PT GEN CHEM

100問 • 3ヶ月前

Liane

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Which of the following practices helps minimize weighing errors due to temperature effects?

Allowing samples to cool in a desiccator before weighing.

Which of the following will NOT cause drift errors in an analytical balance?

The temperature of the balance and the sample is the same.

Which of the following statements about proper weighing techniques is correct?

Weighing by difference is recommended for hygroscopic substances to prevent moisture absorption.

A student is tasked to measure 12 mL of a liquid as precisely as possible. Which measuring device should be selected for this task?

25 mL graduated cylinder

Examine the graduated cylinders shown. What are the correct volumes of the liquids in each?

6.70 mL and 2.30 mL

Which of the following statements about volumetric laboratory equipment is true?

Volumetric flasks are calibrated to contain a specific volume at a specific temperature, typically 20°C.

Which of the following is the most appropriate instrument for determining the density of a liquid?

Pycnometer

Which of the following is true for a 19/22 ground glass joint (stopper)?

Diameter = 19 mm, length of the ground-glass surface = 22 mm

Which technique is recommended for determining the odor of an unknown liquid?

Use your hand to waft (fan) a small amount of vapor from the container toward your nose and then smell cautiously.

A student needs to transfer a precipitate from a beaker to filter paper. Which procedure would best ensure a quantitative transfer of the precipitate?

Decant most of the supernatant, transfer the bulk of the precipitate to the filter paper with a stirring rod, then wash the beaker several times with small volumes of wash solution into the filter to recover any remaining precipitate.

The appropriate sample size for analysis depends on which of the following factors?

All of the above

Which of the following are possible sources of contamination during sample preparation?

All of the above

Which of the following can cause error during sample preparation? I. Loss of sample during weighing or dissolution. II. Contamination by impurities in reagents or the environment. III. Changes in mass due to varying humidity and temperature. IV. Obtaining a nonhomogeneous sample from the bulk.

I, II, III, and IV

Which method involves passing the sample through a metal or plastic mesh of uniform cross-sectional area to separate particles into uniform sizes?

Sieving

Objects should not be weighed when they are hot or warm, as this can affect accuracy. If a sample is left to cool in open air it may absorb moisture. To prevent this, the sample should be placed in a:

Desiccator

Which of the following methods can remove a significant amount of water from a sample containing heat-sensitive material?

Freeze-drying (lyophilization)

Which sample preparation technique is commonly used prior to instrumental analysis of metals?

Acid digestion

Which type of crucible offers the highest maximum temperature resistance and is suitable for high-temperature applications?

Zirconia crucible

What is a key advantage of microwave digestion over traditional wet or dry ashing methods?

Reduced contamination risk and faster processing

Which of the following methods is expensive and often used as a last resort in the decomposition and dissolution of solids during sample preparation?

Fusion (flux fusion)

Which of the following is the correct sequence of steps in a typical liquid–liquid extraction procedure?

Select solvents, add mixture to separatory funnel, mix the layers, allow layers to separate, drain the bottom layer

Which of the following is an important consideration when performing a liquid–liquid extraction?

Label containers clearly to indicate the identity of the layers

In liquid–liquid extraction, to test whether the top layer is aqueous or organic, a drop of water can be added to the top layer. If the drop of water dissolves in the top layer, then the top layer is:

Aqueous

Which of the following is used to remove trace amounts of water from pooled organic layers before removal of the organic solvent?

Drying agent

A 5-g organic solid is dissolved in 100 mL of distilled water. Which method would be most efficient to recover the majority of the organic solid?

Multiple extractions using three 50-mL portions of ether

Which of the following is a correct step during recrystallization?

Use hot gravity filtration to remove insoluble impurities, ensuring the solution is filtered quickly with fluted filter paper.

Why is slow cooling important during recrystallization?

It allows the compound to crystallize slowly, resulting in larger, purer crystals.

Which of the following is true about vacuum (suction) filtration?

A Büchner funnel is typically used along with filter paper to separate solids from liquids under vacuum.

All of the following statements are true, except:

Every pure solid crystalline substance has a characteristic and unique melting point.

Which of the following statements about melting point determination is true?

The melting point range of a pure compound is typically within 1–2 °C, and this can help confirm its identity and purity.

Which of the following best defines the boiling point of a liquid?

The temperature at which the liquid's vapor pressure equals the external (atmospheric) pressure.

A researcher is tasked with purifying a solvent that has a significantly higher boiling point than the nonvolatile impurities mixed with it. Which type of distillation method would be most suitable for this separation?

Simple distillation

A student needs to separate a mixture of ethanol and water but wants to break the constant-boiling azeotrope between them. Which distillation method would be most effective for this purpose?

Azeotropic distillation

A student is conducting a distillation of a substance that requires heating to 250°C. The substance is sensitive to overheating, uniform heat distribution is essential, and a relatively large flask will be used. Which heating method would be most appropriate?

Sand bath

A laboratory technician is attempting to distill a heat-sensitive substance that could decompose at temperatures above 100°C. They want a heating method that provides even and gentle heating without risking overheating. Which heat source is most suitable for this task?

Water bath

During a distillation process, why should the flask not be filled more than two-thirds full?

To allow enough space for the liquid to vaporize and condense without overflowing.

What is the most likely consequence of heating a liquid during distillation without using boiling chips?

The liquid may undergo "bumping," leading to sudden, violent boiling, which can cause spillage and loss of sample.

During fractional distillation, what role does the fractionating column play in achieving a high degree of separation between components with close boiling points?

It provides a large surface area for repeated condensation and vaporization, allowing for gradual separation of components.

Which of the following best describes the primary purpose of heating under reflux in organic synthesis?

To maintain a constant temperature for a reaction while preventing the loss of volatile reactants or solvents through evaporation.

A reaction mixture needs to be cooled to −77°C after refluxing. Which of the following is the most appropriate cooling method?

Place the reaction mixture in a dry ice–acetone mixture.

According to the Aufbau principle, which orbital is filled immediately after the 3p orbital?

Which principle restricts the occupancy of an atomic orbital to no more than two electrons?

Pauli exclusion principle

What is the electron configuration of a neutral chlorine atom (Cl)?

1s2 2s2 2p6 3s2 3p5

Which of the following species is not isoelectronic with neon (Ne)?

Na (neutral)

An atom of which of the following elements has the largest atomic radius?

Arrange Li, Na, K, and Rb in order of increasing first ionization energy.

Rb < K < Na < Li

Which of these elements has the greatest electronegativity?

F (fluorine)

Which pair represents isotopes of the same element?

16O and 17O

What is the product of alpha emission from the isotope uranium-238?

234Th

When carbon-14 (C-14) undergoes beta decay, which of the following is produced?

Nitrogen-14 and a beta particle

Which aspect of metallic bonding best accounts for the characteristic malleability of metals?

Delocalized electrons (a sea of electrons) that allow metal ions to slide past each other

Which of the following best describes a covalent bond?

A pair of electrons is shared between two atoms and attracted to both nuclei

Which of the following compounds contain both ionic and covalent bonds? I. NH4Cl II. NaOH III. MgO

I and II only

Which of the following statements is true about covalent bonds?

Single bonds have lower bond energy than double bonds

Which of the following covalent compounds has a tetrahedral molecular geometry?

CH4

Which of the following is a polar molecule? A. CO B. H2 C. CCl4 D. XeF4

A. CO

Which of the following does NOT describe a nonpolar covalent bond? A. It is a bond between identical nonmetal atoms. B. It involves equal sharing of electrons. C. It has a dipole moment. D. The electronegativity difference between the atoms is zero or very small.

C. It has a dipole moment.

What type of intermolecular force must be overcome when hexane (C6H14) is vaporized? A. Hydrogen bonding B. Covalent bonding between carbons C. Dipole-dipole interactions D. London dispersion forces

D. London dispersion forces

Which compound has the strongest intermolecular forces of attraction? A. CH4 B. CO C. CH3OH D. CH3OCH3

C. CH3OH

The molecules in a sample of pure liquid dichloromethane (CH2Cl2) experience which of the following intermolecular forces? I. Dispersion forces II. Dipole-dipole III. Hydrogen bonding. Options: A. I only B. II only C. I and II only D. I, II, and III

C. I and II only

A solution is:

Unsaturated — it can dissolve more solute at the given temperature.

What generally happens to the solubility of gases in water as the temperature increases?

It decreases because gas molecules escape more easily.

A 100.0 mL solution of H2SO4 has a molarity of 6.00 M and a density of 1.34 g/mL. What is the molality of the solution?

7.98 mol/kg

Which procedure will produce a 0.20 M solution of Na2SO4?

Dissolving 0.100 mol Na2SO4 in 500 mL water., Diluting 400 mL of 0.50 M Na2SO4 to a final volume of 1.0 L.

The lining of the stomach contains cells that secrete hydrochloric acid. Which of the following would best help alleviate heartburn, which is caused by excess stomach acid?

Milk of magnesia (pH 10.5)

Which of the following is NOT an acid–base conjugate pair?

H2S and OH-

The cells lining the stomach secrete HCl at a typical concentration of 1.2 × 10^-3 M. What is the pH of this solution?

2.92

What are the products of the reaction between nitric acid (HNO3) and hydroxylamine (NH2OH)?

NH3OH+ + NO3- (hydroxylammonium nitrate)

A compound has an empirical formula of CH2 and a molar mass of 56.0 g·mol^-1. What is its molecular formula?

C4H8

For which of the following compounds are the empirical and molecular formulas the same?

C2H6O (ethanol)

In the analysis of sugar concentration in a powdered juice solution (powdered juice dissolved in water), which of the following best represents the matrix?

Both the powdered juice and the water (the sample matrix excluding the analyte)

Which type of blank is commonly used in analytical techniques such as spectrophotometry to zero the instrument before measuring test samples and other blanks?

Reagent blank

A balance consistently reads 0.005 g lower than the true mass for all samples, regardless of the operator. What type of error is this, and which aspect of measurement does it primarily affect?

Systematic error; affects accuracy

A student measures the concentration of a salt solution to be 0.98 M, while the true concentration is 1.00 M. What is the percent relative error of the measurement?

2.00%

In a Gaussian (normal) distribution, approximately what percentage of the data values lie within ±1 standard deviation from the mean?

About 68%

Which of the following factors should be considered when selecting an appropriate analytical method?

All of the above.

What is the main principle behind gravimetric analysis?

Determining the mass of a precipitate to find the analyte amount.

Which of the following analytical methods is classified as a primary method?

Gravimetric determination of chloride by precipitation as AgCl.

Which of the following best differentiates a tertiary method from primary and secondary methods?

It is based on simple tools or kits that are often pre-calibrated and designed for rapid field use.

Which of the following is NOT a redox reaction?

AgNO3 + NaCl -> AgCl + NaNO3

Which of the following can be a mobile phase in thin layer chromatography?

A liquid solvent such as isopropanol.

In planar chromatography, a compound shows a very low Rf value. Which of the following best explains this observation?

The compound is strongly retained by the stationary phase due to its high polarity.

A chromatography system uses a silica-based stationary phase and a nonpolar mobile phase. Which compound will elute first in this setup?

A nonpolar compound with a large hydrophobic region.

Which chromatographic method is most commonly used for the analysis and separation of components in industrial products like soaps and synthetic detergents?

Thin layer chromatography

Which of the following is not a characteristic of an open tubular (capillary) column used in gas chromatography?

Poor resolution can be obtained.

An air sample is analyzed by gas chromatography to detect Freon-11 (CCl3F). Which of the following detectors is most suitable for this analysis?

Electron Capture Detector (ECD)

What molecular-weight-related advantage does HPLC have over GC for the separation and analysis of high molecular weight compounds?

Volatile derivatives are not required for analysis in HPLC

Which of the following is NOT an application of HPLC?

Elimination of undesirable substances from blood

Lithium, sodium and potassium ions in an acidified solution can best be separated by which chromatographic technique?

Ion Exchange Chromatography

Which chromatographic method is most suitable for purifying a specific enzyme from a mixture using a substrate or inhibitor immobilized on the stationary phase?

Affinity Chromatography

For the equilibrium Fe3+ + SCN- ↔ FeSCN2+, the equilibrium constant for the reaction can best be determined by means of:

Spectrophotometry

What is the correct order in which light passes through a UV-Vis spectrophotometer?

Source → Monochromator → Sample → Detector

What is the primary purpose of a monochromator in a spectrophotometer?

To allow only light of a certain wavelength to pass from the source to the sample

Which of the following methods demonstrates the proper way to dry a cuvette before use?

Blotting it gently with an absorbent, lint-free towel or tissue and then allowing it to air dry

A student uses Beer's Law to determine the concentration of a colored salt solution by measuring its absorbance at the salt's maximum absorbance wavelength (Amax). Which of the following would most likely cause the calculated concentration to be higher than the actual concentration?

The cuvette is not wiped clean before being placed in the spectrophotometer

Which of the following is the most widely used continuous atomization method in Atomic Absorption Spectroscopy (AAS), known for being the most reproducible technique for liquid-sample introduction to date?

Flame

In a hollow cathode lamp diagram, one component at the end of the lamp (through which the emitted light exits) is unlabeled. Based on its position and function, which of the following best identifies this part?

Quartz window

Which of the following is NOT typically used as an IR sample preparation technique?

Flame ionization

Which functional group typically shows a strong, broad absorption around 3200–3600 cm⁻¹ in an IR spectrum?

O–H (alcohol or carboxylic acid)

100

The molecular ion peak in a mass spectrum corresponds to:

The ion with the highest m/z value representing the whole molecule

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