What mass of sodium hydroxide is needed to prepare 250 cm$^3$ of a 0.2 mol dm$^{-3}$ solution? (NaOH = 40)
A8 g
B2 gCORRECT
C0.05 g
D20 g
JUPEBChemistryStoichiometry and Volumetric Analysis
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Why the answer is B, and why the others tempt you.
Moles $= 0.2 \times \dfrac{250}{1000} = 0.05$ mol, so mass $= 0.05 \times 40 = 2$ g. Leaving the volume in cm$^3$ or forgetting to multiply by the molar mass gives the wrong values.
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