Use this data to rank the following solutions in order of increasing pH. In othe

Question

Use this data to rank the following solutions in order of increasing pH. In other words, select a ' 1 ' next to the solution that will have the lowest pH, a ' 2 ' next to the solution that will have the next lowest pH, and so on.

Consider the following data on some weak acids and weak bases acid base name formula name formula 3 11.8x10-5 methylamine CH3NH24.4 x 104 hydrofluoric acidH 6.8 x 10 ammoniaNH nitrous acidHNO x 10 Use this data to rank the following solutions in order of increasing pH. In other words, select a '1' next to the solution that will have the lowest pH, a '2' next to the solution that will have the next lowest pH, and so on solution pH Vchoose one 1 (lowest) 0.1 M NaCI 0.1 M NaF 4 (highest) 0.1 M KN02 0.1 M NH4CI choose one

Explanation / Answer

Question 1:

The rules are:

1. Bases have higher pH than acids.

2. Stronger acid has lower pH.

3. Stronger acid has hgher Ka. So, the acid with higer Ka has lower pH.

4. Stronger bases have higher pH.

5. Stronger bases have higher Kb. So, bases with higher Kb have higher pH.

Assembling the rules, we get:

HF (1), HNO2 (2), NH3 (3), CH3NH2 (4)

Question 2:

(a) NaCl is a salt of strong acid and strong base. So, it is a neutral salt and its pH is 7.0.

(b) NaF is a salt of strong base (NaOH) and weak acid (HF). So, it is a basic salt.

NaF produces Na+ and F- ions in solution. F-   is a strong conjugate base of weak acid, HF. The Kb of F- is: 10-14/ (6.8 * 10-4) = 1.47 * 10-11

So, [OH-] in 0.1 M NaF solution = ( 0.1 * 1.47 * 10-11 )0.5 = 1.21*10-6 M

pOH = - log[OH-] = - log(1.21*10-6) = 5.92

pH = 14-5.92 = 8.08

(c) KNO2 is a salt of strong base (KOH) and weak acid (HNO2). So, it is a basic salt.

KNO2 produces K+ and NO2- ions in solution. NO2- is a strong conjugate base of weak acid, HNO2. The Kb of NO2- is: 10-14/ (4.5 * 10-4) = 2.22 * 10-11

So, [OH-] in 0.1 M KNO2 solution = ( 0.1 * 2.22 * 10-11 )0.5 = 1.49*10-6 M

pOH = - log[OH-] = - log(1.49*10-6) = 5.83

pH = 14-5.83 = 8.17

(d) NH4Cl is a salt of weak base (NH4OH) and strong acid (HCl). So, it is an acidic salt.

NH4Cl produces NH4+ and Cl- ions in solution. NH4+ is the strong conjugate acid of weak base NH4OH. Ka of NH4+ is: 10-14/ (1.8 * 10-5) = 5.56 * 10-10

[H+] = ( 0.1 * 5.56 * 10-10 )0.5 M = 7.45 * 10-6 M

pH = - log ( 7.45 * 10-6 ) = 5.12

So, the order of pH:

NH4Cl (1), NaCl (2), NaF (3), KNO2 (4)

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