Question 1110771
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Since what is being added is pure water, I would use mental arithmetic to solve this problem.<br>
(1) The amount of acid is 10% of 840 cm^3, or 84 cm^3.
(2) If that same 84 cm^3 of acid is to be 7% of the final mixture, then the total volume of the final mixture must be 84/.07 = 1200 cm^3.
(3) You started with 840 cm^3 of solution and finished with 1200 cm^3; the amount of pure water you added was 1200=840 = 360 cm^3.