How would you prepare 500 ml of a 0.2% solution of the detergent tween-20? this reagent is a liquid in its pure state.

At a 500 mL volumetric ballon, it will be added a little of distilled water, then 1 mL of the pure detergent, and then it will be added water until the meniscus. The solution will be agitated by inverting the flask until it's homogeneous.

Explanation:

In its pure state, the detergent has a 100% concentration. Thus, knowing that the product of the concentration by volume must be constant, we can do:

C1V1 = C2V2

Where C is the concentration, V is the volume, 1 is the pure detergent, and 2 the solution. So, let's find the volume of pure detergent needed to prepare 500 mL of the solution:

100*V1 = 0.2*500

100V1 = 100

V1 = 1 mL

Thus, at a 500 mL volumetric ballon, it will be added a little of distilled water, then 1 mL of the pure detergent, and then it will be added water until the meniscus. The solution will be agitated by inverting the flask until it's homogeneous.

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i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)