Need help on a percent yield question? ?

Question

i know the answer is 57.7%, but i need to know how to get it please.

*determine the percent yield for the reaction between 15.0g H2 and 15g NH3 is produced.

the chemical equation is N2 + 3H2 ====> 2NH3

i appreciate the help =]

Answer 1

when you calculate % yield, the calculation should look like:

(x g experimental product/ y g theoretical product) x 100% = % yield

where "x g experimental product" = how many g NH3 you actually got in your reaction

and

"y g theoretical product" = how many g NH3 you SHOULD've gotten

so for the given chemical reaction, you need to first figure out how many g of NH3 will be formed. you do this by determining the limiting reagent as follows:

conversions: g H2 --> mol H2 --> mol NH3 --> g NH3

15 g H2 x (1molH2 / 2gH2) x (2molNH3 / 3molH2) x (17gNH3 / 1 mol NH3)   =  85 g NH3

so if you have 15 g H2, you'd predict that you would get 85 g NH3.

do the same for N2

conversions: g N2 --> mol N2 --> mol NH3 --> g NH3

15 g N2 x (1molN2 / 28gN2) x (3molNH3 / 1molN2) x (17gNH3 / 1molNH3)  = 18.21 g NH3

therefore, the limiting reagent is N2, since 15g of N2 will theoretically produce 18.21 g NH3.

in the equation for % yield, now you have:

(x g NH3 / 18.21 g NH3) x 100% = % yield

if you know the answer is 57.7%, then your value of x (how much of NH3 was ACTUALLY formed in the reaction) must've been about 10.5 g NH3.