Chem Ch 17

24 July 2022
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question
If solutions of NH4Cl(aq) and NH3(aq) are mixed, which ions in the resulting solution are spectator ions in any acid-base chemistry occurring in the solution?
answer
Cl-
question
What equilibrium reaction determines [OH−] and, therefore, the pH of the solution?
answer
NH3(aq)+H2O(l)⇌OH−(aq)+NH4+(aq)
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Which of the following would occur if solid NH4Cl was added to an aqueous solution of NH3? The equilibrium constant changes. The pH decreases. The pH increases. NH3 will ionize to a greater extent.
answer
The pH decreases.
question
why?
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Whenever a weak electrolyte and a strong electrolyte containing a common ion are together in solution, the weak electrolyte ionizes less that it would if it were alone in solution.
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Buffer solutions can be produced by mixing
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a weak acid with its conjugate base or by mixing a weak base with its conjugate acid.
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The Henderson-Hasselbalch equation, _________ allows you to calculate _______
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pH=pKa+log [base]/[acid] the pH of a buffer.
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Which of the following mixtures may be act as a buffer solution? HCl, NaCl HF, NaF HI, NaI HBr, NaBr
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HF, NaF
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why?
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because it contains a weak acid&-conjugate base pair
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Which solution will have the highest pH? The Ka of HClO is 3.8 × 10−8. A 100 mL buffer solution of 0.1 M HClO, 0.1 MClO− with 20 mL of 0.1 M HCl solution added. A solution of 100 mL of water with 20 mL of 0.1 M NaOH added. A 100 mL buffer solution of 0.1 M HClO, 0.1 M ClO− with 20 mL of 0.1 M NaOH solution added. A buffer solution of 0.1 M HClO, 0.1 M ClO−.
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A solution of 100 mL of water with 20 mL of 0.1 M NaOH added.
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What happens when NaOH is added to a buffer composed of CH3COOH and CH3COO−?
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The OH− of NaOH (a strong base) reacts with the acid member of the buffer (CH3COOH), abstracting a proton. Thus, [CH3COOH] decreases and [CH3COO−] increases.
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What happens when HCl is added to this buffer?
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The H+ of HCl (a strong acid) reacts with the base member of the buffer (CH3COO−). Thus, [CH3COO−] decreases and [CH3COOH] increases.
question
The Ka values for nitrous acid (HNO2) and hypochlorous (HClO) acid are 4.5×10−4 and 3.0×10−8, respectively. Which one would be more suitable for use in a solution buffered at pH = 7.0?
answer
HClO
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What other substance containing sodium would be needed to make the buffer?
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NaClO
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Which of the following conjugate acid-base pairs will function as a buffer? H2O/OH- HBr/OH- HF/F- HCl/Cl-
answer
HF/F-
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why?
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A buffer resists changes in pH because it contains both an acid, to neutralize added OH- ions, and a base, to neutralize added H+ ions. The acid and base must not completely consume each other through a neutralization reaction; therefore, they must both be weak.
question
The pH at the equivalence point is above 7.00 for the weak acid titration. Describe the reasons why this statement is true.
answer
The conjugate base of the weak acid is the majority species in solution at the equivalence point, and this conjugate base reacts with water to produce OH−. Therefore, the pH at the equivalence point for a weak acid/strong base titration is greater than 7.00
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How does the pH at the equivalence point change as the acid being titrated becomes weaker?
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The pH at the equivalence point increases (becomes more basic) as the acid becomes weaker.
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How does the volume of NaOH(aq) needed to reach the equivalence point change? The volume of added base needed to reach the equivalence point increases. The volume of added base needed to reach the equivalence point decreases. The volume of added base needed to reach the equivalence point remains unchanged.
answer
The volume of added base needed to reach the equivalence point remains unchanged.
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What is the pH at the equivalence point when 0.10 M HNO3 is used to titrate a volume of solution containing 0.30 g of KOH?
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pH = 7
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If the acetic acid being titrated here were replaced by hydrochloric acid, would the amount of base needed to reach the equivalence point change?
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will not change
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Would the pH at the equivalence point change? Yes, the pH at the equivalence point will increase. Yes, the pH at the equivalence point will decrease. No, the pH at the equivalence point will not change.
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Yes, the pH at the equivalence point will decrease.
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The pH of the equivalence point of a weak acid-strong base titration is _________. greater than 7 equal to 7 always when equal volumes of acid and base have reacted less than 7
answer
greater than 7
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why?
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At the equivalence point of a strong acid strong base titration, only water and a neutral salt remain.
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Enter the the Ksp expression for the solid AB2 in terms of the molar solubility x.
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Ksp = 4x^3
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Enter the the Ksp expression for C2D3 in terms of the molar solubility x.
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Ksp = ((2x)^2)((3x)^3)
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What is the solubility-product expression for the equilibrium established when calcium phosphate, Ca3(PO4)2(s), is added to pure water at 25°C?
answer
K sp = [Ca2+]3[PO4 3-]2
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Which of the following metal salts will show an increase in solubility with the addition of a strong acid? CaF2 AgI CuBr PbCl2
answer
CaF2
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why?
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A metal salt that contains a basic anion will have an increase in solubility as the pH decreases because the anion will be neutralized by the acid.
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A test tube contains a saturated solution of silver bromide. Which reagent, from the four below, will produce the largest amount of solid precipitate once it is added to the test tube? 0.2 M CaF2 Concentrated HNO3 Concentrated NH3 Concentrated HBr
answer
Concentrated HBr
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Enter the Ksp expression for A2B3(s) in terms of [A] and [B].
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Ksp = [A]^2[B]^3
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Enter the Ksp expression for AC4(s) in terms of [A] and [C].
answer
Ksp = [A][C]4
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If A is slowly added to a solution containing 0.0500 M of B and 0.0500 M of C, which solid will precipitate first? The solubility product constant for A2B3 is 2.30×10−8. The solubility product constant for AC4 is 4.10×10−8. A2B3 AC4
answer
AC4
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why?
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The [A] needed to begin precipitation is lower for AC4, therefore it will precipitate first.
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You can only make buffers with
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WA or WB
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buffers act to
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keep solution at a relatively constant pH
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CH3COOH <=> H+ + CH3COO- what will addition of CH3COO- shifts equilibrium to which side?
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to the left to lower the [H+]
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"ideal buffers" cause
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equal capacity
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buffers make:
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mixture of WA + salt of CB or mixture WB + salt CA +SA and neutralize WB or +SB and neutralize WA
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what is Ka of HA
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HA <=> H+ + A- Ka = [H+][A-] / [HA]
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what is the henderson Hasselbach equation?
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pH = pKa + log ([B]/[A])
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buffer capacity
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between 6 and 8 pH range is usually +- 1 unit from pKa
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Adding a SA or SB =
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neutralization reaction
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Titration of a SA with a SB
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pH = 7 at eq as you add more base, pH levels off **look at graphs
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Titration of SB with SA
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high pH to pH = 7 at eq --> low pH
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titration of WA with SB
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@ eq, pH > 7 as more base is added, pH levels off. same as for SA.
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ways that WA titration differs from SA titration
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1. WA has higher initial pH than SA 2. pH change near eq point is smaller for WA 3. the pH @ eq point is >7 for WA.
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indicators
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color change chemicals you can put into a titration. WA that have a different color than their CB**
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what can indicators be used for?
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can be used to find the equivalence point in a titration
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titrations of polypro tic acids
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where amount of base = amount of acid
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@ 1:1 ratio, pH =
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pKa because log (1/1) = 0
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the common ion effect
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deals with solubility?
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Kf
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formation constant (complex ion formation)
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Lewis base
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donates electron pair
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Lewis acid
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accepts electron pair
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Q = Ksp
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@ eq and solution is saturated
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Q < Ksp
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more solid can dissolve, so no precipitate
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Q > Ksp
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precipitate forms