\textbf{221.} The concentration of sodium ions in a sample of seawater is $10600\ \text{ppm}$. If the density of this water sample is $1.05\ \text{g.mL}^{-1}$, what is the approximate molar concentration of sodium ions?
$(Na = 23 : \text{g.mol}^{-1})$
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(1) $0.22$ \hfill (2) $0.36$ \hfill (3) $0.48$ \hfill (4) $0.65$
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