Hydrogen peroxide (H_2O_2), is sold commercially as a 3.0% (mass/volume) aqueous solution. If you buy a 500 ml bottle of hydrogen peroxide, how many grams of H_2O_2 are dissolved in the bottle?

3% mass/volume means that there are $3$ $g$ of solute in every $100$ $m L$ of solution. That means there will be $3 \times \frac{500}{100}$ or $15$ $g$ of ${H}_{2} {O}_{2}$ in a $500$ $m L$ bottle.