipate on the melting point of a compound if you don’t use a finely powdered solid? A. Higher melting point B. Broader melting range C. No change D. Instant melting Rationale: Coarse solids heat unevenly, leading to a broader observed melting range. What happens if you overload the capillary tube when measuring melting point? A. Lower melting point B. Longer time and broader range C. Sharper melting point D. Faster melting Rationale: Excess sample delays heat transfer, causing a broader range. Which of the melting point capillaries is filled to the proper level for using the Mel-Temp apparatus? A. 3–4 mm of solid B. Less than 1–2 mm of solid C. Entire capillary filled D. Any amount is acceptable Rationale: Overfilling affects heat distribution; proper fill ensures accuracy. Why is it important to heat a sample slowly when determining melting point? A. Avoid explosions B. Ensure accurate melting point range C. Prevent glass breakage D. Decrease melting time Rationale: Rapid heating can overshoot and obscure the true melting range. Why is it sometimes difficult to measure when crystals first liquefy? A. Crystals explode B. Physical changes and solvent impurities ("sweating") C. Crystals melt too fast D. Temperature is too high Rationale: Solvent remnants and subtle physical transitions obscure melting onset. What two effects does a soluble impurity have on the melting point of a compound? A. Increases melting point B. Lowers and broadens the melting point range C. No effect D. Increases sharpness Rationale: Impurities disrupt crystal structure, lowering and broadening the MP. Why should solids be weighed on glazed paper instead of filter paper? A. Glazed paper is heavier B. Filter paper is more accurate C. Filter paper retains some solid D. Filter paper dissolves Rationale: Filter paper can absorb or trap some of the solid, skewing measurements. What is meant by tare? A. Total mass of solid B. Mass of the empty container C. Weight of the solvent D. Boiling point of the substance Rationale: Taring subtracts the weight of the container to get the net weight. What causes liquid to enter a capillary bell during a boiling point measurement? A. External pressure drop B. Vapor pressure equals atmospheric pressure C. Liquid evaporates instantly D. Capillary suction Rationale: When vapor pressure matches atmospheric pressure, liquid reenters capillary. Why are sand baths commonly used in micro-scale organic labs? A. They're easy to clean B. Sand distributes heat evenly and avoids ignition C. Sand cools faster D. Sand is inexpensive Rationale: Sand allows controlled, safe heating without flames. What is the purpose of the cotton plug in a Pasteur pipette during distillation? A. Improve pressure B. Prevent liquid from dripping out C. Absorb vapors D. Remove impurities Rationale: The cotton prevents backflow or dripping. How many mL are occupied by 150 g of cyclohexane (density = 0.779 g/mL)? A. 100 mL B. 192.55 mL C. 300 mL D. 150.00 mL Rationale: Volume = mass/density → 150 / 0.779 ≈ 192.55 mL. What happens if a micro-pipet plunger is pressed fully before filling it? A. Accurate volume is drawn B. It draws air C. More volume than intended is drawn D. Pipette breaks Rationale: Extra suction pulls in too much liquid. Can you measure refractive index if no dark/light line is seen, only a color band? A. Yes, it's temperature-dependent B. Yes, refractive index involves angle change between media C. No, measurement is impossible D. Only with oil immersion Rationale: Refractive index is based on light angle change, not just visual contrast. Why should activated carbon be added to a cool solution before heating? A. To preserve color B. To prevent violent boiling/boil-over C. To speed up boiling D. To reduce solubility Rationale: Adding carbon to boiling solutions causes sudden frothing. What is oiling out? A. Formation of an emulsion B. When BP is above MP, leading to liquid separation C. When impurities crystallize D. Solvent evaporates Rationale: Occurs when the compound becomes a liquid before it can crystallize. What do you use to remove a colored impurity? A. Boiling B. Cooling C. Activated carbon D. Ether Rationale: Activated carbon adsorbs colored impurities. What to do if crystals form prematurely in funnel stem during gravity filtration? A. Add cold solvent B. Preheat funnel to prevent premature crystallization C. Increase pressure D. Stop filtration Rationale: Maintaining temperature prevents blockage. Why shouldn’t you heat organic solvents over an open flame? A. They evaporate too fast B. They decompose C. They are flammable D. They turn acidic Rationale: Most organic solvents are volatile and easily ignite. What to do if crystals fail to form in solution? A. Add base B. Shake it C. Scratch the glass surface to induce crystallization D. Add ice Rationale: Scratching provides a nucleation site for crystals. Which solvent is best for recrystallizing Compound A? A. Water B. Ether C. Methanol D. Acetone Rationale: A good solvent dissolves the compound when hot, but not cold. Why shouldn't boiling chips be added to a boiling liquid? A. They dissolve B. They cause violent boiling/boil-over C. They reduce evaporation D. They stain the solution Rationale: Sudden nucleation leads to frothing and splashing. Where should the thermometer be positioned in distillation? A. Above vapor B. Near the flask C. Bulb submerged at vapor path for accurate boiling point D. Below the heating mantle Rationale: Proper placement ensures accurate temperature of distilling vapor. What is the purpose of the outer jacket in a fractionating column? A. Reduce pressure B. Allow vacuum C. Condense vapor to liquid D. Add air Rationale: Jacket ensures reflux and separation by cooling vapors. How is heating rate adjusted with a sand bath? A. Change power supply B. Adjust exposure area C. Increase sand D. Add oil Rationale: Moving the flask in/out of the hot area changes the heating rate. How is distillate collected in a microscale distillation? A. Graduated cylinder B. Bent-tip Pasteur pipette C. Round bottom flask D. Watch glass Rationale: Pasteur pipettes allow safe, direct collection. What happens to boiling temperature if heating rate increases during distillation? A. BP lowers B. Liquid freezes C. Boil-over and contamination may occur D. Nothing changes Rationale: Excessive heating can cause splashing and mixture contamination. When is the concentration of the more volatile compound increased in fractional distillation? A. When cooled B. During vaporization then condensation cycles C. While pouring D. At the start only Rationale: Repeated vapor-condensation enriches the more volatile component. When is fractional distillation commonly used? A. With compounds with high MP B. When BP difference is <40°C C. When separating solids D. When filtering Rationale: It’s ideal for separating liquids with close boiling points. What are hazards of tert-butyl methyl ether and HCl? A. Explosive B. Radioactive C. Flammable, toxic, and irritating D. Carcinogenic Rationale: Both chemicals pose risks from inhalation, flammability, and skin contact.