Question

Algae can be represented as C106H263O110N16P. Calculate the mass ratio of N/P in algae. If the concentration of the total nitrogen in a lake is 12mg/L and the concentration of total phosphorus is 2mg/L, calculate the mass ratio of N/P in the lake. Is this body of water limited by phosphorus or nitrogen?

Answer 1

Calculate number of moles of N and P present in 1 mole of algae ( C106 H263 O110 N16 P) , & then

multiply with their atomic masses to get the mass of N and P separately,

..................Atomic mass..................Number of moles ...................mass ( = moles x atomic mass )

N.......................14....................................16.....................................224

P.........................31.....................................1......................................31

therefore , Mass ratio of N / P in algae = 224 /31

......................................................................= 7.23 : 1

Calculation of mass ratio in lake water ;

..............Element .......................concentration (mgm / L )

.....................N........................................12

.....................P........................................2

therefore , mass ratio = 12 / 2

...................................=   6:1

Since P cannot be < 1, the body of water is limited by Nitrogen.