Question

A student on a particular diet expends 10,000 kJ/day while excreting 40 g of urea. Assuming that protein is 16% N by weight and that its metabolism yields 18 kJ/g, what percentage of the student

Answer 1

Urea [NH₂ - CO- NH₂] has a molar mass of 60 g/mole. Out of every 60 grams, Nitrogen resent would be 28 g [2 nitrogen atoms * 14g = 28]

40 g urea * 28 g nitrogen = 18.67 g of nitrogen

60 g urea

1gram of nitrogen is roughly equivalent to 6.25 grams of protein.

So 18.67 g of Nitrogen * 6.25 g of protein = 116.67 g of protein

1 g of nitrogen

Mtabolism of 1 g of protein yield -----------> 18 KJ of energy [given info]

116.67 g protein * 18 KJ energy = 2100 KJ energy

1 g of protein

So the protein he consumed provides 2100KJ out of the total energy he expends on a day of 10,000 KJ

Percentage of energy provided by protein = 2100 * 100

10,000

= 21% [answer]