In the United States and Canada, the labeling scheme of fertilizer presents thre

Question

In the United States and Canada, the labeling scheme of fertilizer presents three numbers separated by dashes (e.g. 0-0-48 or 16-4-8); the first number represents the mass percentage of Nitrogen in the product; the second number, Phosphorus; and the third, Potassium. The generalized form is N-P-K. A 50-pound bag of fertilizer labeled 16-4-8 contains 8 pounds of nitrogen (16% of the 50 pounds), 2 pounds of phosphorus (4% of 50 pounds), and 4 pounds of potassium (8% of 50 pounds). In the case of pure chemical compounds, which are often used as fertilizer, the N-P-K rating can be predicted from the formula. For the following pure compounds which are commonly used as fertilizer, predict and write their N-P-K number. Show your work. Ammonia (NH_3) Potassium Nitrate (KNO_3)

Explanation / Answer

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Using the molecular weight of pure chemical compound we can determine its N-P-K number

a) The MW of NH3 is 17 g/mol

We don’t have Phosphorous and Potassium in the compound, so, the N-P-K number is:

For Nitrogen

14/17 = 0.823 = 82%

N-P-K for NH3 --> 82-0-0

b) The MW of KNO3 is 101 g/mol

We don’t have Phosphorous in the compound, thus the N-P-K number is:

For Nitrogen

14/101 = 0.138 = 14 % (where 14 is the molar mass of Nitrogen presented in the compound)

For Potassium

39/101 = 0.386 = 39 % (where 39 is the molar mass of Potassium presented in the compound)

N-P-K for KNO3 --> 14-0-39

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