将 0.4 g 氢氧化钠固体溶于水配成 100 mL 溶液，该溶液的物质的量浓度是多少？（NaOH 摩尔质量为 40 g/mol）
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Correct answer
B. 0.1 mol/L
Principle or equation
物质的量浓度 c = n / V，其中 n = m / M，V 为溶液体积（L）。
Why this answer is correct
n(NaOH) = 0.4 g / 40 g/mol = 0.01 mol，V = 100 mL = 0.100 L，c = 0.01 mol / 0.100 L = 0.1 mol/L。
Example
例如，将 1.6 g CuSO4（M=160 g/mol）配成 200 mL 溶液，n=0.01 mol，c=0.05 mol/L。
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