Question

There is an order of 400ml of a 0.5% solution that needs to be prepared. In stock you have 16 oz. bottle of a 0.75% and 16 oz. bottle of 0.35%. How much do you need to mix from each to fill order? I know the answer is 150ml of .75% and 250ml of .35% but don't know how to get it.

Answer 1

Work in terms of the amount of the substance in the solution. If you need 400 ml of 0.5% solution, that means you need 400 ml * 0.5% * 1/100% = 2 ml of substance in the final mixture. If you use \(a\) to represent the number of ml of the 0.75% solution, and \(b\) to represent the number of ml of the 0.35% solution, the amount of substance contributed by the 0.75% solution will be 0.0075a and the amount of substance contributed by the 0.35% solution will be 0.0035b. The two of them added together will equal 2 ml. You also know that \(a+b = 400 \text{ ml}\). That gives you two equations in two unknowns which you can solve via substitution or elimination. Finally, you'll need to convert your amounts from ml to oz.