To make an aqueous solution of HBr (hydrobromic acid) of concentration 15.0 M from 3.5 M, we can use the dilution ratio: C₁V₁ = C₂V₂.
We can input our data into the above formula, and thus:
15.0×V₁ = 3.5×1 (final volume is a litre), and solving this to get V₁:
V₁ = 0.2333...
therefore, the initial volume, the volume of HBr we start with, is 233.33 mL.
Dilution
1. accurately measure 233 mL of 15.0 M hydrobromic acid into a measuring cylinder, using a pasteur pipette (dropper), to top it up to the mark.
2. pour the hydrobromic acid into a 1 L volumetric flask, using a wash bottle and demineralised water to rinse out the remaining acid from the measuring cylinder into the flask.
3. Using a wash bottle and demineralised water, fill the volumetric flask up to a little bit below the mark on the neck of the flask. Use a dropper to carefully add water to the mark, making sure that the bottom of the meniscus of the solution is on the mark.
4. Stopper the flask, and holding the flask in one hand whilst pressing down the stopper, invert and swirl, and invert again, about 10-15 times. This ensures solution is thoroughly mixed.
5. Your 3.5 M solution of HBr is ready