You must add 3.1 g of urea.

We use the boiling point elevation expression

##T_b = iK_bm##

where

##T_b## = change in boiling point; ##i## is the vant Hoff factor; ##K_b## is the molal boiling point elevation constant for the ; and ##m## is the of the solution.

Step 1. Determine the ##i## value.

Since urea is a nonionic compound ##i## = 1.

Step 2. Calculate the molality of the solution.

##T_b =T_b T_b^o## = (79.6 – 76.5) C = 3.1 C

##T_b = iK_bm##

##m = (T_b)/(iK_b) = (3.1 C)/(1 1.22 Ckgmol)## = 2.5 molkg

Step 3: Calculate the moles of the urea.

##m = moles of urea/kilograms of ethanol##

##n## = moles of urea = ##m## kilograms of ethanol = 2.5 molkg 0.0200 kg = 0.051 mol.

Step 4. Calculate the mass of urea.

The formula of urea is NHCONH = CHNO. Molar mass = 60.06 g/mol

Molar mass = ##mass/moles##

Mass = moles molar mass = 0.051 mol ##(60.06 g)/(1 mol)## = 3.1 g