Question

A 20% ethanol solution is mixed with another ethanol solution, say, S of unknown concentration in the proportion 1:3 by volume. This mixture is then mixed with an equal volume of 20% ethanol solution. If the resultant mixture is a 31.25% ethanol solution, then the unknown concentration of S is

• A. 52%
• B. 50%
• C. 55%
• D. 48%

Answer 1

Answer: (B)

50%

Answer 2

The correct answer is (B):
Let the volume of the first and the second Solution be 100 and 300.
When they are mixed, quantity of ethanol in the mixture = (20 + 300S)
Let this Solution be mixed with equal volume i.e. 400 of third Solution in which the strength of ethanol is 20%
So, the quantity of ethanol in the final Solution
= (20 + 300S + 80) = (300S + 100)
It is given that, 31.25% of 800 = (300S + 100)
or, 300S + 100 = 250
or S = 1/2 = 50%
Hence, 50 is the correct answer.