Lecture 31 Acids and Bases III Tutorial

www.apchemsolutions.com Lecture 31 Acids and Bases III Tutorial

Write balanced net ionic equations for the reactions that occur in questions 1-5.

1) Equal volumes of 0.15 M sulfurous acid and 0.1.5 M lithium hydroxide are mixed.

- - H2SO3(aq) + OH (aq) Æ HSO3 (aq) + H2O(l)

2) Solutions of ammonium nitrate and sodium cyanide are poured into a beaker.

+ - NH4 (aq) + CN (aq) Æ NH3(aq) + HCN(aq)

3) Carbon dioxide gas is bubbled through a solution of 0.5 M potassium hydroxide.

- - CO2(g) + OH (aq) Æ HCO3 (aq)

4) Hydrogen bromide gas is mixed with ammonia gas.

HBr(g) + NH3(g) Æ NH4Br(s)

5) Solid sodium acetate is stirred into a beaker of distilled water.

+ - NaCH3CO2(s) + H2O(l) Æ Na (aq) + CH3COOH(aq) + OH (aq)

6) Which mixture, 1.5 M NH4Cl or 1.5 M NaCl, has the lowest pH? Justify your answer.

+ 1.5 M NH4Cl will have the lowest pH. Its pH will be less than 7. NH4 is an acidic anion. Acidic anions lower the pH.

Cl- is a neutral anion, which will not affect the pH.

1.5 M NaCl will have the highest pH. NaCl and water produces a neutral solution with a pH of 7, as both ions are neutral. Na+ is the conjugate acid of a strong base, and Cl- is the conjugate base of a strong acid.

[OH-] = 0.3 – 0.4 = 0.1 M

pOH = -log[OH− ] =− log(0.1) = 1

pH = 14 - pOH = 14 - 1 = 13

8) The acid dissociation constants for hypoiodous acid, HOI, and lactic acid, -11 -4 HC3H5O3, at 298 K are 2 x 10 and 1.38 x 10 respectively. Which solution is more basic: 1.0 M NaOI or 1.0 M NaC3H5O3? Justify your answer.

HOI is the weaker acid, as 2 x 10-11 < 1.38 x 10-4. This means that its conjugate base, OI- - , is stronger than C3H5O3 . Weaker acids have stronger conjugate bases.

- - Because OI is a stronger base than C3H5O3 , the equilibrium for - - OI (aq) + H2O(l) U HOI(aq) + OH (aq) - - lies further to the right than it does in C3H5O3 (aq) + H2O(l) U HC3H5O3(aq) + OH (aq).

Thus, 1.0 M NaOI produces a higher concentration of OH-, making a more basic solution.