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All questions of Ions in Solutions (GC, BC) for MCAT Exam

If 40 ml of 0.2 M KOH is added to 160 ml of 0.1 M HCOOH [Ka = 2 × 10-4]. The pOH of the resulting solution is
  • a)
    3.4
  • b)
    3.7
  • c)
    7
  • d)
    10.3
Correct answer is option 'D'. Can you explain this answer?

Kalyan Desai answered
m. equivalent of KOH = 8
m. equivalent of HCOOH = 16
Remaining m. eq. (HCOOH) = 8
Formed m. eq. (HCOOK) = 8
⇒ Acidic Buffer
pH = pKa = 4 – log2
= 3.7
pOH = 10.3

The precipitate of CaF2 (Ksp = 1.7 × 10-10) is obtained when equal volumes of the following are mixed
  • a)
    10-4 M Ca3+ + 10-4 M F-
  • b)
    10-2 M Ca2+ + 10-3 M F-
  • c)
    10-5 M Ca2+ + 10-3 M F-
  • d)
    10-3 M Ca2+ + 10-5 M F-
Correct answer is option 'B'. Can you explain this answer?

Pranav Saha answered
For ppt Qsp > Ksp
CaF2→ Ca2++2F
Qsp  = (Ca2+) (F)2
(A) Qsp  = 12.5 × 10–14
(B) Qsp  = 12.5 × 10–10
(C) Qsp  = 12.5 × 10–13
(D) Qsp  = 12.5 × 10–15
Only (B) option will get precipitate.

1 c.c. of 0.1N HCl is added to 99 CC solution of NaCl. The pH of the resulting solution will be
  • a)
    7
  • b)
    3
  • c)
    4
  • d)
    1
Correct answer is option 'B'. Can you explain this answer?

Given:
- Volume of 0.1N HCl = 1 cc
- Volume of NaCl solution = 99 cc

To find:
The pH of the resulting solution

Solution:
Step 1: Calculate the moles of HCl
- Concentration of HCl = 0.1 N
- Volume of HCl = 1 cc

Moles of HCl = Concentration × Volume
= 0.1 N × 1 cc
= 0.1 moles

Step 2: Calculate the moles of NaCl
- Concentration of NaCl = 0.1 N
- Volume of NaCl = 99 cc

Moles of NaCl = Concentration × Volume
= 0.1 N × 99 cc
= 9.9 moles

Step 3: Calculate the moles of H+ ions
- HCl dissociates in water to give H+ and Cl- ions
- The moles of H+ ions will be equal to the moles of HCl added

Moles of H+ ions = Moles of HCl
= 0.1 moles

Step 4: Calculate the total volume of the resulting solution
- The total volume of the resulting solution will be the sum of the volumes of HCl and NaCl solution

Total volume = Volume of HCl + Volume of NaCl solution
= 1 cc + 99 cc
= 100 cc

Step 5: Calculate the concentration of H+ ions
- Concentration is defined as moles of solute divided by the volume of the solution

Concentration of H+ ions = Moles of H+ ions / Total volume
= 0.1 moles / 100 cc
= 0.001 moles/cc

Step 6: Calculate the pH of the resulting solution
- pH is defined as the negative logarithm of the concentration of H+ ions

pH = -log10(Concentration of H+ ions)
= -log10(0.001 moles/cc)
= -(-3)
= 3

Answer:
The pH of the resulting solution will be 3.

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