Question

    A chemist has 40 litres of a 25% acid solution. How many

    litres of a 60% acid solution must be added so that the resulting mixture has 40% acid?
    A 20 litres Correct Answer Incorrect Answer
    B 30 litres Correct Answer Incorrect Answer
    C 60 litres Correct Answer Incorrect Answer
    D 19 litres Correct Answer Incorrect Answer

    Solution

    Let x litres of 60% solution be added. Pure acid initially = 25% of 40 = 10 litres Pure acid added = 60% of x = 0.6x litres Total acid = 10 + 0.6x Total volume = 40 + x Required concentration = 40%: (10 + 0.6x) / (40 + x) = 0.4 10 + 0.6x = 0.4(40 + x) 10 + 0.6x = 16 + 0.4x 0.6x − 0.4x = 16 − 10 0.2x = 6 ⇒ x = 30 litres.

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