You have 900mL of a 1:400 (w/v) stock solution. How would you prepare 1 gallon of a1:2000 (w/v) solution?

To prepare 1 gallon of a 1:2000 (w/v) solution from a 1:400 (w/v) stock solution, follow these steps:

1. First, convert the desired final volume to the same units as the stock solution. 1 gallon is approximately 3785 mL.

2. Next, calculate the amount of solute needed for the final solution. A 1:2000 w/v solution means there is 1 gram of solute for every 2000 mL of solution. So, for 3785 mL, you would need 3785/2000 = 1.8925 grams of solute.

3. Now, determine how much of the stock solution contains this amount of solute. In a 1:400 w/v solution, there is 1 gram of solute in 400 mL of solution. So, 1.8925 grams of solute would be in 1.8925 * 400 = 757 mL of the stock solution.

4. Measure out 757 mL of the stock solution. This contains the amount of solute needed for the final solution.

5. Finally, add enough solvent to this to make up the final volume of 3785 mL. This will dilute the solute to the desired concentration of 1:2000 w/v.

Please note that this assumes the volumes are additive, which may not be the case if the solute and solvent are not miscible.