CENEVAL EGEL Plus Química (EGEL Plus QUIM)

A 25.00 mL aliquot of 0.100 M acetic acid (CH3COOH, Ka = 1.8 × 10⁻⁵) is titrated with 0.100 M NaOH. What is the pH of the solution at the half-equivalence point (after the addition of 12.50 mL of NaOH)?

• A4.74
• B7.00
• C2.87
• D8.72

A standard solution of potassium permanganate (KMnO4) with a concentration of 2.00 × 10⁻⁴ M yields an absorbance of 0.440 at 525 nm in a 1.00 cm cuvette. An unknown solution of KMnO4 analyzed under identical optical conditions gives an absorbance of 0.660. Assuming adherence to the Beer-Lambert law, what is the concentration of the unknown sample?

• A1.33 × 10⁻⁴ M
• B3.00 × 10⁻⁴ M
• C4.40 × 10⁻⁴ M
• D2.50 × 10⁻⁴ M

A 0.5000 g solid sample containing sulfate is dissolved and precipitated quantitatively as barium sulfate (BaSO4, molar mass = 233.39 g/mol). The ignited precipitate weighs 0.3500 g. What is the percentage by mass of sulfate (SO4²⁻, molar mass = 96.06 g/mol) in the original sample?

• A14.41%
• B28.81%
• C41.16%
• D70.00%

In a gas chromatographic separation using a capillary column, an unretained solute elutes at 1.20 min. A target analyte elutes at 4.80 min with a baseline peak width of 0.30 min. What is the retention factor (capacity factor, k) of the target analyte?

• A4.00
• B3.00
• C0.75
• D2.75

In an HPLC analysis, a chromatographic peak has a retention time (tR) of 6.00 min and a width at half-height (w1/2) of 0.15 min. What is the theoretical plate count (N) for this column?

• A8872
• B1600
• C25600
• D6400

An unknown compound has the molecular formula C8H8O2. How many degrees of unsaturation (rings plus π bonds) does it contain?

• A4
• B5
• C6
• D9

What is the potential of a zinc indicator electrode immersed in a 0.0050 M Zn²⁺ solution at 25 °C, given that E°(Zn²⁺/Zn) = -0.763 V? (Use E = E° - (0.0592/n) · log(1/[Zn²⁺]))

• A-0.695 V
• B-0.831 V
• C-0.763 V
• D-0.899 V

When titrating Ca²⁺ with standard EDTA at pH 10 in an ammoniacal buffer using Eriochrome Black T (EBT) as an indicator, what causes the sharp color transition from wine-red to pure blue at the endpoint?

• AThe direct oxidation of the EBT dye by atmospheric oxygen
• BThe displacement of free EBT from the wine-red Ca-EBT complex as EDTA binds all available…
• CThe precipitation of calcium hydroxide which removes the red chromophore
• DThe complete protonation of EDTA into its fully acidic H4Y form

Which of the following compounds is commonly used as an authoritative primary standard for the standardization of aqueous sodium hydroxide (NaOH) solutions?

• APotassium hydrogen phthalate (KHP)
• BAnhydrous sodium carbonate (Na2CO3)
• CConcentrated hydrochloric acid (HCl)
• DSodium thiosulfate pentahydrate

The solubility product constant (Ksp) of lead(II) iodide (PbI2) is 7.1 × 10⁻⁹ at 25 °C. What is the molar solubility of PbI2 in pure water?

• A8.43 × 10⁻⁵ M
• B1.21 × 10⁻³ M
• C1.92 × 10⁻³ M
• D1.53 × 10⁻³ M

In flame atomic absorption spectrometry (FAAS), what is the primary function of the hollow cathode lamp (HCL)?

• ATo atomize the liquid sample into the gas phase
• BTo emit narrow, element-specific atomic spectral lines matching the analyte's absorption…
• CTo serve as a continuous broadband light source across the entire UV region
• DTo detect transmitted photons and amplify the electrical signal

In the permanganometric titration of iron(II) with potassium permanganate in acidic media (5 Fe²⁺ + MnO4⁻ + 8 H⁺ -> 5 Fe³⁺ + Mn²⁺ + 4 H2O), what volume of 0.0200 M KMnO4 is required to completely titrate 25.00 mL of 0.100 M Fe²⁺?

• A5.00 mL
• B125.0 mL
• C25.00 mL
• D50.00 mL

A buffer solution is prepared by dissolving 0.20 mol of sodium acetate (CH3COONa) and 0.10 mol of acetic acid (CH3COOH, Ka = 1.8 × 10⁻⁵) in water to yield 1.00 L of solution. What is the resulting pH of this buffer? (log 1.8 = 0.255, log 2 = 0.301)

• A4.44
• B5.05
• C4.75
• D5.35

In a potentiometric determination of fluoride using an ion-selective electrode, a 50.0 mL sample yields a potential of 120.0 mV. When 1.00 mL of a 100.0 mg/L fluoride standard is added, the potential shifts to 102.0 mV (ΔE = -18.0 mV). Assuming a Nernstian slope of S = -59.2 mV/decade, what was the initial concentration of fluoride in the sample?

• A0.97 mg/L
• B1.90 mg/L
• C0.64 mg/L
• D1.97 mg/L

In reversed-phase high-performance liquid chromatography (RP-HPLC), what are the typical characteristics of the stationary and mobile phases?

• APolar stationary phase (silica gel) and nonpolar mobile phase (hexane)
• BNonpolar stationary phase (e.g., C18-bonded silica) and polar mobile phase (e.g., water/a…
• CCation-exchange resin and acidic buffer mobile phase
• DChiral stationary phase and supercritical fluid carbon dioxide mobile phase

In analytical method validation according to the ICH Q2 guideline, how is the limit of detection (LOD) defined in terms of the blank standard deviation (s_blank) and the calibration sensitivity (slope, m)?

• ALOD = 3.3 · s_blank / m
• BLOD = 10 · s_blank / m
• CLOD = s_blank · m / 3
• DLOD = 2 · s_blank + m

In an electron-ionization mass spectrum, the molecular ion of an unknown compound containing only C, H, N and O appears at m/z 121. What does the nitrogen rule indicate?

• AThe compound contains no nitrogen or an even number of nitrogen atoms
• BThe compound contains an odd number of nitrogen atoms
• CThe compound must contain a halogen
• DThe compound contains exactly two oxygen atoms

An organic analyte has a distribution ratio (D) of 9.0 between diethyl ether and water. If 100.0 mL of an aqueous solution containing 1.00 g of the analyte is extracted with two successive 50.0 mL portions of ether, what mass of analyte remains in the aqueous phase?

• A0.1818 g
• B0.0331 g
• C0.0100 g
• D0.1000 g

During the washing of a freshly filtered colloidal precipitate in gravimetric analysis, why is a dilute electrolyte solution used instead of pure distilled water?

• ATo dissolve residual impurities through complex ion formation
• BTo prevent peptization (the redispersal of the coagulated colloid into fine particles)
• CTo increase the solubility of the desired precipitate
• DTo neutralize the acidic indicator adhering to the filter paper

How many signals appear in the proton-decoupled ¹³C NMR spectrum of p-xylene (1,4-dimethylbenzene)?

• A2
• B3
• C4
• D8

For N2 + 3 H2 → 2 NH3, 28.0 g of N2 reacts with 9.0 g of H2 and 25.5 g of NH3 is obtained. What is the percent yield? (Molar masses: N2 28.02, H2 2.016, NH3 17.03 g/mol)

• A74.9%
• B50.3%
• C68.9%
• D149.7%

A compound contains 40.0% C, 6.7% H and 53.3% O by mass, and its molar mass is 180 g/mol. What is its molecular formula?

• ACH2O
• BC2H4O2
• CC3H6O3
• DC6H12O6

In electron ionization mass spectrometry (EI-MS), the presence of an isotope peak at [M+2]⁺ with approximately one-third (33%) the intensity of the molecular ion [M]⁺ is diagnostic for the presence of which element?

• ABromine (Br)
• BChlorine (Cl)
• CSulfur (S)
• DSilicon (Si)

In the Mohr method for the argentometric determination of chloride with AgNO3 titrant, why must the titration be performed strictly within the neutral pH range of 6.5 to 9.0?

• ASilver chloride decomposes photolytically outside this pH range
• BAt pH < 6.5 the chromate indicator is protonated to HCrO4⁻ suppressing end-point precipit…
• CChloride ions are oxidized to chlorine gas under basic conditions
• DPotassium chromate forms a soluble blue complex with silver above pH 9.0

In analytical data evaluation, what is Dixon's Q-test used for?

• AComparing the precisions of two different analytical methods (variance ratio)
• BDetermining whether an outlying numerical data point should be rejected or retained at a…
• CTesting whether two population means are statistically significantly different
• DCalculating the calibration slope of a nonlinear standard curve

What volume does 0.500 mol of an ideal gas occupy at 25 °C and 1.00 atm? (R = 0.08206 L·atm/(mol·K))

• A1.03 L
• B11.2 L
• C12.2 L
• D1.24 × 10³ L

A solution is 0.010 M in both Cl⁻ and CrO4²⁻. Solid AgNO3 is added slowly (ignore volume change). Which salt precipitates first, and at what [Ag⁺] does it begin? (Ksp AgCl = 1.8 × 10⁻¹⁰; Ksp Ag2CrO4 = 1.1 × 10⁻¹²)

• AAgCl, when [Ag⁺] reaches 1.8 × 10⁻⁸ M
• BAg2CrO4, because its Ksp is smaller
• CAg2CrO4, when [Ag⁺] reaches 1.1 × 10⁻¹⁰ M
• DBoth at the same time, because the anion concentrations are equal

For the reaction A + B → P, initial-rate experiments gave: (1) [A] = 0.10 M, [B] = 0.10 M, rate = 2.0 × 10⁻³ M/s; (2) [A] = 0.20 M, [B] = 0.10 M, rate = 8.0 × 10⁻³ M/s; (3) [A] = 0.10 M, [B] = 0.20 M, rate = 4.0 × 10⁻³ M/s. What is the rate law?

• Arate = k[A][B]
• Brate = k[A]²[B]
• Crate = k[A][B]²
• Drate = k[A]²[B]²

Given C(s) + O2(g) → CO2(g), ΔH° = −393.5 kJ and CO(g) + ½ O2(g) → CO2(g), ΔH° = −283.0 kJ, what is ΔH° for C(s) + ½ O2(g) → CO(g)?

• A−676.5 kJ
• B−110.5 kJ
• C+110.5 kJ
• D−55.3 kJ

Which ligand acts as a chelating agent, binding a metal ion through two donor atoms at once to form a ring?

• AAmmonia (NH3)
• BChloride (Cl⁻)
• CEthylenediamine (H2N–CH2–CH2–NH2)
• DThiocyanate (SCN⁻)

A mixture of benzoic acid and naphthalene dissolved in diethyl ether is shaken in a separatory funnel with 5% aqueous NaOH. After the layers separate, where is each compound?

• ABenzoic acid is in the aqueous layer as sodium benzoate; naphthalene stays in the ether l…
• BNaphthalene passes into the aqueous layer; benzoic acid stays in the ether layer
• CBoth stay in the ether layer, because NaOH does not react with organic compounds
• DBoth pass into the aqueous layer as sodium salts

Why can't simple or fractional distillation of an ethanol–water mixture at atmospheric pressure yield ethanol purer than about 95.6% by mass?

• AEthanol decomposes above its boiling point of 78 °C
• BEthanol and water have the same boiling point
• CThe mixture forms a minimum-boiling azeotrope whose vapor has the same composition as the…
• DWater forms a solid hydrate with ethanol that distills along with it

Which property should an ideal solvent for recrystallizing a solid organic product have?

• AIt dissolves the product very well at room temperature
• BIt dissolves the product well when hot and poorly when cold, while impurities either stay…
• CIts boiling point is higher than the melting point of the product
• DIt reacts with the impurities so that they precipitate together with the product

Which of these substances can be purified by sublimation, passing directly from solid to vapor and back to solid on a cold surface?

• ASodium chloride
• BIodine
• CSucrose
• DGlycerol

Why does a rotary evaporator remove a solvent at a temperature well below its normal boiling point?

• ARotation heats the solvent by friction until it boils
• BThe vacuum raises the vapor pressure of the solvent
• CReducing the pressure lowers the temperature at which the solvent's vapor pressure equals…
• DThe condenser chemically absorbs the solvent vapor

A stream of 100 kg/h of brine containing 20.0% NaCl by mass is mixed with 300 kg/h of brine containing 4.0% NaCl. At steady state, what is the NaCl mass fraction in the outlet stream?

• A8.0%
• B12.0%
• C24.0%
• D10.7%

A reactor is fed 100 mol of A. At the outlet, 20 mol of A remain unreacted; 60 mol of the desired product B (A → B) and the rest as by-product C (A → C) were formed. What are the conversion of A and the yield of B based on A fed?

• AConversion 80%, yield 60%
• BConversion 60%, yield 80%
• CConversion 80%, yield 75%
• DConversion 20%, yield 60%

How much heat is needed to raise 50.0 kg of water in a jacketed vessel from 20.0 °C to 80.0 °C, ignoring losses? (cp = 4.18 kJ/(kg·°C))

• A12.5 kJ
• B1.25 × 10⁴ kJ
• C1.67 × 10⁴ kJ
• D7.38 × 10⁴ kJ

Green chemistry favors reactions with high atom economy. What is the atom economy of C2H5OH + HCl → C2H5Cl + H2O? (Molar masses: C2H5OH 46.07, HCl 36.46, C2H5Cl 64.51, H2O 18.02 g/mol)

• A100%
• B78.2%
• C64.2%
• D21.8%

Under NOM-018-STPS-2015, which adopts the Globally Harmonized System (GHS) in Mexican workplaces, how many sections does a Safety Data Sheet (Hoja de Datos de Seguridad, HDS) have, and which section specifies exposure controls and personal protective equipment?

• A16 sections; section 8
• B16 sections; section 4
• C12 sections; section 8
• D9 sections; section 2

Under NOM-052-SEMARNAT-2005, a waste is classified as hazardous when it appears in the standard's lists or when it has at least one of which characteristics, known by the acronym CRETIB?

• ACarcinogenic, radioactive, ecotoxic, teratogenic, irritant and bioaccumulative
• BCorrosive, reactive, explosive, toxic, flammable and biological-infectious
• CCombustible, refrigerated, evaporable, transportable, industrial and bulk
• DOnly wastes generated in laboratories, regardless of their properties

Which pair of chemicals must not be stored together on the same shelf?

• ASodium chloride and potassium chloride
• BConcentrated nitric acid and acetone
• CAcetone and ethanol in a flammable-liquids cabinet
• DSodium sulfate and magnesium sulfate

What is the correct way to dilute concentrated sulfuric acid in the laboratory?

• AAdd water to the acid quickly so that the dilution happens as fast as possible
• BAdd the acid slowly to the water, with stirring and cooling, working in a fume hood with…
• CMix equal volumes of acid and water in a closed container to avoid splashes
• DHeat the water first so the acid dissolves faster

Hexane in a beaker catches fire in the laboratory. Under the fire classification of NOM-002-STPS-2010, what class of fire is it, and which extinguishing agent is appropriate?

• AClass A; a jet of water
• BClass B; dry chemical powder (ABC or BC) or carbon dioxide
• CClass C; water fog
• DClass D; special agent for combustible metals

Read the Spanish passage and answer. 'La química verde no busca únicamente sustituir disolventes tóxicos por otros más benignos. Su propuesta es rediseñar los procesos desde el inicio: elegir rutas con menos pasos, aprovechar al máximo los átomos de los reactivos y evitar la generación de residuos, en lugar de tratarlos una vez producidos. Por ello, se dice que es preferible prevenir un residuo que remediarlo.' What is the main idea of the passage?

• ALa química verde consiste solo en reemplazar disolventes tóxicos por otros menos dañinos.
• BLa química verde propone rediseñar los procesos para prevenir los residuos, en lugar de t…
• CEl tratamiento de residuos es la etapa más importante de un proceso químico.
• DLas rutas sintéticas con muchos pasos son las más sustentables.

Read the Spanish passage and answer. 'Las plantas de tratamiento convencionales fueron diseñadas para eliminar materia orgánica y microorganismos, no fármacos. Por esa razón, en el efluente de muchas de ellas se detectan antibióticos en concentraciones de nanogramos por litro. Aunque esas cantidades no representan un riesgo tóxico inmediato para las personas, su presencia continua podría favorecer la selección de bacterias resistentes en ríos y lagos.' Which statement can be inferred from the passage?

• ALas plantas de tratamiento convencionales eliminan por completo los antibióticos del agua.
• BLos antibióticos en el efluente representan un riesgo tóxico inmediato para la población.
• CPara reducir el riesgo de resistencia bacteriana, podrían requerirse tratamientos adicion…
• DLa resistencia bacteriana en ríos se debe exclusivamente a las plantas de tratamiento.

When (S)-2-bromobutane is treated with sodium cyanide (NaCN) in dimethyl sulfoxide (DMSO), what is the stereochemical outcome of the resulting 2-methylbutanenitrile?

• AComplete retention of configuration to give (S)-2-methylbutanenitrile
• BInversion of configuration via backside attack to give (R)-2-methylbutanenitrile
• CComplete racemization giving a 1:1 mixture of enantiomers
• DElimination yielding a 1:1 mixture of 1-butene and 2-butene

When 2-bromo-2-methylbutane is treated with potassium tert-butoxide in tert-butanol, what is the major alkene product and which selectivity rule governs it?

• A2-Methyl-2-butene governed by Zaitsev's rule
• B2-Methyl-1-butene governed by Hofmann's rule
• C3-Methyl-1-butene governed by Markovnikov's rule
• Dtert-Butyl 2-methylbutan-2-yl ether formed by SN2 substitution