Question 862967
Do it like this one.
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If I have a 25 mil test tube of 72% nitric acid and I want to reduce the concentration to 42%, how much water would I need to add ? 
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w = ml of water
25*72 + 0*w = (25+w)*42
1800 = 42w + 1050
42w = 750
w = 125/7 ml
=~ 17.86 ml
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This is safe on paper.
If it's real liquids, add the acid to the water, not the water to the acid.
And don't drink any.