# A 2400-gram sample of an aqueous solution contains 0.012 gram of NH_3. What is the concentration of NH_3 in the solution, expressed as parts per million?

$\text{Parts per million "= " Milligram per litre of solution}$. The concentration of the solution is $5$ $\text{ppm}$.
At $\text{ppm}$ concentrations, we don't have to bother with consideration of the density of the solvent versus the density of the solution; in any case we don't have these details.
There are $12 \times {10}^{-} 3$ $g$ ammonia in a $2400$ $g$ mass of solution. We assume (reasonably) that the volume of the solution is $2.400 \cdot L$.
So $\text{concentration}$ $=$ $\frac{12 \times {10}^{-} 3 g \times {10}^{3} \cdot m g \cdot {g}^{-} 1}{2.400 \cdot L}$ $=$ $\frac{12 \cdot m g}{2.400 \cdot L}$ $=$ $5 \cdot m g \cdot {L}^{-} 1$ $=$ $5$ $\text{ppm}$