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Some Basic Concepts of Chemistry question

2021 · 16 Mar · Shift 1 · Q19
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Some Basic Concepts of Chemistry question

2021 · 16 Mar · Shift 1 · Q19

JEE MainChemistrySome Basic Concepts of ChemistryNumerical+4 / −1
A 6.50 molal solution of KOH (aq.) has a density of 1.89 g cm −-− 3. The molarity of the solution is ‾\underline{\hspace{2cm}}​ mol dm −-− 3. (Round off to the Nearest Integer). [Atomic masses : K : 39.0 u; O : 16.0 u; H : 1.0 u]
Numerical answer
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Correct answer: 9

  1. Given data

    • Molality of KOH solution: m=6.50 mol kg−1m = 6.50\ \text{mol kg}^{-1}m=6.50 mol kg−1
    • Density of solution: ρ=1.89 g cm−3\rho = 1.89\ \text{g cm}^{-3}ρ=1.89 g cm−3
    • Molar mass of KOH: MKOH=39+16+1=56 g mol−1M_{\text{KOH}} = 39 + 16 + 1 = 56\ \text{g mol}^{-1}MKOH​=39+16+1=56 g mol−1
  2. Take 1 kg of solvent Since molality is defined as moles of solute per kg of solvent, for 1 kg1\ \text{kg}1 kg water: moles of KOH=6.50\text{moles of KOH} = 6.50moles of KOH=6.50

  3. Mass of solute mass of KOH=6.50×56=364 g\text{mass of KOH} = 6.50 \times 56 = 364\ \text{g}mass of KOH=6.50×56=364 g

  4. Total mass of solution mass of solution=1000+364=1364 g\text{mass of solution} = 1000 + 364 = 1364\ \text{g}mass of solution=1000+364=1364 g

  5. Volume of solution using density Volume=massdensity=13641.89=721.69 cm3\text{Volume} = \frac{\text{mass}}{\text{density}} = \frac{1364}{1.89} = 721.69\ \text{cm}^3Volume=densitymass​=1.891364​=721.69 cm3 Convert to dm3^33 (or L): 721.69 cm3=0.72169 dm3721.69\ \text{cm}^3 = 0.72169\ \text{dm}^3721.69 cm3=0.72169 dm3

  6. Calculate molarity Molarity=moles of solutevolume of solution in dm3\text{Molarity} = \frac{\text{moles of solute}}{\text{volume of solution in dm}^3}Molarity=volume of solution in dm3moles of solute​ M=6.500.72169≈9.01 mol dm−3M = \frac{6.50}{0.72169} \approx 9.01\ \text{mol dm}^{-3}M=0.721696.50​≈9.01 mol dm−3

  7. Nearest integer 9\boxed{9}9​

  8. Comparison with stored answer Stored correct answer = 999

    Our derived answer also is 999, so they agree.

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