PREPARATIONS 123 water-condenser by means of a bent delivery (or "knee") tube, and distil off about 10-15 ml. of the liquid. Confirm the presence of acetic acid in this distillate by the tests given on p. 347. Urea (or Carbamide). CO(NH 2 ) 2 . Urea (the diamide of carbonic acid) can be prepared by the historic method of Wohler. When an aqueous solution of ammonium cyanate is allowed to stand, the cyanate undergoes molecular rearrangement to urea, and an equilibrium mixture containing about 93% of urea is thus formed. Urea is NH 4 CNO ^ NH 2 CONH 2 less soluble in water than ammonium cyanate; therefore if the solution is evaporated, urea begins to separate, and consequently the ammonium cyanate undergoes steady conversion to urea in order to maintain the above equilibrium. Ultimately, if the solution is evaporated to dryncss, the conversion to urea becomes complete. Ammonium cyanate, because of its instability in solution, is usually prepared (NH 4 ) 2 SO 4 + 2KCNO = 2NH 4 CNO + K 2 SO
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