tugas kimia semester 2

1.      A solution has a pH of 8.5.  What is the Molarity of hydrogen ions in the solution?

ANSWER:

pH = - log [H⁺]

8.5 = - log [H⁺]

-8.5 = log [H⁺]

Antilog -8.5 = antilog (log [H⁺])

10^-8.5 = [H⁺]

3.16 10^-9 = [H⁺]

2.      Hydrochloric acid reacts with potassium hydroxide according to the following reaction:

HCl  +  KOH  à  KCl  +  H₂O

If 15.00 cm³ of 0.500 M HCl exactly neutralizes 24.00 cm³ of KOH solution,  what is the concentration of the KOH solution?

ANSWER:

V_acid C_acid  =   V_base C_base

(15.00cm³ )(0.500M)  =  (24.00 cm³ ) C_base

C_base    =   (15.00 cm³ )(0.500 M)

                                    (24.00cm³ )

C_base    =  0.313 M

3.      Sulfuric acid reacts with sodium hydroxide according to the following reaction:

H2SO4  +  2 NaOH  à  Na2SO4  +  2 H2O

If 20.00 cm3 of 0.400 M H2SO4 exactly neutralizes 32.00 cm3 of NaOH solution,  what is the concentration of the NaOH solution?

ANSWER:

In this case the mole factor is 2 and it goes on the acid side, since the mole ratio of acid to base is 1 to 2.  Therefore

2 V_acid  C_acid  =   V_base  C_base

2 (20.00 cm³ )(0.400 M)  =  (32.00 cm³ ) C_base

C_base    =   (2) (20.00 cm³ )(0.400 M)

                            (32.00 cm³ )

C_base    =  0.500 M

4.      Calculate the pH of the following of a mixture that contains  0.75 M lactic acid (HC₃H₅O₃) and 0.25 M sodium lactate (Ka = 1.4 x 10^-4)

HC₃H₅O₃ +  H₂O  ^à_ß H₃O⁺  +  C₃H₅O₃^-

ANSWER:

Using the Henderson-Hasselbach equation

pH =  - log (1.4 x 10^-4) +  log (  0.25/0.75 )

=   3.85   +  (-0.477)    =  3.37

5.      Calculate the pH of a solution that contains 0.25 M NH₃ and 0.40 M NH₄Cl (K_b = 1.8 x 10^-5)

ANSWER:

pOH =  - log(1.8 x 10^-5) + log (0.40/0.25)

         =   4.74  +  0.204 =  4.94

=  14 - pOH  =  14 - 4.94  =   9.06