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Percentage by Mass chemistry

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ATB_roo | 22:00 Mon 15th Oct 2007 | Science
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How do I find the percentage by mass ethanoic acid (CH3COOH) in the vinegar solution I have been given. Through titration I have found that the concentration of the vinegar is 8.45moldm-3, 0.21125 moles are present and I used 25cm3 of vinegar.I know the answer is 5% ethanoic acid in the vinegar (said on the bottle), but how do I arrive at this answer from the values I have?

Thank you very much,
Ruth

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I think either you or I have a decimal point out.

Atomic mass of Carbon:12, Oxygen 16 Hydrogen:1
2x12+2x16+2x12 =60.

So 1 mol of CH3COOH is 60g

If you've got about 1/5 of a mol (0.21125) that's about 12g

25cm3 of water is 25g so it sounds more like 50% than 5%

But titrations were never my strong suit
Question Author
Ahh wow thanks jake-the-peg =] I think it's me with the decimal point out, because I multiplied all the values by 10 as the vinegar we used in the experiment had been diluted to a 10th of the concentration. I wasn't really sure if I was supposed to have done this or not, but now I guess I shouldn't have.
Thanks again, you'll have made my chemistry teacher very happy =]

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