# The mass of the ocean is about 1.8 x 10^21 kg. If the ocean contains 1.076% by mass sodium ions, Na^+, what is the mass in kilograms of Na^+ in the ocean?

There are 1.8xx10^21*kgxx1.076% $k g$ of sodium ions.
Because 1.076% $\cong$ ${10}^{-} 2$, there are thus at least a $1.8 \times {10}^{21} \cdot k g \times {10}^{-} 2$ mass of sodium ions, i.e. $=$ $1.8 \times {10}^{19} \cdot k g$, which is not an inconsiderable quantity.