Urea, chemical formula H2N-CO-NH2, is a metabolite or waste product eliminated by the kidneys. It is a colourless solid and an important source of nitrogen in fertilisers. Although it may be applied to the ground as a solid, it is often applied as a water-based solution of specific concentration. Making such a solution is not difficult to accomplish if one has the minimal equipment needed and is familiar with the concept of molecular weight. There are two methods of identifying urea concentration in solution: percentage by weight---whether of urea or "as nitrogen"---and molarity.
Look up the atomic weights of the elements in urea and calculate its molecular weight. Doing so gives hydrogen, 1; nitrogen, 14; carbon, 12; and oxygen, 16. Since there are four hydrogen atoms, two nitrogen atoms, one carbon atom and one oxygen atom, the molecular weight of urea calculates as: (4 x 1) + (2 x 14) + 12 + 16 = 60.
Calculate the percentage of nitrogen in urea and look up the definition of molarity. Of urea's 60 molecular weight, 28 is nitrogen, and the percentage of nitrogen in urea is calculated as: (28 / 60) x 100 per cent = 47 per cent.
The definition of molarity is, according to Princeton's Wordnet Search: "concentration measured by the number of moles of solute per litre of solution."
The word "mole" is short for a molecular weight in grams of a substance. For urea, this is 60g per litre of solution.
Make the solution in one of three ways (examples provided below):
Per cent-by-weight: if, for example, a 4 per cent solution is desired, place the beaker on the scale and using the scoop, weigh 40g of urea and 960g of water. Stir them together until thoroughly dissolved and uniform, and the job is complete.
By molarity: if a one-quarter molar solution is desired (0.250 molar), weigh into the beaker 15 grams urea (one-fourth of one molecular weight of urea) and, while stirring, add water until reaching the 1-liter mark of the beaker.
Per cent-by-weight nitrogen: if a 3 per cent-by-weight solution, by weight as nitrogen is desired, first calculate what the percentage would be as per cent urea: 3 weight per cent nitrogen x (60 / 28) = 6.5 weight per cent urea.
Thus add 65g urea to 935g of water, using the methodology in item 1.
Things you need
- Weighing scoop
- Scale or balance
- Graduated beaker (1-liter graduated beaker, for example)
- Stirring rod or stir plate with magnetic stir bar