Question 166168
Let x=amount of pure acid needed

Now we know that the amount of pure acid added (x) plus the amount of pure acid in the 100 ml solution (0.10*100) has to equal the amount of pure acid in the final solution (0.20(100+x)).  So our equation to solve is:

x+0.10*100=0.20(100+x)  get rid of parens (distributive law) and simplify
x+10=20+0.20x  subtract 0.20x and also 10 from each side
x+10-10-0.20x=20-10+0.20x-0.20x  collect like terms

0.80x=10  divide each side by 0.80
x=12.5 ml-----------------amount of pure acid needed

CK
12.5+10=0.20(100+12.5)=0.20(112.5)
22.5=22.5

Hope this helps----ptaylor