\textbf{221.} With 4 milligrams of sodium hydroxide, approximately how many grams of solution of 50 ppm can be prepared, and this solution contains how many moles of sodium hydrogen sulfate?
$$(\mathrm{H=1,\ O=16,\ Na=23\,:\,g.mol^{-1}})$$
(1) $10^{-3}$, $\Delta.5$ \quad (2) $10^{-4}$, $\Delta.5$ \quad (3) $10^{-3}$, $8.0$ \quad (4) $10^{-4}$, $8.0$
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