We had a question like this on a previous test but I've forgotten how to do it

Question

How much pure HCL acid must be added to 2 liters of a solution that is 15% HCL acid to make it 20% HCL acid?

Answer 1

Since "pure HCl" is a gas, I assume we can neglect the volume it would add to the system. I guess, we can also assume the density of our solution to be 1 g/mL.

Now, 15% HCl solution means that for every 100g of your solution you have 15g of HCl, which would correspond to 300g of HCl in 2L (2000g) of your current solution. 

We need to have 20% HCl, which would mean we need 400g of HCl in the solution. Since we already have 300g, we just need to add the difference, which is 100g, which would be a rough estimation that doesn't take into consideration the added mass of the solution due to adding HCl to it.

If we need it to be more precise (taking into consideration the added mass) then we would need to solve the following equation:

 

(300+X)/(2000+X) = 0.2, from where X = 125 g.