Water to add to dilute 0.7% saline to 0.07%

There are 50 grams of saline solution with a concentration of 0.7%. How many grams of water must be added to dilute it to a concentration of 0.07%?

Step-by-step

1. 35 / 0.07 - 50 = 450
   Divide the salt mass by the new concentration to get the total solution, then subtract the original saline

Why it works

The salt is 50 × 0.7% = 0.35 g and does not change; at 0.07% the whole solution must weigh 500 g, so 500 - 50 = 450 g of water is added.

Common mistakes

• Assuming the concentration halves when the same mass of water is added.
• Forgetting that the salt stays at 0.35 g and only the water changes.
• Subtracting the concentrations instead of using the salt mass.