First off, please don’t add water to acid, ever. Add the acid to
the water, and do it carefully. If the solution bumps, it’s a lot
better to have it blowing up in your face if you’ve only managed to
add a little acid to a lot of water, versus the other way 'round.
Next, it’s just a matter of percentages and simultaneous equations,
but since you’d probably rather just get the answer, here’s a site
that you’ll find useful for that.
The specific link you want on that page is
It’s in the "Chemical Calculators section, maybe halfway down, and
the entry is “Chemical Mixing Calculator for Adjusting the Active
Ingredient Concentration of a Batch”.
You didn’t specify how much, or what the final percentage was, but
if you have 20 grams of the 55% and want to get it down to 20%…
well, here’s how that example would work out:
Weight of Initial Mixture 20
Percent Active Ingredient in Initial Mixture 55
Percent Active Ingredient in Adjustment Mixture 0
Percent Active Ingredient in Final Mixture 20
Weight of Adjustment Mixture 35
Weight of Final Mixture 55
So, you’d take 35ml (that’d be 35 grams) of water, and add,
carefully, 20 grams (probably less than 20ml) of your 55% solution
to it. The final weight would be 55 grams, with a concentration of
Use your own numbers to get the results you really need.