The equivalent weight of Ba(OH)2 is (given, atomic weight of Ba is 137.3)
- A.85.7
- B.137.3
- C.154.3
- D.171.3
Correct answer
Explanation
The correct answer is A, 85.7. The equivalent weight of a base is its molecular weight divided by its acidity, which is the number of replaceable hydroxide ions in one molecule. Barium hydroxide, Ba(OH)2, has two OH groups, so its acidity is 2. Its molecular weight is 137.3 for barium plus 2 × 17 for the two hydroxide groups (16 for oxygen and 1 for hydrogen), which gives 171.3. Dividing 171.3 by 2 gives 85.65, which rounds to 85.7. Option B is wrong because 137.3 is only the atomic weight of barium. Option C is wrong because 154.3 counts just one hydroxide group, 137.3 + 17. Option D is wrong because 171.3 is the full molecular weight, not divided by the acidity. The same rule gives 40 for NaOH, which has one OH group, and 37 for Ca(OH)2, whose molecular weight is 74 and which has two OH groups. Exam tip: the equivalent weight of a base is its molecular weight divided by its number of OH ions.
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