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A laboratory needs 250 mL of 2.00 M HCl. How many millilitres of 11.9 M HCl must be diluted to prepare it?
A420 mL
B42.0 mL
C4.20 mL
D1490 mL
Answer & Solution
Correct answer: B. 42.0 mL
1. Dilution keeps the amount of solute fixed, so C1V1 = C2V2.
2. Solve for the stock volume: V1 = C2V2 / C1 = 2.00 M x 250 mL / 11.9 M.
3. The result is 42.0 mL of the concentrated acid.
4. Sanity check: the stock is about six times stronger, so about a sixth of 250 mL is needed.
5. The value 1490 mL swaps the two concentrations in the formula, which would be a further dilution, not a preparation.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.3 Molarity_
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