102.     The amino acid alanine has the structure of ^- O²C.CH(NH₃⁺).CH₃ in a neutral buffer. What would be the net charge the amino acid in a strong alkali (pH>11)?

1.        +2
2.        +1
3.        0
4.        –1
5.        –2

103.     The net charge at a neutral pH (~7) of the amino acid lysine, HO₂C.CH NH₂.CH₂.CH₂.CH₂.CH₂.NH₂ is:

1.        +2
2.        +1
3.        0
4.        –1
5.        –2

104.     In solution at pH 11 the compound COOH.CH₂.CHNH₂.COOH has a net charge of:

1.        +2
2.        +1
3.        0
4.        -1
5.        -2

105.     In highly acidic conditions, the compound COOH.CH₂.CHNH₂.COOH has a net charge of:

1.        +2
2.        +1
3.        0
4.        -1
5.        -2

106.     The amino acid glycine has pKs of 2.3 and 9.6. Which one of the following is FALSE?:

1.        At a pH of about 7 glycine is H₃N⁺.CH₂.CO₂^-
2.        The pK of 2.3 refers to the amino group
3.        When pH < 2.3 the dominant form of the molecule is H₃N⁺.CH₂.CO₂H
4.        When pH > 9.6 the dominant form of the molecule is H₂N.CH₂.CO₂^-
5.        When pH = 9.6 the net charge on the molecule is - ½

107.     The amino acid glycine has pKs of 2.3 and 9.6. Which one of the following is FALSE?:

1.        At a pH of about 7 glycine is H₂N.CH₂.CO₂H
2.        The pK of 2.3 refers to the carboxyl group
3.        When pH < 2.3 the dominant form of the molecule is H₃N⁺.CH₂.CO₂H
4.        When pH > 9.6 the dominant form of the molecule is H₂N.CH₂.CO₂^-
5.        When pH = 9.6 the net charge on the molecule is -½

108.    With regard to a pH buffer, which one of the following is FALSE?:

1.        It consists of a weak acid and its conjugate base
2.        It obeys the equation

3.        It is most effective when the pH is equal to the pK of the weak acid
4.        Its pH does not change when it is diluted
5.        When pH = pK, [base] = [acid]
 

109.     Ethanoic acid (acetic acid) has a pK of 4.6. Which one of the following statements about an ethanoic acid/sodium ethanoate buffer is FALSE (for concentrations in mol.l^-1)?

1.    At pH 3.6 [ethanoic acid] = 10 ´ [sodium ethanoate]
2.    The buffer works well at pH 4.6
3.    At pH 5.6 [ethanoic acid] = 0.1 ´ [sodium ethanoate]
4.    The buffer works well at pH 7.0
5.    At pH 4.6 [ethanoic acid] = [sodium ethanoate]

110.     Oxalic acid has two acid groups with pKs of 1.3 and 4.3 respectively, and the dissociation can be represented by
                HO₂C-CO₂H « HO₂C-CO₂^-+ H⁺ « ^-O₂C- CO₂^-+ 2H⁺
            Which one of the following statements is FALSE?

1.        The solution is a good buffer at pH 4.3
2.        HO₂C-CO_2-can both donate and accept hydrogen ions
3.        At pH 5, the dominant form is HO₂C-CO₂^-
4.        The concentration of HO₂C-CO₂H is negligible when the pH is greater than 4.3
5.        At pH 2.8 (half way between the 2 pKs) the net charge on the oxalate is –1.

111.     The tribasic acid H₃PO₄ has pKs of 2.1, 6.7 and 12.5. Which one of the following is FALSE?

1.        Its dissociation can be represented by:
            H₃PO₄ « H⁺ + H₂PO_4-« 2H⁺ + HPO₄^2-« 3H⁺ + PO₄^3-
2.        At a pH of less than 2.1 the dominant form is PO₄^3-
3.        H₂PO₄^-can both accept and donate hydrogen ions
4.        A mixture of H₂PO₄^-and HPO₄^2-buffers best around pH 6.7
5.        The concentration of H₃PO₄ is negligible when the pH is greater than 12.5

112.    Succinic acid has two acid groups with pKs 4.2 and 5.6 respectively. Its dissociation is represented by
            HO₂C-CH₂-CH₂-CO₂H « HO₂C-CH₂-CH₂-CO₂^-+ H+ « ^-O₂C-CH₂-CH₂-CO₂^- + 2H⁺
            Which of the following statements is FALSE?

1.        At pH 3, the uncharged form predominates.
2.        Succinic acid/sodium succinate make a good buffering solution at a pH of 5.6
3.        The concentration of HO₂C-CH₂-CH₂-CO₂H is negligible at a pH greater than 5.6
4.        At pH 4.2, the concentrations of HO₂C-CH₂-CH₂-CO₂H and ^-O₂C-CH₂-CH₂-CO₂^- are equal.
5.        HO₂C-CH₂-CH₂-CO₂^- is both an acid and a base.
 
 

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