Question

Find the pH for the following solutions:

a) A solution that is 0.115 M in HBr and 0.130 M in HCHO2

b) A 0.25 M CH3NH3I solution? (K_b = 4.4 x 10^-4 M for CH₃NH₂)

c) A 0.0180 M solution of HClO₂? (K_a = 1.1 x 10^-2 M)

d) A 0.0180 M solution of Sr(OH)₂

e) A 0.170 M solution of KOCl? (For HOCl, K_a = 3.5 x 10^-8 M)

f) A solution that is 3.00×10−2M in HI and 9.00×10−3M in HF. (For HF, pK_a = 3.14)

g) A solution that is 0.105 M in KF and 0.105 M in KCl? (for HF, pK_a = 3.14)

I'm having a hard time in chem class, please help understand how to know if it's an acid or base, and what the steps are to find pH. Thanks!

Answer 1

solution:

a)

b)

0.25 M CH3NH3I

it is a salt oe weakamine,strongacid.So that it is acidic.

pH = 7-1/2(pkb+logC)

pkb of CH3CH2NH2 = 3.19

   = 7 - 1/2 (3.19+Log(0.25))

   = 5.7

c)

HClO2 + H2O ---> H3O+ + ClO2-

ka = [H3O+] [ClO2-] /[HClO2]

1.1*10^-2= [x][x]/[0.15]

1.1*10^-2*0.15= x²

0.00165 = x²

taking square rrot of boht the sides we get x= 0.0406 M

x = [H3O+] = [H+]

so, p^H = - log[H+]

p^H = - log[0.0406] = 1.34

d)

Sr(OH)₂--------> Sr²⁺ + 2OH-

so, [OH^-] =2*0.018

[OH^-] = 0.036 M

so pOH = -log[OH] = - log[0.036]

pOH = 1.44

so, Ph = 14 - pOH = 14 - 1.44

pH = 12.6

=13.7