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Gen Chem 3 > Exam 2 Flashcards > Flashcards

Exam 2 Flashcards

Weak acids, weak base, buffers, and strong acids/bases added to those. Indicators. pH/pOH/[H3O+]/[OH-] conversions. pKa/pKb/Ka/Kb conversions. ICE tables. Beginning and End tables. Polyprotic acids.

1
Q

If you “undo” a pH or a pOH by raising ten to the negative, what UNIT are you in now?

A

Molarity

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2
Q

How do you turn pH into molarity of H3O+?

A

10^-pH

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3
Q

How do you turn pOH into molarity of OH-?

A

10^-pOH

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4
Q

How do you turn a molarity of H3O+ into a pH?

A

Take the negative log of the molarity.

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5
Q

How do you turn a molarity of OH- into a pOH?

A

Take the negative log of the molarity.

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6
Q

How do you turn a pH into a pOH or vice versa?

A

pH + pOH = 14

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7
Q

How do you turn an OH- molarity into a H3O+ molarity or vice versa?

A

[OH-] * [H3O+] = 10^-14

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8
Q

You’ve noticed that you are dealing with just a weak acid in water. What three things will you need to solve this problem?

A

(1) A balanced equation
(2) An ICE table
(3) A Ka expression

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9
Q

You’ve noticed that you are dealing with just a weak acid in water. What unit should you be using in your ICE table?

A

You’ll usually be using molarity, because after this ICE table you’ll need to use a Ka expression, and that NEEDS molarity.

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10
Q

You’ve noticed that you are dealing with just a weak acid in water. You have the initial molarity of the weak acid and the Ka. What assumption do you try in order to solve for x in your ICE table?

A

You set up a Ka expression and try the “denominator assumption.” You assume that because x is hopefully small, not much of the weak acid has decomposed.

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11
Q

You’ve noticed that you are dealing with a weak acid in water. You have the initial molarity of the weak acid and the pH. How do you figure out what x is in your ICE table?

A

x is the molarity of hydronium in this ICE table. It is 10^-pH

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12
Q

What is the symbolic reaction for a weak acid in water?

A

(HX) + (H2O) –> (H3O+) + (X-)

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13
Q

What is the abbreviation typically used for a weak acid?

A

HX. (Some books use HA, or HY)

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14
Q

In the Bronstead-Lowery definition, do acids give away a hydrogen, or do they take a hydrogen from something else?

A

Acids give away a H+ to a different molecule.

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15
Q

You’ve noticed that you are dealing with a weak acid in water. You have the pH and the Ka but do not know the original molarity of the weak acid. How do you solve for the original molarity of the weak acid?

A

Write a balanced reaction.
Set up an ICE table with an unknown starting molarity, like “G” Work the ICE table normally. The x in the ICE table will be 10^-pH
Set up a Ka expression to solve for your only unknown “G”

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16
Q

How do you solve for a percent ionization (also called percent dissociation) for a weak acid?

A

Divide the how much weak acid decomposed (the “x” you found in the ICE table) by how much weak acid was originally present, and multiply by 100. (If you have made a denominator assumption, this value must be less than 5%, or you have to go back and do the quadratic)

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17
Q

How do you turn a pKa into a pKb, or vice versa?

A

pKa + pKb = 14

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18
Q

How do you turn a Ka into a Kb, or vice versa

A

(Ka) * (Kb) = 10^-14

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19
Q

How do you turn a Ka into a pKa?

A

Take the negative log of the Ka

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20
Q

How do you turn a pKa into a Ka?

A

Raise 10^-pKa

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21
Q

How do you turn a Kb into a pKb?

A

Take the negative log of the Kb

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22
Q

How do you turn a pKb into a Kb?

A

Raise 10^-pKb

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23
Q

What is the symbol I have typically used for a weak base?

A

X-
(Really it can be anything. Just something that could steal an H+ from something else)

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24
Q

In the Bronstead-Lowery definition, do bases give away a hydrogen, or do they take a hydrogen from something else?

A

Bases take a H+ from something else.

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25
Q

What is the symbolic reaction for a weak base in water?

A

(X-) + (H2O) –> (HX) + (OH-)

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26
Q

What three things do you always need to solve a weak base in water problem?

A

(1) A balanced equation
(2) An ICE table
(3) A Kb expression

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27
Q

You’ve noticed that you have just a weak base in water. What unit should you do your ICE table in?

A

You’ll usually be using molarity, because after this ICE table you’ll need to use a Kb expression, and that NEEDS molarity.

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28
Q

You’ve noticed that you are dealing with just a weak base in water. You have the initial molarity of the weak base and the Kb. What assumption do you try in order to solve for x in your ICE table?

A

You set up a Kb expression and try the “denominator assumption.” You assume that because x is hopefully small, essentially none of the weak acid has decomposed.

29
Q

You’ve noticed that you are dealing with a weak base in water. You have the initial molarity of the weak base and the pOH. How do you figure out what x is in your ICE table?

A

In this ICE table, x will be the molarity of OH-. So you raise 10^-pOH

30
Q

You’ve noticed that you are dealing with a weak base in water. You have the pOH and the Kb, but do not know the original molarity of the weak base. How do you solve for the original molarity of the weak base?

A

Write a balanced reaction.
Set up an ICE table with an unknown starting molarity, like “G” Work the ICE table normally. The x in the ICE table will be 10^-pOH
Set up a Kb expression to solve for your only unknown “G”

31
Q

You’ve noticed that you are dealing with just a weak acid in water…but it’s weird because after giving you the starting molarity and volume of weak acid, it said then a certain amount of extra water was added. How do you account for this extra water?

A

Do an M1V1 = M2V2 calculation first, to see what the revised starting molarity is, then work the problem like a normal weak acid in water problem.
(You must do this dilution calculation at the START of your ICE table, not at the end. If you wait until the end to do it, the dilution will disturb the equilibrium and cause a new shift)

32
Q

What is the Henderson–Hasselbalch equation?

A

pH = pKa + log (X- / HX)

33
Q

When can you use the Henderson-Hasselbalch equation?

A

When ALL you have is a buffer (weak base and weak acid conjugate pair)

34
Q

What is a buffer made of? Be able to recognize some examples

A

It is a weak acid and a weak base conjugate pair. Or to put it another way, they both have to be weak, and look identical except that one has an extra H+
(…and possibly a neutral ion might be present as well on one to cancel out a charge)

35
Q

What unit goes into the log term of the Henderson-Hasselbalch equation?

A

You can either use moles (safer), or you can use molarity. Just make sure you are not mixing the two units in the same long term. And make sure that the molarities you use are the molarities IN the buffer, not molarities of solutions used to MAKE the buffer

36
Q

You see any of these words/prases in the problem:
“Mixed” “Combined” “Added To” “Poured together”
What unit should you first convert everything to?

A

moles. When you combine solutions, the volume changes, so molarities get messed up.

37
Q

What is the rule for when an indicator is at its intermediate color (if it has one)? Or to put it another way, what is the rule for when indicators change color?

A

Indicators are at their intermediate color when pH = + or - 1 from their pKa values

38
Q

What is the rule for picking which chemicals to use in a buffer?

A

Buffers work best when the pKa of their weak acid component is as close as possible to the pH where the buffer is going to be used.

39
Q

How do you solve for B in the following equation? A = log(B)

A

10^A
(Note that there is no “extra”: negative sign here. We are undoing a logarithm, not a pH or pKa)

40
Q

What is the symbol typically used for a strong acid in a balanced reaction?

A

H3O+ (or H+)

41
Q

What is the symbol typically used for a strong base in a balanced reaction?

A

OH-

42
Q

If you are adding a strong acid to a buffer, what is the symbolic balanced reaction that you should write?

A

(H3O+) + (X-) –> (HX) + (H2O)

43
Q

If you are adding a strong acid to a weak base, what is the symbolic balanced reaction that you should write?

A

(H3O+) + (X-) –> (HX) + (H2O)

44
Q

If you are adding a strong base to a buffer, what is the symbolic balanced reaction that you should write?

A

(OH-) + (HX) –> (X-) + (H2O)

45
Q

If you are adding a strong base to a weak acid, what is the symbolic balanced reaction that you should write?

A

(OH-) + (HX) –> (X-) + (H2O)

46
Q

You are adding a strong base to a buffer. What unit should go in your ICE table?

A

moles. Because you were mixing at least two solutions together.

47
Q

You are adding a strong acid to a buffer. What unit should go in your ICE table?

A

moles. Because you were mixing at least two solutions together.

48
Q

You are adding a strong acid to a weak base. How do you finish the last row of your ICE table?

A

Use up whatever reactant started with the smallest number of moles.

49
Q

You are adding a strong base to a buffer. How do you finish the last row of your ICE table?

A

Use up whatever reactant started with the smallest number of moles.

50
Q

You are adding a strong base to a weak acid. How do you finish the last row of your ICE table?

A

Use up whatever reactant started with the smallest number of moles.

51
Q

You are adding a strong acid to a buffer. How do you finish the last row of your ICE table?

A

Use up whatever reactant started with the smallest number of moles.

52
Q

List the 7 strong acids. Yes all of them.

A

HI
HBr
HCl
HClO3
HClO4
HNO3
H2SO4

53
Q

List the 8 strong bases. Yes all of them

A

LiOH
NaOH
KOH
RbOH
CsOH
Ca(OH)2
Ba(OH)2
Sr(OH)2

54
Q

Ibele was a dick and wrote a problem that has Ca(OH)2, Ba(OH)2, or Sr(OH)2 as the strong base. What extra step does he foolishly think you might forget in the problem?

A

Diprotic bases make twice as much OH- as you’d expect. Double the molarity given for that chemical before using it in calculations. And I was just being through in my assessment of your brilliance. I wasn’t being a dick.

55
Q

List all 8 of the ions which do not affect pH and which are cations. Yes all of them. (Reminder: Cats are great POSITIVE creatures. Cations have positive charges)

A

Li+
Na+
K+
Rb+
Cs+
Ca(2+)
Ba(2+)
Sr(2+)

56
Q

List 6 ions which do not affect pH and which are anions.
(Reminder: Anions are negative charges)

A

I-
Br-
Cl-
ClO3-
ClO4-
NO3-

57
Q

Name 3 ways of noticing that something is a weak acid just by looking at its formula

A

1) It has COOH at the end
2) It begins with H but isn’t a strong acid
3) It is nitrogen with 4 things around it, and one is a hydrogen (except for HNO3, which is strong)

58
Q

Name 2 ways of noticing that something is a weak base

A

1) It would have been a weak acid, but one of its hydrogens was replaced with a neutral ion (i.e. it is the conjugate base of a weak acid)
2) It is nitrogen with three things around it (except HN3 and HNO2)

59
Q

You have almost finished working a problem where a strong base was added to a buffer. You know the Ka of the weak acid part of the buffer. At the end of the ICE table, all that is left is weak acid (HX) and weak base (X-). How do you find the pH of the solution?

A

Use the Henderson–Hasselbalch equation

60
Q

You have almost finished working a problem where a strong base was added to a buffer. You know the Ka of the weak acid part of the buffer. At the end of the ICE table, all that is left is strong base (OH-) and weak base (X-). How do you find the pH of the solution?

A

Ignore the weak base. Turn the left over strong base moles into a left over strong base molarity by dividing by the total volume. Then convert that [OH-] into a pOH then pH.

61
Q

You have almost finished working a problem where a strong base was added to a buffer. You know the Ka of the weak acid part of the buffer. At the end of the ICE table, all that is left is the weak base product (X-). How do you find the pH of the solution?

A

1) Start a brand new reaction of a weak base in water.
2) Convert your moles of X- from the end of your first ICE table into a molarity of X- by dividing by the total volume, and use that to start your second ICE table
3.) Convert Ka to Kb and set up a Kb expression.
Try the denominator assumption to solve for x.
That x will be the molarity of OH-. Convert it to pH.

62
Q

You have almost finished working a problem where a strong acid was added to a buffer. You know the Ka of the weak acid part of the buffer. At the end of the ICE table, all that is left is weak acid (HX) and weak base (X-). How do you find the pH of the solution?

A

Use the Henderson–Hasselbalch equation

63
Q

You have almost finished working a problem where a strong acid was added to a buffer. You know the Ka of the weak acid part of the buffer. At the end of the ICE table, all that is left is strong acid (H3O+) and weak acid (HX). How do you find the pH of the solution?

A

Ignore the weak acid. Turn the leftover moles of strong acid into leftover molarity of strong acid by dividing by the total volume. Take the negative log of that [H3O+] to find pH.

64
Q

You have almost finished working a problem where a strong acid was added to a buffer. You know the Ka of the weak acid part of the buffer. At the end of the ICE table, all that is left is the weak acid product (HX). How do you find the pH of the solution?

A

1) Start a brand new reaction of a weak acid in water.
2) Convert your moles of HX from the end of your first ICE table into a molarity of HX by dividing by total volume, and use that HX molarity at the start of your second ICE table
3.) Set up a Ka expression and try the denominator assumption to solve for x.
That x will be the molarity of H3O+. Convert it to pH.

65
Q

You see that I’ve asked a dreaded polyprotic acid problem with the acid H3X. How many ICE tables are needed if I ask you to solve for the pH?

A

Just the first ICE table. The second and third reactions do not affect the amount of H3O+ present much, and they can be ignored.

66
Q

You see that I’ve asked a dreaded polyprotic acid problem with the acid H3X. I give you the initial molarity of H3X and ask you to solve for the molarity of H3X at equilibrium. How many ICE tables are needed for this?

A

Just one. H3X will not show up in the 2nd or 3rd reactions at all.

67
Q

You see that I’ve asked a dreaded polyprotic acid problem with the acid H3X. How many ICE tables are needed in order to solve for the molarity of HX(2-)?

A

Two if you don’t know the shortcut. None if you do. The molarity of HX(2-) will be equivalent to the second Ka value (but with molarity units, of course).

But use this short cut with caution! If you mess it up at all (like claiming its the wrong Ka, or if I had asked for moles instead of molarity), you’ll get no partial credit!!!

68
Q

You see that I’ve asked a dreaded polyprotic acid problem with the acid H3X. How many ICE tables are needed in order to solve for the molarity of H2X(-)?

A

Just one. The first reaction makes H2X(-), and while it does show up in the second reaction as well, it will change by a negligible amount.

69
Q

You see that I’ve asked a dreaded polyprotic acid problem with the acid H3X. How many ICE tables are needed in order to solve for the molarity of X(3-)?

A

All three. If you are cleaver any you’ve practiced and know the shortcut, you can sort of skip the second ICE table. But you’ll definitely need the 1st and 3rd ICE table.

Gen Chem 3 (4 decks)

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