Question #e2397

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Answer 1 · 2014-08-10T17:42:19

The mass of urea is 26.1 g.

In your problem, we have to work backwards from the freezing point to calculate the mass of solute.

Step 1. Calculate $T_{f}$ $T_"f"$

$Δ T_{f} = T_{f} ° - T_{f}$ $ΔT_"f" = T_"f"° - T_"f"$ = 0.00 °C – (-2.38) °C = 2.38°C

Step 2. Calculate $i$ $i$ .

Since urea is a nonelectrolyte, $i$ $i$ = 1.

Step 3. Calculate the molality

$Δ T_{f} = i K_{f} m$ $ΔT_"f" = iK_"f"m$

$m = (ΔT_"f")/(iK_"f") = (2.38"°C")/( 1 × 1.86"°C·kg·mol⁻¹")$ = 1.28 mol·kg⁻¹

Step 4. Calculate the moles of urea

$m = "moles of urea"/"kilograms of water"$

Moles of urea = $m$ × kilograms of water = 1.28 mol·kg⁻¹ × 0.340 kg = 0.435 mol

Step 5. Calculate the mass of urea

The formula of urea is NH₂CONH₂. The molar mass is 60.06 g·mol⁻¹.

Moles = $"mass"/"molar mass"$

Mass = moles × molar mass = 0.435 mol × 60.06 g·mol⁻¹ = 26.1 g

The mass of urea is 26.1 g.