Asked by danielle p.

what mass of water must be added to 425 g of formaldehyde to prepare a 40.0% (by mass) solution of formaldehyde? this solution, called formalin, is used to preserve biological specimens.

(hint:) substitute the known quantities into the definition for mass percent, and then solve for the unknown quantity (mass of solvent)
Answered by DrBob222
The hint isn't quite right but close.
You want 40% formaldehyde (w/w) which means 40g HCHO/100 g solution =
40 g HCHO/(40g HCHO + 60 H2O)
[425g HCHO/x]*100 = 40%
Solve for x which is the TOTAL grams solution. The amount of H2O to add will be x-425 = ? or 637.5g H2O.
Check it to see if that will do it.
[425/(425+637.5)]*100 =
(425/1062.5)*100 = 40%
Answered by danielle p.
ohhh yes, you are right. thank you very much I see my mistake now thank you again. have a nice day (:
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