Deck 6: Reactions

Question 1

Write the autoprotolysis reaction for liquid ammonia.

Answer 1

2NH₃(l).NH₂ ^{$\Leftrightarrow$} (solvated)+ NH₄⁺(solvated)

Question 2

Calculate the hydroxide ion concentration for an aqueous solution that has a pH of 3.45.

A) 3.2 * 10^-2 M
B) 0.54 M
C) 3.5 * 10^-4 M
D) 2.8 * 10^-11 M
E) 2.6 *10^-5 M

Answer 2

2.8 * 10^-11 M

Question 3

Which of the following produces the strongest conjugate base?

A) HF (pK_a = 3.45)
B) HClO (pK_a = 7.53)
C) HCOOH (pK_a = 3.75)
D) CH₃COOH (pK_a = 4.75)
E) HIO (pK_a = 10.64)

Answer 3

E

Question 4

When sulfur trioxide dissolves in water, sulfuric acid is produced.An intermediate in the reaction is H₂O-SO₃.In the reaction of the intermediate to produce sulfuric acid,

A) water acts both as an acid and a base.
B) water acts as a proton donor only.
C) water acts as a proton acceptor only.
D) the intermediate undergoes an intramolecular rearrangement to form the product.

Question 5

The conjugate acid of HPO₄^2- is

A) HPO₄^2-.
B) PO₄^3-.
C) H₂PO₄^-.
D) H₃O⁺.
E) H₃PO₄.

Question 6

Of the following, which does not have an amphoteric oxide?

A) BeO
B) PbO
C) SnO
D) Al₂O₃
E) MgO

Question 7

The conjugate base of ammonia is

A) NH₂OH.
B) NH₂-^-.
C) NH₄⁺.
D) NH₃.
E) OH^-.

Question 8

Which of the following is the weakest acid?

A) HNO₃
B) HBr
C) HCl
D) HF
E) HI

Question 9

What is the conjugate acid of O^2-?

Question 10

What is the conjugate base of H₂PO₄^- ?

A) HPO₄^2-
B) OH^-
C) H₃PO₄
D) H₂PO₄^-
E) PO₄^3-

Question 11

In a solution labeled "0.10 M HNO₃," which of the following is correct?

A) [HNO₃] = 0.10 M
B) [H₃O⁺] = 0.10 M, [NO₃^-] = 0.10 M
C) [H₃O⁺] = 0.090 M, [NO₃^-] = 0.010 M
D) [HNO₃] = 0.050 M, [H₃O⁺] = 0.050 M, [NO₃^-] = 0.050 M
E) [H₃O⁺] = 0.10 M, [OH^-] = 1.0 * 10^-7 M

Question 12

Calculate the hydrogen ion concentration for an aqueous solution that has a pH of 3.45.

A) 0.54 M
B) 3.5 * 10^-4 M
C) 2.8 * 10F^-11
D) 3.2 * 10^-2 M
E) 1.22 M

Question 13

Strong acids are leveled in water to the strength of the acid H₃O⁺.

Question 14

All of the following are strong bases in water except

A) NaHCO₃
B) CaO
C) Na₂O
D) Na₂SO₄

Question 15

Which of the following is the weakest acid?

A) HCN (pK_a = 9.31)
B) HIO₃ (pK_a = 0.77)
C) HF (pK_a = 3.45)
D) CH₃COOH (pK_a = 4.75)
E) HNO₂ (pK_a = 3.37)

Question 16

What is the pK_a of the conjugate acid of hydrazine,given that the pK_b of hydrazine is 5.77? Write the formula of the conjugate acid of hydrazine.

Question 17

The conjugate base of OH^- is

A) H⁺.
B) OH^-.
C) O^2-.
D) H₃O⁺.
E) H₂O.

Question 18

In the following reaction
SO₂(g)+ H₂O(l) $\rightarrow$ H₂SO₃(aq),
identify the Lewis acid and base.

Question 19

Which of the following is the strongest base?

A) methylamine (pK_b = 3.44)
B) morphine (pK_b = 5.79)
C) urea (pK_b = 13.90)
D) ammonia (pK_b = 4.75)
E) pyridine (pK_b = 8.75)

Question 20

The pH of a 0.0050 M aqueous solution of calcium hydroxide is

A) 11.40
B) 2.00
C) 12.00
D) 12.70
E) 11.70

Question 21

When CaO(s) is dissolved in water,Which of the following is true?

A) The solution contains O^2-(aq), OH^-(aq), and Ca²⁺(aq).
B) The solution contains CaO(aq).
C) CaO(s) does not dissolve in water.
D) The solution contains O^2-(aq) and Ca²⁺(aq).
E) The solution contains OH^-(aq) and Ca²⁺(aq).

Question 22

If the value of K_b for pyridine is 1.8 *10^-9, calculate the equilibrium constant for
C₅H₅NH⁺(aq)+ H₂O(l) $\Leftrightarrow$ C₅H₅N(aq)+ H₃O⁺(aq)

A) -1.8 * 10^-9
B) 1.8 * 10^-16
C) 5.6 * 10⁸
D) 1.8 * 10^-9
E) 5.6 * 10^-6

Question 23

Write the charge balance equation for a dilute aqueous solution of HI.

A) [I^-] = [OH^-] + [H₃O⁺]
B) [H₃O⁺] = [OH^-]
C) [H₃O⁺] = [I^-]
D) [H₃O⁺] = [I^-] + [OH^-]
E) [HI]_initial = [I^-]

Question 24

The pH of 0.010 M aniline(aq) is 8.32.What is the percentage aniline protonated?

A) 2.1%
B) 0.69%
C) 0.021%
D) 0.12%
E) 0.21%

Question 25

Write the charge balance equation for a dilute aqueous solution of KOH.

A) [KOH]_initial = [K⁺]
B) [OH^-] = [H₃O⁺] + [K⁺]
C) [H₃O⁺] = [OH^-]
D) [K⁺] = [OH^-] + [H₃O⁺]
E) [OH^-] = [K⁺]

Question 26

Which of the following aqueous solutions gives a pH greater than 7?

A) 10^-8 M NH₄Cl
B) None of the solutions gives a pH greater than 7.
C) 10^-8 M CH₃COOH
D) 10^-8 M HCl
E) 10^-8 M HCOOH

Question 27

The pH of 0.10 M pyridine(aq) is 9.13.What is the value of K_b for pyridine?

A) 2.7 * 10^-4
B) 7.4 * 10^-10
C) 2.7 * 10^-5
D) 1.8 * 10^-10
E) 1.8 * 10^-9

Question 28

For a 0.10 M solution of a weak acid, HA,With pK_a = 10,Which of the following is true?

A) [HA] $\cong$ 0
B) [HA] = [A^-]
C) [HA] = [H₃O⁺]
D) [HA] = K_a
E) [HA] $\neq$ [H₃O⁺]

Question 29

In liquid ammonia,the base B is a strong base if it is a stronger proton acceptor than NH₂^-.

Question 30

If the pK_a of acetic acid is 4.75, the pK_a of CH₃CH₂OH is

A) about 4.
B) much less than 4.75.
C) about 16.
D) also 4.75.
E) about 7.

Question 31

Which of the following 0.10 M aqueous solutions has the lowest pH?

A) B(OH)₃
B) HIO
C) C₂H₅NH₃Cl
D) C₆H₅OH

Question 32

What is the pH of 0.025 M (CH₃)₃N(aq) (K_b = 6.5 * 10^-5)?

A) 11.91
B) 12.40
C) 11.11
D) 8.29
E) 9.81

Question 33

In liquid ammonia,the acid HB is a strong acid if it is a weaker proton donor than NH₄⁺.

Question 34

The pH of 0.80 M benzenesulfonic acid is 0.51 .What is the percentage ionization of benzenesulfonic acid?

A) 25%
B) 39%
C) 51%
D) 5.0%
E) 64%

Question 35

For the following acids, which has the highest pK_a?

A) HIO
B) HClO₃
C) HClO
D) HBrO
E) HClO₄

Question 36

Which of the following 0.10 M aqueous solutions gives the lowest pH?

A) CCl₃COOH (pK_a = 0.52)
B) Because all are acids, the pH is the same for all solutions.
C) HF (pK_a = 3.45)
D) CH₃COOH (pK_a = 4.75)
E) H₃PO₄ (pK_a1 = 2.12)

Question 37

Which of the following is the strongest acid?

A) CH₃CH₂OH
B) CH₃COOH
C) CHCl₂COOH
D) CH₂ClCOOH
E) CCl₃COOH

Question 38

A flask of 0.25 M HBrO(aq) has what pH? (pK_a = 8.69)

A) 5.90
B) 0.60
C) 8.10
D) 4.65
E) 9.30

Question 39

The pH of 0.800 M aqueous benzenesulfonic acid is 0.51 .What is the value of K_a for benzenesulfonic acid?

A) 0.19
B) 0.12
C) 0.90
D) 0.44
E) 0.51

Question 40

Estimate the pH of 10^-7 M HClO₄(aq).

A) 6.8
B) 8.0
C) 1.0
D) 5.0
E) 7.0

Question 41

The equation that represents K_a2 for phosphoric acid is

A) HPO₄^2-(aq) + H₂O(l) ? PO₄^3-(aq) + H₃O⁺(aq)
B) H₂PO₄^-(aq) + H₂O(l) ? HPO₄^2-(aq) + H₃O⁺(aq).
C) H₃PO₄(aq) + 2H₂O(l) ? HPO₄^2-(aq) + 2H₃O⁺(aq).
D) HPO₄^2-(aq) + H₂O(l) ? H₂PO₄^-(aq) + OH^-(aq).
E) H₃PO₄(aq) + H₂O(l) ? H₂PO₄^-(aq) + H₃O⁺(aq).

Question 42

For a solution labeled "0.10 M H₂SO₄(aq),"

A) [HSO₄^-] is greater than 0.10 M.
B) the pH is less than 1.0.
C) [SO₄^2-] = 0.10 M.
D) the pH equals 1.0.
E) the pH is greater than 1.0.

Question 43

If pK_a1 and pK_a2 for H₂CO₃ are 6.37 and 10.25, respectively,Calculate the equilibrium constant for the reaction below:
H₂CO₃(aq)+ 2H₂O(l) $\Leftrightarrow$ 2H₃O⁺(aq)+ CO₃^2-(aq)

A) 4.1 * 10^-11
B) 4.3 * 10^-7
C) 5.6 * 10^-11
D) 2.3 * 10^-8
E) 2.4 * 10^-17

Question 44

If pK_a1 and pK_a2 for H₂S are 6.88 and 14.15, respectively,Calculate the equilibrium constant for the reaction below:
H₂S(aq)+ 2H₂O(l) $\Leftrightarrow$ 2H₃O⁺(aq)+ S^2-(aq)

A) 1.3 * 10^-7
B) 1.1 * 10^-7
C) 7.7 *10^-8
D) 9.2 * 10^-22
E) 7.1 * 10^-15

Question 45

The following 0.1 M aqueous solutions are arranged in order of increasing pH, with the highest pH on the far right. $$\begin{array} { | l | l | l | l | l | } \hline \mathrm { C } _ { 5 } \mathrm { H } _ { 5 } \mathrm { NHCl } & \mathrm { HClO } & \mathrm { KNO } _ { 3 } & \mathrm { NaClO } & \\\hline\end{array}$$
Which one of the following 0.10 M aqueous solutions should be placed in the empty box?

A) NaCN
B) CH₃COOH
C) KNO₂
D) NaBr
E) NaHSO₄

Question 46

The following 0.1 M aqueous solutions are arranged in order of increasing pH, with the highest pH on the far right. $$\begin{array} { | l | l | l | l | l | } \hline \mathrm { HNO } _ { 3 } & \mathrm { HCOOH } & & \mathrm { KBr } & \mathrm { KNO } _ { 2 } \\\hline\end{array}$$
Which one of the following 0.10 M aqueous solutions should be placed in the empty box?

A) NaHSO₄
B) KF
C) HNO₂
D) CH₃NH₂
E) (CH₃)₃NHCl

Question 47

The following 0.1 M aqueous solutions are arranged in order of increasing pH, with the highest pH on the far right. $$\begin{array} { | l | l | l | l | l | } \hline \mathrm { HClO } _ { 4 } & & \mathrm { KNO } _ { 3 } & \mathrm { KNO } _ { 2 } & \mathrm { RbOH } \\\hline\end{array}$$
Which one of the following 0.10 M aqueous solutions should be placed in the empty box?

A) CuSO₄
B) NaNO₂
C) CH₃NH₂
D) NaHCO₃
E) Na₂HPO₄

Question 48

The equation that represents K_a2 for sulfurous acid is

A) HSO₃^-(aq) + H₂O(l) ? H₂SO₃(aq) + OH^-(aq).
B) HSO₃^-(aq) + H₂O(l) ? SO₃^2-(aq) + H₃O⁺(aq).
C) H₂SO₃(aq) + 2H₂O(l) ? SO₃^2-(aq) + 2H₃O⁺(aq).
D) SO₃^2-(aq) + H₂O(l) ? HSO₃^-(aq) + OH^-(aq).
E) H₂SO₃(aq) + H₂O(l) ? HSO₃^-(aq) + H₃O⁺(aq).

Question 49

The following 0.1 M aqueous solutions are arranged in order of increasing pH, with the highest pH on the far right. $$\begin{array} { | l | l | l | l | l | } \hline \mathrm { HNO } _ { 3 } & \mathrm { C } _ { 5 } \mathrm { H } _ { 5 } \mathrm { NHCl } & \mathrm { HClO } & & \mathrm { NaClO } \\\hline\end{array}$$
Which one of the following 0.10 M aqueous solutions should be placed in the empty box?

A) NH₄Cl
B) NaCN
C) Al₂(SO₄)₃
D) CH₃NH₂
E) CH₃COOH

Question 50

Calculate the equilibrium constant for the reaction
HS^-(aq)+ H₂O(l) $\Leftrightarrow$ H₂S(aq)+ OH^-(aq), Given K_a1 = 1.3 * 10^-7 and K_a2 = 7.1 * 10^-15 for H₂S.

A) 1.3 * 10^-7
B) 7.7 * 10^-8
C) 9.2 * 10^-22
D) 7.1 * 10^-15
E) 1.4

Question 51

A 0.0010 M solution of a weak acid, HA,With K_a = 2 * 10^-10 produces [H₃O⁺] < 10^-6 M.Which of the following equations can be used to determine [H₃O⁺]?

A) The acid is so weak that the pH is about 7.
B) [H₃O⁺]² + K_a [H₃O⁺] - [HA]_initialK_a = 0
C) [H₃O⁺] = (K_w + K_a[HA]_initial)^½
D) [H₃O⁺] = [HA]_initial
E) [H₃O⁺] = (K_a[HA]_initial)^½

Question 52

Write the charge balance equation for a solution that is 0.0010 M phenol(aq) .
Let phenol be represented by HA(aq).

A) [H₃O⁺] = [OH^-]
B) K_w = [H₃O⁺][OH^-]
C) [H₃O⁺] = [OH^-] + [A^-]
D) K_a = K_w/K_b
E) 0.0010 = [HA] + [A^-]

Question 53

What is the [H⁺] for a solution labeled "0.0500 M H₂SO₃(aq)" if pK_a1 = 1.81, and pK_a2 = 6.91?

A) 0.021 M
B) 0.029 M
C) 0.015 M
D) 0.025 M
E) 0.050 M

Question 54

Estimate the pH of 10^-7 M KOH(aq).

A) 6.9
B) 9
C) 13
D) 7.2
E) 7.0

Question 55

Calculate the equilibrium concentration of sulfurous acid in a solution labeled "0.100 M H₂SO₃(aq)" if pK_a1 = 1.81, and pK_a2 = 6.91.

A) 0.068 M
B) 0.015 M
C) 0.100 M
D) 0.050 M
E) 0.032 M

Question 56

The following 0.1 M aqueous solutions are arranged in order of increasing pH, with the highest pH on the far right. $$\begin{array} { | l | l | l | l | l | } \hline \mathrm { HClO } _ { 4 } & & \left( \mathrm { CH } _ { 3 } \right) _ { 3 } \mathrm { NHCl } & \mathrm { KBr } & \mathrm { KNO } _ { 2 } \\\hline\end{array}$$
Which one of the following 0.10 M aqueous solutions should be placed in the empty box?

A) NaI
B) HCOOH
C) C₆H₅NH₂
D) CH₃NH₃Cl
E) NaClO

Question 57

For a solution labeled "0.10 M H₂SO₃(aq)," pK_a1 = 1.81 and pK_a2 = 6.91, which of the following is true?

A) [H⁺] = 0.2 M.
B) The pH is 1.0.
C) The pH ~1.5.
D) The pH is 0.70.
E) The pH ~ 4.4.

Question 58

The K_a of phenol is 1.3 * 10^-10 .For a solution labeled "1.0 * 10^-3 M aqueous phenol,"

A) [H₃O⁺] = [H₃O⁺]²/[phenol]_initial.
B) [H₃O⁺] << [phenol]_initial.
C) [H₃O⁺] > 10 ^-6.
D) pH ~ 4.
E) K_w/[H₃O⁺] >> [phenol]_initial.

Question 59

For a solution labeled "0.10 M H₃PO₄(aq),"

A) [H₂PO₄^?] is greater than 0.10 M.
B) [H⁺] = 0.30 M.
C) [PO₄^3-] = 0.10 M.
D) [H⁺] = 0.10 M.
E) [H⁺] is less than 0.10 M.

Question 60

The K_a of phenol is 1.3 * 10^-10 .For a solution labeled "1.0 * 10^-3 M aqueous phenol,"

A) K_w/[H₃O⁺] >> [phenol]_initial.
B) K_w/[H₃O⁺] << [phenol]_initial.
C) [H₃O⁺] > 10^-6.
D) pH ~ 4.
E) [H₃O⁺] = [H₃O⁺]²/[phenol]_initial.

Question 61

The amino acid alanine,HOOC-CH(CH₃)NH₃⁺,Has K_a1 = 4.5 * 10^-3 and K_a2 = 1.4 * 10^-10.Calculate $\alpha$ (^-OOC-CH(CH₃)NH₃⁺)At pH 10.

A) 0.42
B) 0.29
C) 1.0
D) 0
E) 0.58

Question 62

The boxes below contain a series of 0.1 M aqueous solutions of increasing pH where A is the solution of lowest pH and E is the solution of highest pH.
Match each box with the correct compound.
phenol,pK_a = 9.89
cyanide ion,pK_b = 4.69
pyridine,pK_b = 8.75
hydrogen sulfate ion,pK_a = 1.92
sodium nitrate

Question 63

The fractional composition diagram for the amino acid alanine is given below.
Write the structure of the dominant species at pH 1,6,and 12,respectively.

Question 64

The pH of 0.010 M H₃PO₄(aq) is 2.24.Estimate the concentration of HPO₄^2- in the solution.For H₃PO₄,The values of K_a1,K_a2,And K_a3 are 7.6 * 10^-3,6.2 * 10^-8,And 2.1 * 10^-13,Respectively.

A) 5.8 * 10^-3 M
B) 7.6 * 10^-3 M
C) 0.010 M
D) 6.2 * 10^-8 M
E) 2.1 *10^-13 M

Question 65

Of the following, which is not a Lewis acid?

A) H₃O⁺
B) SO₃
C) NO₂^-
D) BF₃
E) None of the above.

Question 66

The amino acid alanine, HOOC-CH(CH₃)NH₃⁺,Has K_a1 = 4.5 * 10^-3 and K_a2 = 1.4 * 10^-10.Calculate $\alpha$ (^-OOC-CH(CH₃)NH₃⁺)At pH 3.

A) 0
B) 0.82
C) 0.18
D) 0.29
E) 0.58

Question 67

Calculate the equilibrium constant for the reaction
S^2-(aq)+ H₂O(l) $\Leftrightarrow$ H^-(aq)+ OH^-(aq),
Given K_a1 = 1.3 * 10^-7 and K_a2 = 7.1 * 10^-15 for H₂S.

A) 1.3 * 10^-7
B) 9.2 *10^-22
C) 7.7 * 10^-8
D) 7.1 * 10^-15
E) 1.4

Question 68

If $\alpha$ (HSO₃^-) = 0.45 at pH 7.0,What are $\alpha$ (H₂SO₃)And $\alpha$ (SO₃^2-)At this pH? For H₂SO₃,PK_a1 and pK_a2 are 1.81 and 6.91,Respectively.

A) $\alpha$ (H₂SO₃) = 0.225 and $\alpha$ (SO₃^2-) = 0.55
B) $\alpha$ (H₂SO₃) = 0.55 and $\alpha$ (SO₃^2-) ~ 0
C) $\alpha$ (H₂SO₃) ~ 0 and $\alpha$ (SO₃^2-) = 0.225
D) $\alpha$ (H₂SO₃) ~ 0 and $\alpha$ (SO₃^2-) = 0.55
E) $\alpha$ (H₂SO₃) = 0.45 and $\alpha$ (SO₃^2-) = 0.55

Question 69

If $\alpha$ (HSO₃^-) = 0.83 at pH 2.5,What are $\alpha$ (H₂SO₃)And $\alpha$ (SO₃^2-)
At this pH? For H₂SO₃,PK_a1 and pK_a2 are 1.81 and 6.91,Respectively.

A) $\alpha$ H₂SO₃) ~ 0 and $\alpha$ (SO₃^2-) = 0.17
B) $\alpha$ (H₂SO₃) = 0.415 and $\alpha$ (SO₃^2-) ~ 0
C) $\alpha$ (H₂SO₃) = 0.0.085 and $\alpha$ (SO₃^2-) = 0.085
D) $\alpha$ (H₂SO₃) = 0.17 and $\alpha$ (SO₃^2-) ~ 0
E) $\alpha$ (H₂SO₃) = 0.17 and $\alpha$ (SO₃^2-) ~ 1

Question 70

In a solution labeled "0.0018 M barium hydroxide" what is the molarity of OH^-?

A) 0.0018 M
B) 0.00090 M
C) 0.0036 M
D) 0.0072 M
E) None of the above.

Question 71

All of the following are Lewis bases except

A) OH^-
B) H₂O
C) SO₃
D) Br^-

Question 72

Both H₂O and OH^-can act as a Brønsted acid and a Brønsted base but not as a Lewis acid.

Question 73

All of the following acids have the same strength in water except

A) HNO₃
B) HClO₃
C) HBr
D) HF

Question 74

The pH of 0.010 M H₃PO₄(aq) is 2.24; estimate the concentration of PO₄^3- in the solution.For H₃PO₄,The values of K_a1,K_a2,And K_a3 are 7.6 * 10^-3,
6.2 * 10^-8,And 2.1 * 10^-13,Respectively.

A) 5.8 * 10^-3 M
B) 2.1 * 10^-13 M
C) 7.6 * 10^-3 M
D) 6.2 *10^-8 M
E) 2.3 * 10^-18 M

Question 75

For a solution of phosphoric acid, write the equation for (HPO₄^2-).

A) K_a3/([H₃O⁺]³ + K_a1[H₃O⁺]² + K_a1K_a2[H₃O⁺] + K_a1K_a2K_a3)
B) [H₃O⁺]/([H₃O⁺]³ + K_a1[H₃O⁺]² + K_a1K_a2[H₃O⁺] + K_a1K_a2K_a3)
C) K_a1K_a2[H₃O⁺]/([H₃O⁺]³ + K_a1[H₃O⁺]² + K_a1K_a2[H₃O⁺] + K_a1K_a2K_a3)
D) K_a1K_a2K_a3/([H₃O⁺]³ + K_a1[H₃O⁺]² + K_a1K_a2[H₃O⁺] + K_a1K_a2K_a3)
E) K_a1[H₃O⁺]²/([H₃O⁺]³ + K_a1[H₃O⁺]² + K_a1K_a2[H₃O⁺] + K_a1K_a2K_a3)

Question 76

The amino acid alanine, HOOC-CH(CH₃)NH₃⁺,Has K_a1 = 4.5 *10^-3 and K_a2 = 1.4 * 10^-10.Calculate $\alpha$ (HOOC-CH(CH₃)NH₃⁺)At pH 3.

A) 0
B) 0.82
C) 0.58
D) 0.29
E) 0.18

Question 77

True or false: the pH of 0.10 M and 0.40 M NaHCO₃(aq) solutions is 8.31 for both?

Question 78

Estimate the pH of 0.10 M Na₂HPO₄(aq) given pK_a1 = 2.12,
PK_a2 = 7.21,And pK_a3 = 12.68 for phosphoric acid.

A) 12.68
B) 9.94
C) 7.40
D) 4.67
E) 2.12

Question 79

The amino acid methionine, HOOC-CH(CH₂CH₂SCH₃)NH₃⁺,Has pK_a1 = 2.2 and pK_a2 = 9.1.If this amino acid is represented by H₂L⁺,The major species at pH 6 is

A) HL
B) H₂L⁺
C) L^-
D) HL and L^-

Question 80

The fractional composition diagram for the amino acid alanine is shown below.
What do the two points represent where alpha is 0.5?