A stock solution has a concentration of 0.625 M. A 0.0800 M solution is required. If you use a 50.0 mL volumetric flask for the dilution, what volume
(in mL) needs to be taken from the stock solution?

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Explanation:

After the solution is prepared, the volumetric flask is expected to contain of the solution

The concentrations of the solutions are given in the unit moles-per-liter. Hence, convert the unit of the volume of the volumetric flask to liters:

.

Calculate the number of moles of the solute in the volumetric flask:

.

All these of the solute would have come from the stock solution that was initially added to the flask.

The concentration of the stock solution is . Calculate the volume of the stock solution that would contain of the solute:

.

Convert the unit of the required stock solution to milliliters:

.

nacl is added to the aqueous of what? ? in order to get a proper answer you have to specify the exact composition of the aqueous solution.