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Chemistry Question 132 – AP-EAMCET 2025

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Which of the following aqueous solutions has highest pH ? (Given log⁡2=0.30, log⁡3=0.48, log⁡4=0.60, log⁡5=0.70)

A higher pH value indicates a more basic solution. All given solutions are strong bases, so we need to calculate the concentration of hydroxide ions ([OH−]) for each.

Step 1: Determine Hydroxide Ion Concentration for Each Solution✦ Active

We need to calculate the hydroxide ion concentration ([OH−]) for each given solution. For strong bases, the dissociation is complete.

For Ba(OH)2, it dissociates as Ba(OH)2→Ba2++2OH−. So, [OH−]=2×[Ba(OH)2]. For NaOH, it dissociates as NaOH→Na++OH−. So, [OH−]=[NaOH]. If normality (N) is given for a base, [OH−]=N (assuming the n-factor is the number of OH− ions).

Option 1: 0.2 M Ba(OH)2

[OH−]=2×0.2 M=0.4 M

Option 2: 0.02 N Ba(OH)2

For Ba(OH)2, the n-factor is 2. So, Molarity = Normality / n-factor = 0.02 N/2=0.01 M.

[OH−]=2×0.01 M=0.02 M

Option 3: 0.1 M NaOH

[OH−]=0.1 M

Option 4: 0.05 M Ba(OH)2

[OH−]=2×0.05 M=0.1 M
💡 Teacher's Secret Hint

Remember that Normality is concentration of equivalents. For Ba(OH)2, 1 mole provides 2 equivalents of OH−. So, N=M×2.

Step 2: Calculate pOH for Each Solution○ Expand

Now, we calculate the pOH for each solution using the formula pOH=−log⁡[OH−] and the given logarithm values.

Option 1: [OH−]=0.4 M

pOH=−log⁡(0.4)=−log⁡(4×10−1)=−(log⁡4+log⁡10−1)=−(log⁡4−1) pOH=−(0.60−1)=−(−0.40)=0.40

Option 2: [OH−]=0.02 M

pOH=−log⁡(0.02)=−log⁡(2×10−2)=−(log⁡2+log⁡10−2)=−(log⁡2−2) pOH=−(0.30−2)=−(−1.70)=1.70

Option 3: [OH−]=0.1 M

pOH=−log⁡(0.1)=−log⁡(10−1)=1.00

Option 4: [OH−]=0.1 M

pOH=−log⁡(0.1)=−log⁡(10−1)=1.00
💡 Teacher's Secret Hint

Be careful with the negative sign when calculating logarithms of numbers less than 1. log⁡(A×10−B)=log⁡A−B.

Step 3: Calculate pH for Each Solution and Identify the Highest pH○ Expand

Finally, we calculate the pH for each solution using the relationship pH=14−pOH and compare them to find the highest pH.

Option 1: pOH=0.40

pH=14−0.40=13.60

Option 2: pOH=1.70

pH=14−1.70=12.30

Option 3: pOH=1.00

pH=14−1.00=13.00

Option 4: pOH=1.00

pH=14−1.00=13.00

Comparing the pH values: 13.60, 12.30, 13.00, 13.00. The highest pH is 13.60, which corresponds to Option 1.

💡 Teacher's Secret Hint

A common mistake is to forget that Ba(OH)2 is a diprotic base, meaning it releases two hydroxide ions per formula unit, doubling the effective hydroxide concentration compared to its molarity.

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