Test: Titrations - MCAT MCQ

The balanced reaction is  HCl + NαOH → H₂O + NαCl

Convert the molarity into a number of mols 33 mL x 3 M ≈ 100 mmol

Using the balanced equation, one mol of sodium hydroxide is required for each mol of hydrochloric acid consumed

The reaction consumes 100 mmol of sodium hydroxide

The formulae for barium hydroxide and sulfuric acid are

The balanced reaction is

Because the stoichiometric ratio of the reactants is 1 : 1, we can use the shortcut formula M_bV_b = M_αV_α

In a weak-strong titration, the equivalence point is shifted from neutralization towards the pH of the strong reagent (a base here)

The equivalence point occurs when the mols of acid added stoichiometrically balances the moles of base added.

The pH of a weak acid, strong base titration is greater than 7 at the equivalence point.

During a titration, as titrant is added the pH gradually changes from the pH of the initial solution to being the pH of the titrant.

The pH takes on maximal values only at maximal values on the horizontal (volume) axis

The “crossover” between the portion of the graph where the solution has pH closer to the initial solution, and the portion in which the titrant is in excess, is known as the equivalence point

The magnitude of the slope of the graph is greatest at the equivalence point.

Unlike a strong acid, a weak acid does not completely neutralize; instead an Initial-Change-Equilibrium (ICE) table may be constructed for this problem.

For every unit x of H Z that reacts, a unit x of both H₃O⁺ and Z^- is created due to the balanced reaction equation.

The expression for the dissociation constant of an acid in water is Substituting our values into this expression results in the equation 10e - 8 = 

Because H Z is a weak acid, approximate 1.00 - x ≈ 1.00  and solve the resulting equation x² = 10e - 8 ⇒ 10e - 4, where x is the equilibrium concentration of acid The pH is thus pH = -log(10e - 4) = 4

Because this question asks about the limiting case of an excess of titrant, you can pick any titrant volume that is in excess of that needed to obtain the equivalence point. Not knowing anything about the species being titrated, a safe bet is to use the case where an infinite amount of titrant has been added to the solution.

In the limit of an infinitely amount of titrant has been added to the solution, the pH should become just the pH of the titrant---the original solution gets diluted away.

We thus expect the asymptotic pH of a titration curve to approach that of the titrant.

While an acid with a pH less than 0 is likely a strong acid, many strong acids can have pH greater than zero depending on their concentration

Proton donation versus electron acceptance differentiate Bronsted-Lowry and Lewis acids, but not their relative strengths

Complete versus partial ionization in solution is the primary determinant of whether an acid is considered weak.

The buffer region derives its name because the pH of the solution does not greatly change when the titrant is added to the solution.

The buffer region does not correspond to the region of the curve where an excess of titrant has been added, such that the pH of the solution approaches that of the titrant

The buffer region thus corresponds to the flat portion of the curve just before the equivalence point.

The balanced reaction is

First find the number of mols that reacted, 0.5 x 10 = 5 mmol. According to the balanced reaction, this requires 10mmol of HCl

Find the concentration of the acid using the definition of molarity, 10/50 = 1/5 M

The equivalence point of the titration of a weak base by a strong acid will occur at a pH less than 7

A good indicator will be able to indicate the equivalence point right as it occurs, making a narrow range preferable to a large range like 5 - 8

The best indicator has a pH in the range 4 - 6