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300 g of a 30% (w/w) NaOH solution is mixed with 500 g of a 40% (w/w) NaOH solution. If the density of the resulting solution is 2.0 g/mL, what is the % (w/v) concentration of NaOH in the final solution?
- 72.5
- 65
- 62.5
- 60
Correct answer: 72.5
Solution
Mass of NaOH from first solution: 0.30 * 300 = 90 g. Mass of NaOH from second solution: 0.40 * 500 = 200 g. Total NaOH = 290 g. Total mass of mixed solution = 300 + 500 = 800 g. Density of final solution = 2.0 g/mL => volume = 800/2.0 = 400 mL. % (w/v) = (mass of solute / volume of solution) * 100 = (290/400) * 100 = 72.5%.
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