# Chemistry

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I am using 85% ACS grade phosphoric acid. I need to make a 21% phosphoric acid solution with a total volume of 250ml. I have been told that 37ml of the 85% ACS grade phosphoric acid plus 188ml of Deionized water will create the 21% phosphoric acid solution I need to make. I'm having trouble figuring out how to write out the equation/equations. If the calculations were just based on volume, it would be easy because 21% of 250ml is 52.5ml, but I can't figure out how to factor in the concentration of the phosphoric acid. I know that 85% ACS grade phosphoric acid solution has a density of 1.685g/ml and if I multiply the 37ml x 1.685g/ml, I get 62.345g, if I multiply that mass by the concentration of the ACS grade phosphoric acid (85%), so 62.345g x .85 = 52.99325g. If I divide that mass by the total volume I desire (250ml), I get 0.211973g/ml. How do I need to write this/these equation/equations so that they make sense? Help please.

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