WEBVTT

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Here is the problem: How much 5 M stock solution do you need to make 400 mL of 1.25 M solution?

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Diluting adds solvent but not solute, so the moles of solute before and after are equal.

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Moles of solute stay the same when you dilute.

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Rearrange for initial volume V₁.

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Substitute.

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Solvent to add. Start with V₁ of the stock and add solvent up to V₂.

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So the answer is initial volume V₁ equals 100 mL; add 300 mL of solvent.

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Any volume unit works in M₁V₁ equals M₂V₂ as long as both volumes use the same one. Moles of solute: 0.5 mol.

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M₁V₁ equals 500 and M₂V₂ equals 500, which checks out.