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Correct Answer : A

Glass membrane resistance roughly triples for each ten degree fall, so the time constant of the measuring circuit lengthens and readings take much longer to settle, sometimes minutes near zero degrees. Low temperature electrodes use a lower resistance glass. Buffer values do change but that is a calibration matter, crystallisation blocks the junction rather than slowing the glass, and the Nernst slope falls with temperature but never reverses. Patience, or a warmer sample, is the practical answer.

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Correct Answer : B

Conductance is conductivity times area divided by separation, so bringing the electrodes closer raises it, as does making them larger. Reducing the area lowers it, frequency affects polarisation error rather than the true conductance, and dilution lowers the conductivity itself. Recognising which changes belong to the cell and which to the solution is the practical content of the cell constant: the instrument reports conductance, and only division by the cell constant yields a property of the liquid.

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Correct Answer : A

Potassium chloride solutions of 0.1, 0.01 and 0.001 molar have certified conductivities tabulated to high accuracy at 25 degrees, so measuring the conductance of one and dividing the known conductivity by it gives the cell constant directly. Sodium hydroxide absorbs carbon dioxide and is unstable, distilled water is far too variable and too resistive to serve as a standard, and sulfuric acid is neither certified nor stable enough. The choice again turns on equal ionic mobilities and chemical stability.

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Correct Answer : B

The hydrogen ion moves by the Grotthuss mechanism, in which a proton hops along a chain of hydrogen bonded water molecules rather than the ion itself migrating, giving it a mobility some five to seven times that of sodium. Hydroxide moves by a related mechanism and is second. This is why acids and bases dominate the conductivity of a solution containing them, and it is the basis of the very characteristic V shaped conductometric titration curve of a strong acid against a strong base.

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Correct Answer : B

A two point calibration sets the offset from the near neutral buffer and the slope from a second buffer on the side where the samples lie, acid or alkaline. One point fixes the offset but leaves the slope assumed, which is unsafe with an ageing electrode. Three points are worth using when samples span a wide range, but four is unusual, and no meter is accurate without calibration because the asymmetry potential is specific to the electrode and changes with time.

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Correct Answer : D

Broad underlying humps come from species that tumble slowly, such as polymers, aggregates or particulate matter, whose short T2 gives very wide lines. Filtering the sample or diluting it often removes them. A phase error deforms individual peaks rather than adding a featureless hump, truncation gives sinc wiggles beside sharp peaks, and excess TMS gives a large sharp line at zero. Distinguishing a broad genuine signal from a baseline fault is a matter of seeing whether it survives a change of processing.

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Correct Answer : A

COSY, HSQC and NOESY all build a second frequency dimension by incrementing an evolution period, whereas DEPT is a one dimensional experiment that uses a variable final pulse angle to edit the carbon spectrum by attached proton count. Its output is a single spectrum with peaks pointing up or down, not a contour map. The distinction matters when planning instrument time, since DEPT takes minutes and a good HMBC can take hours.

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Correct Answer : A

The gyromagnetic ratio is the magnetic moment divided by the spin angular momentum, a fixed property of each isotope. It sets the resonance frequency in a given field through the Larmor equation and it sets the sensitivity, which scales as its cube. The proton has the largest value of the common nuclei, which is why it is the workhorse. Mass to charge ratio belongs to mass spectrometry, level populations follow from the Boltzmann law, and linewidth is a relaxation matter. Knowing the ratio for a nucleus lets you predict its frequency at any field.

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Correct Answer : D

The Larmor equation says the resonance frequency is directly proportional to the applied field, the constant of proportionality being the gyromagnetic ratio of the nucleus divided by two pi. Doubling the field doubles the frequency, which is exactly why instruments are named by the proton frequency they reach: a 400 MHz spectrometer holds about 9.4 tesla. Neither an inverse nor a square root dependence appears anywhere in the treatment, and a field-independent frequency would make the whole technique useless for chemistry.

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Correct Answer : D

The term is k over one plus k, which approaches one asymptotically, so beyond about ten there is almost nothing left to gain while the analysis time grows without limit. Selectivity and plate number are not systematically affected, and detector response does not fall. Recognising that a term is saturating is what tells you to stop pushing it and move to another lever.

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Correct Answer : A

Sending the whole sample onto the column suits trace work, where discarding ninety-nine per cent of it would put the analytes below detection. A concentrated sample would overload the column, packed columns take direct injection, and thermal lability calls for on-column injection. Which injection mode to use is decided by concentration first and by thermal stability second, and the two questions are quite separate. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

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Correct Answer : A

Partition takes time, and at high velocity a molecule is swept along before it has settled into equilibrium, so some molecules run ahead and others lag. Phase stripping, turbulence and pressure drop are not the mechanism. A term proportional to velocity, opposing one inversely proportional to it, is exactly what produces a minimum, and the balance point is the optimum flow. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

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Correct Answer : C

Applying the standard relation for a base-width measurement:

\[ N = 16\left(\frac{t_{R}}{w}\right)^{2} = 16(30)^{2} = 14400 \]

giving 14400. The factor of sixteen belongs to the base width; a half-height measurement uses 5.54 instead. Quoting which width was used is essential, since the two constants differ by nearly threefold and a plate number without its method is meaningless.

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Correct Answer : C

In this context mass defect means how far an element's exact mass departs from the nearest integer - hydrogen is 1.00783 and oxygen 15.9949, one above and one below. The nuclear binding energy interpretation is the physicist's usage of the same term. Because hydrogen-rich compounds have positive defects and oxygen-rich ones negative, the decimal part of a measured mass itself constrains the formula. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

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Correct Answer : B

Arsenic, selenium, antimony, bismuth and tin form volatile hydrides readily, which is what the technique depends on. Alkali metals, refractory elements and common transition metals do not. That an entire analytical approach is available for one chemical family and useless for the rest is worth knowing before a method is sought for an unfamiliar element.

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Correct Answer : C

Diamond withstands hard samples clamped at high pressure and is chemically inert, which zinc selenide and germanium are not. Its index is lower than germanium's, it has a weak absorption of its own near 2000, and it is far more expensive. A material chosen for durability rather than optical performance is the right trade when the alternative is replacing a soft crystal regularly.

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Correct Answer : A

Water's hydroxyl bands dominate the infrared spectrum and swamp the sample's, while its Raman scattering is weak and leaves the spectrum clear. It does dissolve salt cells, which is a separate practical nuisance. It certainly has Raman-active vibrations, just weak ones. That one technique tolerates the most important biological solvent and the other does not is a large part of why Raman is used for aqueous and biological samples at all.

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Correct Answer : B

Smoothing averages neighbouring points, and if the window is wider than a genuine feature that feature is broadened and reduced along with the noise. Baseline, wavelength scale and absolute readings are not systematically altered. Choosing a smoothing width smaller than the narrowest real feature is the rule, and violating it produces a clean-looking spectrum that has quietly lost the information it was run to find.

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Correct Answer : C

A polar or hydrogen-bonding solvent interacts strongly with the lone pair in the ground state, lowering its energy and widening the gap - the opposite of what happens to a pi to pi star band. The excited state here is less polar rather than more. That the two transitions move in opposite directions with solvent polarity is the most reliable way to tell them apart, and it costs only a second spectrum in a different solvent. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

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Correct Answer : B

Photon energy is Planck's constant times frequency, and frequency is the speed of light over wavelength, so energy goes as one over wavelength - which is exactly what wavenumber is. The counting description is true but is not the reason, units are irrelevant, and energy is linear in frequency rather than quadratic. A unit chosen so that a scale becomes linear in the quantity of interest is worth adopting, and it is why infrared spectra are plotted the way they are.