Question

Caffeine (C₈H₁₀N₄O₂) is a stimulant found in coffee and tea. A solution of caffeine in the solvent chloroform (CHCl₃) has a concentration of 0.0490 m.a. Calculate the percentage of caffeine by mass in the solution.b. Calculate the mole fraction of caffeine in the solution.

Answer 1

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) = 0.00952 kg caffeine

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 = 194.22g/mol)

Percentage by mass is:

0.00952 kg Caffeine / kg of solution ×100 = 0.952%

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) = 8.30 moles of chloroform

Molar mass of CHCl₃ 12,01g/mol×1 + 1.01×1 + 35.45g/mol×3 = 119.37g/mol

Thus, mole fraction of caffeine is:

0.0490 mol Caffeine / 8.30 moles chloroform + 0.0490 moles of caffeine = 0.00587

I hope it helps!