<PRE><B>Question 564714</B>
Solution A
Amount = 12 gallons
Concentration =30% =0.3

Solution B
Amount = x 
Concentration =100% = 1 (Pure Hydrochloric acid)

Resultant Solution
Amount =12+x 
Concentration =40%=0.4

[Amount Solution A * Concentration A] + [Amount Solution B * Concentration of B] = Amount of Resultant * Concentration of resultant

12*0.3  +  x*1  = (12+x)*0.4
3.6+x=4.8+0.4x
x-0.4x=4.8-3.6
0.6x=1.2
0.6x/0.6=1.2/0.6
x=2


2 gallons of hydrochloric acid (pure) must be added to 12 gallons of a 30% solution to obtain a 40% solution.
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