How to calculate n-percent solution given impure chemical? I couldn't find a Stack Exchange site for computational chemistry, so hope this is apropos for this forum. I can get $80\%$ pure sodium chlorite and I want to make a $28\%$ solution of it. Though I'm getting some conflicting information on the web, this site seems to indicate that $28\,\rm g$ of _pure_ $\mathrm{NaClO_2}$ to $72\,\rm g$ of water would give me what I want, and since a gram of pure water is $1\,\rm mL$ at STP, I could measure the water as volume rather than weight. But, since I have $80\%$ $\mathrm{NaClO_2}$ powder, does that mean I can just multiply the amount by $1/.8 = 1.25$, adding $35\,\rm g$ of powder to $72\,\rm mL$ of water, to get a $28\%$ nominal solution?

I'm assuming your sodium chlorite is $80\%$ pure by mass, and you want your solution to be $28\%$ pure by mass as well.

Let's say you have $1\,\rm g$ of powder and let's calculate how much water you need to add to make a $28\%$ solution (you can then proportionally scale up both amounts.)

Let $y$ be the number of grams of water added. Then the total mass is $1+y$. The mass of $\mathrm{NaClO}_2$ stays constant at $.8$. So, we want to solve $$\frac{0.8}{1+y} = 0.28$$ which has solution $y \approx 1.86\,\rm g$.

So if you have e.g. $35\,\rm g$ of powder, you will need to add about $65.1\,\rm mL$ of water to make a $28\%$ solution.