Question 713055
About half of the water.  Are you trying to ask what was the volume of the 1% acid solution to begin with?   We can work backward from what the solution became.  You doubled the concentration so evaporated half the water.


Result is 2% acid, size is 10 liters.  Let w be the liters to add back to get 1% acid.


{{{highlight((10*2+0*w)/(10+w)=1)}}}
{{{(10*2)/(10+w)=1}}}
{{{10*2=1(10+w)}}}
{{{20=10+w}}}
20-10=w
{{{highlight(w=10)}}} liters would be added to return to the 1% acid concentration.
 So "some" 1% acid solution was 10 liters + 10 liters = 20 liters.