Chemistry: Acids & Bases Flashcards Preview

MCAT > Chemistry: Acids & Bases > Flashcards

Flashcards in Chemistry: Acids & Bases Deck (59)
Loading flashcards...
1

Strong Acids

Strongest: HCl, H2SO4, HNO

HBr, HI, HClO4 (HPO4?), HClO3

Strong acid Formula
Perchloric acid HClO4
Hydroiodic acid HI
Hydrobromic acid HBr
Sulfuric acid H2SO4
Hydrochloric acid HCl
Nitric acid HNO3
Hydronium ion H3O+ or H+

2

Strong Bases

Strong bases Formula
Lithium hydroxide LiOH
Sodium hydroxide NaOH
Potassium hydroxide KOH
Rubidium hydroxide RbOH
Cesium hydroxide CsOH
Calcium Hydroxide Ca(OH)2
Strontium hydroxide Sr(OH)2
Barium hydroxide Ba(OH)2

3

What is the ionization constant of H2S?

much less than 1
WEAK ACID!

4

-SH vs. -OH

Because of charge density (charge per volume):
-SH: weak
-OH: strong

Share ~same charge but S has larger volume it has lower charge density... means more stable and therefore less reactive
charge density= charge/volume

5

pH

-log[H+]

6

Assuming 25 degrees Celsius:
pH + pOH =

14

7

What is the relationship between the variables on the L and R sides of the equations?
pH=-log[H+]
pOH=-log[OH]
pKa = -log[Ka]
pKb = -log[Kb]

inverse:
ie: if pH goes down, [H+] goes up

8

a) What is pH if [H+] = 1.0 x 10^-1?
b) What is pOH if 1.0 x 10^8

(@25 degrees C)

a) 1
b) -8

9

Steps to convert concentrations of H+ and OH- into pH and pOH (@25 degrees C)

Examples
A) H+: 6.4 x 10^-3
B) OH-: 2.1 x 10^-8

A) H+: 6.4 x 10^-3
1) For ones value of answer (pH from [H+]): Lock in on the power of 10. Flip the sign and subtract 1.
(10^-3) --> -3 --> +2

2) For decimal of answer: Subtract the digits (before the power of 10) from 10.
(6.4 ~ 6) --> 10-6 = 4

3) Put it together: 2.4 = pH
_______________
B) OH-: 2.1 x 10^-8
pOH=7.8

10

(In 3 steps)
Convert pH into [H+]
a) pH: 13.3
b) pH: 1.8

(also works for pOH-->OH, pKa-->Ka, pKb--Kb, @25 degrees C)

A)pH: 13.3
1) 13 --> 10^-14
2) .3 -->10-3 = 7
3) 7 x 10^-14 = [H+]

A)pH: 1.8
1) 1 --> 10^-2
2) .8 -->10-8 = 2
3) 2 x 10^-2 = [H+]

11

Based on the pKa, which of the following species is the strongest base?
pKa of ammonium ion for:
*NH3: 9.26
*(CH3CH2)2NH: 10.64
*(Ch3CH2)2NH: 10.98
*(CH3CH2)3N: 10.76

Answer: (CH3CH2)2NH:

pKa --> pKb of ammonium ion: (14-pKa=pKb)

So,
*NH3: 9.26 (pKb: 4.74)
*(CH3CH2)2NH: 10.64 (pKb: 3.36)
*(CH3CH2)2NH: 10.98 (pKb: 3.02)
*(CH3CH2)3N: 10.76 (pKb) 3.24

Smallest Kb: 4.74
***Largest Kb: 3.02**

Largest pKa = Smallest pKb = Largest Kb = strongest base

12

The bigger the pKa, the ____ the base?

Bigger the pKa -->
Smaller the Ka

Smaller the Ka -->
Bigger the Kb

Bigger the Kb -->
Stronger the base

13

Reacting equivalents

make stoich corrections for how many mol acid/base are produced per mol of reacting source

Ex:
HCl --> H+ + Cl-
(1 mol HCl --> 1 eq acid)

H2SO4 --> 2H+ + SO4^2-
(1 mol HsSO4 --> 2 eq acid)

Ca(OH)2 --> Ca^2+ + 2OH-
(1mol Ca(OH)2 --> 2 eq base)
----------------------
(MCAT limits to Acid-Bases... but it applies elsewhere)

14

Gram-Equivalent Weight

Ex: NaOH and Na2CO3

Stoich correction of molecular weight
g/eq = g/mol X mol/eq

NaOH (1mol:1eq ratio)
40 g/mol x 1 mol/1eq= 40 g/eq

Na2CO3 (1mol:2eq ratio)
106 g/mol x 1 mol/2 eq = 53 g/eq

15

Normality (N)

Ex: H2SO4

stoich corrections for molality--needed for world of equivalents in world of eq points (as opposed to world of mols)

eq/L = mol/L x eq/mol

1O^-1 mol/L x 2 eq/mol = 2 x 10^-1 eq/L

16

Titration Curves

Strong Acid-Strong Base: steeper

Weak Acid-Strong Base: less steep

17

Titrant

solution of known concentration

18

Equivalence Point Formula

WHEN you are at the equivalence point:
Eq acid = Eq base

*Use this formular to find the M? or vol? or grams?
*Does not care about pH if asking for volume of base added!!!
*If everything is monoprotic/monobasic, you can interchange M and N... but be read JIC

19

Eq Pt Formula: Both aq

(Normality of Acids)(Volume of Acid) = (Normality of Base)(Volume of Base)
(eq/L)(L) = (eq/L)(L)

20

Eq Pt Formula: aq/solid

(Normality of Acids)(Volume of Acid) = mass base/gram eq wt = mass/mw x eq/mol

21

Titration Question Types
(pH question families)

1) What is the pH at X point in the curve?

2) What is the volume/grams/moles of the solution ?

22

Eq Pt Formula: if using both solids

mass of acid/gram eq wt = mass of base/ gram eq wt

23

Set up the formula to find:
How many mg of NaOH (s) is required to neutralize 408 mg of KHP (s), a monoprotic weak acid with a mw of 204 amu?

Monoprotic means Gram Eq Wt (GEW)= mw

Mass of acid/ GEW = mass of base / GEW

408/204 = x/40
2=x/40
x=80

24

Set up the formula to find:
How many mg of NaOH (s) is required to neutralize 40 mL of a 10^-1 M HCl solution?

(N of acid) * (V of acid) = mass/GEW

(10^-1)(40) = x/40
x=160

25

In Weak Acid-Strong Base (or SA-WB--same)Titrations, how are the following affected?
*pH initial
*pH Halfway
*pH at eq pt

e.g.
HA + NaOH --> H2O + A-Cl+

Buffer solution: weak acid with its weak CB in solution at the same time (Henderson-Hasselbach time)

Simulation:
-Initial: HCl=30eq, NaCl=0eq
-Halfway: HCl=15eq, NaCl=15eq
-Eq Pt: HCl=0eq, A-Cl+=30eq

*pH initial: must use Ka expression to solve

*pH Halfway: buffer soln (must use H-H eqn to solve)

*pH at eq pt: HA neutralized, A- = wk base, pH > 7

25

In Weak Acid-Strong Base (or SA-WB--same)Titrations, how are the following affected?
*pH initial
*pH Halfway
*pH at eq pt

e.g.
HA + NaOH --> H2O + A-Cl+

Buffer solution: weak acid with its weak CB in solution at the same time (Henderson-Hasselbach time)

Simulation:
-Initial: HCl=30eq, NaCl=0eq
-Halfway: HCl=15eq, NaCl=15eq
-Eq Pt: HCl=0eq, A-Cl+=30eq

*pH initial: must use Ka expression to solve

*pH Halfway: buffer soln (must use H-H eqn to solve)

*pH at eq pt: HA neutralized, A- = wk base, pH > 7

26

In Strong Acid-Strong Base Titrations, how are the following affected?
*pH initial
*pH Halfway
*pH at eq pt

e.g.
HCl + NaOH --> H2O + NaCl

*Assume 100% disassociation.

Simulation:
-Initial: HCl=30eq, NaCl=0eq
-Halfway: HCl=15eq, NaCl=15eq
-Eq Pt: HCl=0eq, NaCl=30eq

*pH initial: easy to solve bc [HCl]initial = [H+]

*pH Halfway: NaCl is inert, so [HCl]halfway = [H+]

*pH at eq pt (look at the products): HCl neutralized, NaCl inert, pH = 7

Aside: how do we know NaCl is inert? it is a conjugate of strong species. That is HCl is strong so produces inert Cl. NaOH is strong so produces inert Na.

26

In Strong Acid-Strong Base Titrations, how are the following affected?
*pH initial
*pH Halfway
*pH at eq pt

e.g.
HCl + NaOH --> H2O + NaCl

*Assume 100% disassociation.

Simulation:
-Initial: HCl=30eq, NaCl=0eq
-Halfway: HCl=15eq, NaCl=15eq
-Eq Pt: HCl=0eq, NaCl=30eq

*pH initial: easy to solve bc [HCl]initial = [H+]

*pH Halfway: NaCl is inert, so [HCl]halfway = [H+]

*pH at eq pt (look at the products): HCl neutralized, NaCl inert, pH = 7

Aside: how do we know NaCl is inert? it is a conjugate of strong species. That is HCl is strong so produces inert Cl. NaOH is strong so produces inert Na.

27

What is the pH of a 10^-3 HCl soln (pKa = -8)

Strong acid soln: 100% disassociation

[HCl]initial --> [H3O+]
[H+] = 10^-3
pH = 3

27

What is the pH of a 10^-3 HCl soln (pKa = -8)

Strong acid soln: 100% disassociation

[HCl]initial --> [H3O+]
[H+] = 10^-3
pH = 3