Answer:
a. 0.952%
b. Mole fraction 0.00587.
Explanation:
In chemistry, concentration is defined as an amount of solute per amount of solution.
In the problem, caffeine is acting as solute and chloroform is the solvent.
0.0490m is 0.0490 mol of solute / kg of solution.
a. To obtain percentage by mass it is necessary to convert moles of caffeine in kg of solute, thus:
0.0490 mol Caffeine × (194.22g / mol) × (1kg / 1000g) = <em>0.00952 kg caffeine</em>
194.22 g/mol is molar mass of Caffeine C₈H₁₀N₄O₂ (12,01g/mol×8 + 1.01g/mol×10 + 14.01g/mol×4 + 16.00g/mol×2 = <em>194.22g/mol</em>)
Percentage by mass is:
0.00952 kg Caffeine / kg of solution ×100 = <em>0.952%</em>
b. To obtain mole fraction it is necessary to find moles of chloroform, thus:
1 kg solution = Xkg chloroform + 0.00952 kg caffeine
0.99048 kg of chloroform × (1000g / 1kg) × (1mol / 119.37g) = <em>8.30 moles of chloroform</em>
Molar mass of CHCl₃ 12,01g/mol×1 + 1.01×1 + 35.45g/mol×3 = <em>119.37g/mol</em>
Thus, mole fraction of caffeine is:
0.0490 mol Caffeine / 8.30 moles chloroform + 0.0490 moles of caffeine = <em>0.00587</em>
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I hope it helps!
