Problem: Knowing that NaNO2 is the salt of a strong base and weak acid calculate : a: pH of 0.100 M NaNO2 solution. Ka of HNO2 = 4.6 x 10-4.b: pH of the solution obtained on adding 20.0 ml of a 0.100 M HCl solution to 60.0 ml of a 0.100 M NaNO2 .c: pH of the solution obtained on adding 120.0 ml of a 0.100 M HCl solution to 60.0 ml of a 0.100 M NaNO2

FREE Expert Solution

To calculate pH of starting NaNO2 solution and after each addition of HCl


a) NaNO2 dissociates into ions: Na+ + NO2- and reacts with water: 

  • NO2- is conj. base of weak acid HNO2


Step 1: ICE Chart 



The Kb expression for NO2-  is:


Kb=productsreactants=[HNO2][OH-][NO2-]


Calculate Kb:  


Assuming the reaction is at 25°C:  Kw = 1.0x10-14


KwKa=Ka·KbKaKb=KwKaKb=1.0×10-144.6×10-4

Kb = 2.17 x 10-11


Step 2: Solve for x = [|OH-]


Kb=[HNO2][OH-][NO2-]

2.17 × 10-11=xx0.100-x


NO2-iKb=0.100 M2.17×10-11500;     we can ignore x in the denominator

2.17 × 10-11=xx0.100-x0.100 2.17 × 10-11=x20.1000.1002.17 × 10-12=x2

x = [OH-]  = 1.47 x 10-6 M


Step 3: Calculate pOH:


pOH=-log OH-pOH=-log 1.47×10-6 M

pOH = 5.83

89% (67 ratings)
View Complete Written Solution
Problem Details

Knowing that NaNO2 is the salt of a strong base and weak acid calculate : 

a: pH of 0.100 M NaNO2 solution. Ka of HNO2 = 4.6 x 10-4.

b: pH of the solution obtained on adding 20.0 ml of a 0.100 M HCl solution to 60.0 ml of a 0.100 M NaNO2 .

c: pH of the solution obtained on adding 120.0 ml of a 0.100 M HCl solution to 60.0 ml of a 0.100 M NaNO2

Frequently Asked Questions

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Weak Base Strong Acid Titrations concept. You can view video lessons to learn Weak Base Strong Acid Titrations. Or if you need more Weak Base Strong Acid Titrations practice, you can also practice Weak Base Strong Acid Titrations practice problems.

What professor is this problem relevant for?

Based on our data, we think this problem is relevant for Professor Hjorth-Gustin's class at San Diego Mesa College.