SOLUTION: how much pure acid should be mixed with 2 gallons of a 40% acid solution in order to get a 70% acid solution?

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Question 900947: how much pure acid should be mixed with 2 gallons of a 40% acid solution in order to get a 70% acid solution?
Found 2 solutions by josgarithmetic, richwmiller:
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
Two gallons of 40% acid,
You must include the density of the acid solutions. IFF the concentrations are as volume per volume, then the density is not really necessary; but then the answer to the question must be in GALLONS of the pure acid.

ONLY if you have all concentrations as volume per volume, then:
Let Q be the VOLUME of pure acid to add.
%282%2A40%2BQ%2A100%29%2F%28Q%2B2%29=70

Answer by richwmiller(17219) About Me  (Show Source):
You can put this solution on YOUR website!
x+2*.40=.7*(2+x)
x=2 gallons
check
2+.8=.7*4
2.8=2.8
from view of water
2*0.6=0.3*(2+x)
1.2=0.6+0.3*x
1.2-0.6=0.3*x
0.6=0.3*x
x=2
check
2*0.6=0.3*(2+x)
2*0.6=0.3*(4)
1.2=1.2
ok
or from view of acid
1*y+0.4*2=0.7(2+y)
1*y+0.8=1.4+0.7y
1*y-0.7y=1.4-0.8
0.3y=0.6
y=0.6/0.3
y=2