The answer is 280 cm^3.
A solution's percent concentration by mass is defined as the mass of the solute divided by the mass of the solution and multiplied by 100%.
c = m_(solute)/m_(solution) * 100%, where m_(solution) = m_(solvent) + m_(solute).
We know that m_(solution1) = 70g and the the concentration is equal to c_1 = 10%, which means we can determine the mass of KCl used for the mixture
m_(KCl) = (c_1 * m_(solution))/100 = (10 * 70)/100 = 7g
Now, let's say that after adding a certain mass of water - m_(added) - we would get a new concentration of c_2 = 2%. This means that
c_2 = m_(KCl)/m_(solution2) * 100, where
m_(solution2) = m_(solutiuon1) + m_(added)
Therefore, m_(solution2) = (m_(KCl) * 100)/c_2 = 7 * 100/2 = 350g
This means that m_(added) = m_(solution2) - m_(solution1) = 350 - 70 =280g
Assuming we're at room temperature, we can determine the volume of water by using its known density of rho = 1g/(cm^3)
V_(water) = m_(added)/rho = (280g)/(1 g/(cm^3)) = 280 cm^3
