A pH meter must be calibrated with standard buffers before use because the electrode's
Answer: A
A real electrode departs from the theoretical 59 millivolts and carries an offset that changes as the glass ages, so two buffers fix both the slope and the intercept. Calibration is the largest single source of error in routine pH work.
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An ion selective electrode differs from a glass pH electrode in responding to
Answer: B
Correct answer: an ion other than hydrogen
An ion selective electrode works on the same principle as the glass electrode - a membrane potential responding to the activity of one ion - but with a membrane chosen for a different species: fluoride using a lanthanum fluoride crystal, and others for nitrate, calcium, potassium and so on. Each has an interference from ions of similar size and charge, which is the practical limit on its selectivity.
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Specific conductance of a solution is the conductance of a portion
Answer: C
It is the reciprocal of resistivity, has units of siemens per centimetre, and rises with concentration because more ions are present in the fixed volume. The equivalent and molar conductances refer instead to a fixed amount of solute.
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Equivalent conductance is obtained from specific conductance by
Answer: D
Correct answer: multiplying by 1000 and dividing by the normality
Equivalent conductance is
\[ \Lambda = \frac{1000\kappa}{N} \]where the 1000 converts litres to cubic centimetres. It gives the conductance of all the solution containing one gram equivalent, which is why it can be compared between electrolytes and between concentrations while specific conductance cannot.
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Equivalent conductance increases on dilution while specific conductance decreases because dilution
Answer: A
Correct answer: reduces the ions per unit volume but increases dissociation
The two quantities move oppositely because they measure different things. Specific conductance is per unit volume, and dilution puts fewer ions in each cubic centimetre, so it falls. Equivalent conductance refers to a fixed quantity of solute, and dilution increases the degree of dissociation and reduces the interionic attraction, so more of that fixed quantity conducts and it rises.
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The cell constant of a conductance cell is obtained by measuring
Answer: B
Standard potassium chloride solutions have tabulated specific conductance, and dividing that by the observed conductance gives the constant in reciprocal centimetres. Measuring the geometry directly is neither accurate nor practical.
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Conductance is measured with alternating current rather than direct current to avoid
Answer: C
Correct answer: polarisation of the electrodes
Direct current would electrolyse the solution, changing its composition and building up layers of product at the electrodes whose back EMF would corrupt the measurement. Alternating current at about a kilohertz reverses before any appreciable electrolysis occurs, so the electrodes are not polarised. Platinised platinum electrodes are used for the same reason, their large real area reducing the current density further.
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Kohlrausch's law of independent migration states that the limiting equivalent conductance of an electrolyte is the
Answer: D
At infinite dilution each ion migrates without regard to its partner, so ionic conductances are additive and can be tabulated separately. It allows a weak electrolyte's limiting value to be assembled from strong electrolyte data.
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The limiting equivalent conductance of a weak acid cannot be found by extrapolation because its equivalent conductance
Answer: A
Correct answer: rises steeply and without limit near infinite dilution
A strong electrolyte’s equivalent conductance varies linearly with the square root of concentration and can be extrapolated to zero. A weak electrolyte is barely dissociated at measurable concentrations and approaches complete dissociation only asymptotically, so its curve turns sharply upward near the axis and no extrapolation is possible. Kohlrausch’s law of independent migration supplies the limiting value instead.
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The degree of dissociation of a weak electrolyte is estimated as the ratio of its equivalent conductance at the given concentration to its value at
Answer: B
Correct answer: infinite dilution
At infinite dilution the electrolyte is completely dissociated, so the equivalent conductance there represents all the solute conducting. The ratio
\[ \alpha = \frac{\Lambda_c}{\Lambda^\circ} \]therefore gives the fraction actually dissociated at the concentration measured, and from it the dissociation constant follows through Ostwald’s dilution law.
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