Question 1

Which of the following compounds would not have a pH dependent solubility?

Accepted Answer

The answer of Which of the following compounds would not...

Question 2

Acid-base indicators need to have very intense colors so that a very low concentration is visible. Could there be a problem in using too much indicator?

Accepted Answer

Using too much indicator could affect the acid-base chemistry being measured, as the indicator itself could contribute to the overall pH of the solution, leading to inaccurate results.

Question 3

A phosphate buffer solution (25.00 mL sample) used for a growth medium was titrated with 0.1000 M hydrochloric acid. The components of the buffer were sodium monohydrogenphosphate and sodium dihydrogenphosphate. The first endpoint occurred at a volume of 10.32 mL; the second occurred after an additional 18.62 mL was added (or total volume = 28.94 mL). What was the approximate ratio of HPO₄²^- to H₂PO₄^-in the buffer?

Accepted Answer

The answer of A phosphate buffer solution (25.00 mL sample)...

Question 4

As the pH decreases, the solubility of __________ would increase.

Accepted Answer

The answer of As the pH decreases, the solubility of...

Question 5

Which of the following is not a buffer system? A solution containing roughly equal concentrations of __________

Accepted Answer

A buffer system consists of a weak acid and its conjugate base or a weak base and its conjugate acid. Option B contains a strong acid (hydrobromic acid) and its corresponding salt (bromide ion), making it not a buffer system.

Question 6

A solution that contains a weak acid and its conjugate base in roughly equal concentrations is __________

Accepted Answer

A solution containing a weak acid and its conjugate base in roughly equal concentrations is a buffer solution. A buffer solution can resist changes in pH when small amounts of acid or base are added to it, making it important in many biological and chemical processes.

Question 7

The following titration curve is most likely to be associated with

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The answer of The following titration curve is most likely...

Question 8

The most suitable acid-base indicator for a titration of acetic acid with NaOH has __________

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The answer of The most suitable acid-base indicator for a...

Question 9

A 0.500 g sample of an unknown substance was titrated with a 0.1 M HCl solution. Another 0.500 g sample of it was titrated with a 0.1 M NaOH solution. The resulting titration curves are illustrated here. Given the following possibilities, what is the sample?

Accepted Answer

The answer of A 0.500 g sample of an unknown...

Question 10

Halfway to the equivalence point in a titration curve of a weak acid with a strong base, __________

Accepted Answer

Halfway to the equivalence point, the number of moles of the strong base added equals half of the initial number of moles of weak acid. At this point, the concentration of weak acid is equal to its initial concentration divided by 2, and the concentration of its conjugate base is equal to the initial concentration of weak acid divided by 2. Using the Henderson-Hasselbalch equation, we can calculate the pH at this point as pK_a of the weak acid.

Question 11

What are the characteristics of a pH indicator? A pH indicator __________ (I) is a weak acid. (II) has different characteristic colors in protonated and unprotonated forms. (III) changes color when the pH is near its pK_a.

Accepted Answer

A pH indicator is a weak acid (I), which can donate a proton and undergo protonation (H+) to form a conjugate base. The conjugate base has a different color than the protonated form, and thus pH indicators have different characteristic colors in protonated and unprotonated forms (II). When the pH of a solution is near the pKa of the indicator, it undergoes a color change, which makes it useful for determining the pH of a solution. Therefore, a pH indicator changes color when the pH is near its pKa (III). All three characteristics are true for pH indicators, so the best choice is D.

Question 12

When an acetic acid solution is titrated with sodium hydroxide, the slope of the titration curve (pH vs volume of NaOH added) increases when sodium hydroxide is first added. This change shows that __________.

Accepted Answer

The increase in the slope of the titration curve indicates that acetic acid is being neutralized by sodium hydroxide to form sodium acetate, which results in a change in pH.

Question 13

The following titration curve is most likely to be associated with __________

Accepted Answer

The curve shows a gradual increase in pH at the beginning, indicating the presence of a weak acid. As the titrant is added, there is a sharp increase in pH around the equivalence point, which suggests the strong base titrant has neutralized the weak acid. Therefore, this is most likely a titration of a weak acid with a strong base.

Question 14

A solution of sulfuric acid (H₂SO₄, 25.00 mL) was titrated to completion with 34.55 mL of 0.1020 M sodium hydroxide. What was the concentration of the sulfuric acid?

Accepted Answer

The balanced chemical equation for the reaction between sulfuric acid (H₂SO₄) and sodium hydroxide (NaOH) is: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. This indicates that 1 mole of sulfuric acid reacts with 2 moles of sodium hydroxide. Given that 34.55 mL of 0.1020 M NaOH is used, the moles of NaOH are (34.55 mL) * (0.1020 mol/L) = 0.035241 mol. Since the stoichiometry of the reaction is 1:2, the moles of H₂SO₄ are half of the moles of NaOH, which is 0.035241 mol / 2 = 0.0176205 mol. The concentration of sulfuric acid is then the moles of H₂SO₄ divided by the volume of the solution in liters, which is 0.0176205 mol / 0.025 L = 0.70482 M, or 0.07048 M when rounded to the correct number of significant figures.

Question 15

In a titration of monoprotic acids and bases, there is a large change in pH __________

Accepted Answer

The answer of In a titration of monoprotic acids and...

Question 16

What is indicated by the shape of the titration curve?

Accepted Answer

The answer of What is indicated by the shape of...

Question 17

A solution of hydrochloric acid (HCl, 25.00 mL) was titrated to completion with 34.55 mL of 0.1020 M sodium hydroxide. What was the concentration of the hydrochloric acid?

Accepted Answer

The concentration of hydrochloric acid can be calculated using the formula $C_1V_1 = C_2V_2$, where $C_1$ and $C_2$ are the concentrations of the acid and base, respectively, and $V_1$ and $V_2$ are the volumes of the acid and base, respectively. Given that $V_1 = 25.00$ mL, $V_2 = 34.55$ mL, and $C_2 = 0.1020$ M, we can solve for $C_1$ as follows: $C_1 = \frac{C_2V_2}{V_1} = \frac{0.1020 \, \text{M} \times 34.55 \, \text{mL}}{25.00 \, \text{mL}} = 0.1410 \, \text{M}$.

Question 18

A phosphate buffer solution (25.00 mL sample) used for a growth medium was titrated with 0.1000 M hydrochloric acid. The components of the buffer were sodium monohydrogenphosphate and sodium dihydrogenphosphate. The first endpoint occurred at a volume of 10.32 mL, and the second occurred after an additional 18.62 mL was added, for a total volume of 28.94 mL. What was the total concentration of phosphate (in any form) in the buffer?

Accepted Answer

First, we need to determine the pH at each endpoint to identify which phosphate species is acting as the acid and which is acting as the base.

At the first endpoint:

pH = pKa + log([acid]/[base])
pH = 7.21 + log([NaH2PO4]/[Na2HPO4])

Since we know that at the first endpoint, [H2PO4-] = [NaH2PO4] and [HPO42-] = [Na2HPO4], we can substitute in:

pH = 7.21 + log([H2PO4-]/[HPO42-])
pH = 7.21 + log(1/2.07)
pH = 7.21 - 0.32
pH = 6.89

Therefore, at the first endpoint, H2PO4- acted as the acid and HPO42- acted as the base.

At the second endpoint:

pH = pKa + log([base]/[acid])
pH = 12.32 + log([NaH2PO4]/[Na2HPO4])

Again, substituting in [HPO42-] = [Na2HPO4] and [H2PO4-] = [NaH2PO4]:

pH = 12.32 + log([HPO42-]/[H2PO4-])
pH = 12.32 + log(1/0.42)
pH = 12.32 - 0.38
pH = 11.94

Therefore, at the second endpoint, HPO42- acted as the acid and H2PO4- acted as the base.

Next, we need to use the Henderson-Hasselbalch equation to determine the concentration of phosphate species in the buffer.

At the first endpoint:

pH = pKa + log([H2PO4-]/[HPO42-])
6.89 = 7.21 + log([H2PO4-]/[HPO42-])
log([H2PO4-]/[HPO42-]) = -0.32
[H2PO4-]/[HPO42-] = 0.464

At the second endpoint:

pH = pKa + log([HPO42-]/[H2PO4-])
11.94 = 12.32 + log([HPO42-]/[H2PO4-])
log([HPO42-]/[H2PO4-]) = -0.38
[HPO42-]/[H2PO4-] = 0.44

We can use these ratios to set up equations for the concentrations of the two species in terms of the total phosphate concentration:

[H2PO4-] = 0.464([total])
[HPO42-] = 0.44([total])

Substituting in the volumes and concentrations at each endpoint:

At the first endpoint (10.32 mL):

[H2PO4-] + [HPO42-] = 0.1 M x 0.01.32 L = 0.001032 mol
0.464([total]) + 0.44([total]) = 0.001032 mol
[total] = 0.001032/[0.464 + 0.44] = 0.002627 M

At the second endpoint (28.94 mL):

[H2PO4-] + [HPO42-] = 0.1 M x 0.01862 L = 0.001862 mol
0.464([total]) + 0.44([total]) = 0.001862 mol
[total] = 0.001862/[0.464 + 0.44] = 0.004725 M

Therefore, the total concentration of phosphate in the buffer is the average of these two values:

[total] = (0.002627 + 0.004725)/2 = 0.003676 M

Since the question asks for the total concentration of phosphate in any form, the answer is:

0.003676 M, which is closest to answer choice D (0.07448 M).

Question 19

Bromocresol green is yellow in its acidic form and blue in its basic form. When is it green?

Accepted Answer

Bromocresol green changes color in the pH range around its pKa value, which is when it is in its transition state between the acidic (yellow) and basic (blue) forms, appearing green. This is not necessarily at pH 7 or specifically at the equivalence or midpoint of a titration unless those points coincidentally match its pKa.

Question 20

Acid-base indicators change color __________

Accepted Answer

Acid-base indicators change color over a range of 1 or 2 pH units, depending on the specific indicator. The transition from one color to another occurs gradually as the pH of the solution changes. The exact pH at which the indicator changes color varies depending on the type of indicator used. Therefore, A, C, D, and E are not correct choices.

Quiz 17: Equilibrium in the Aqueous Phase