chemistryhelp needed
posted by Minnie .
How do I prepare 250ml of 10% phosphoric acid from 85% phosphoric acid(density 1.69g/ml)?
Thanks.

Minnie, you don't mention if this is percent by mass or percent by volume.For the moment, I will assume this is 85% H3PO4 by mass with a density of 1.69 g/mL.
Work by mass.
85 g/100 g soln is 85% by mass = 0.85 g/g soln.
We want 25 g/250 g soln for 10%
(85 g/100 g soln) x ?? g soln = 25 g and solve for ??g. That gives me 29.41 g of the soln. Weigh out 29.41 g of that soln, add water to make to a total mass of 250 g (i.e., add 220.6 g H2O) and you will have 25 g H3PO4/250 g soln which is 10%. Let me caution you, however, that this will NOT be 250 mL volume but somewhat less than that depending upon the density of the 10% solution.
If you want to do it by volume, then
100 g soln with a density of 1.69 occupies a volume of 100/1.69 = 59.17 mL or 85/59.17 = 1.4365 g/mL.
Then 1.4365 mL x ?? mL = 25 g
??Ml = 25/1.4365 = 17.4 mL. Therefore, pipet 17.4 mL of the 85% soln into the flask and add water to make 250. That will give you 250 mL of a 10% w/v solution. 
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