25.0 cm$^3$ of 0.1 mol dm$^{-3}$ NaOH was exactly neutralised by 20.0 cm$^3$ of hydrochloric acid. Calculate the concentration of the acid.
A0.25 mol dm$^{-3}$
B0.125 mol dm$^{-3}$CORRECT
C0.1 mol dm$^{-3}$
D0.08 mol dm$^{-3}$
JUPEBChemistryStoichiometry and Volumetric Analysis
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Why the answer is B, and why the others tempt you.
NaOH + HCl react 1:1, so $C_A V_A = C_B V_B$: $C_A = \dfrac{0.1 \times 25}{20} = 0.125$ mol dm$^{-3}$. Inverting the volume ratio gives 0.08.
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