15 g of hydrogen reacts with 25 g of nitrogen. how much excess reagent will be left over when the reaction stops?
5.36 g of excess
7.23 g of excess
12.12 g of excess
19.27 g of excess

1) Chemical reaction: 3H2 + N2 -> 2NH3

2) proportions 3 moles H2 ; 1 mol N2 : 2 mol NH3

3) molar masses

H2: 2*1g/mol = 2 g/mol

N2: 2* 14 g/mol = 28 g/mol

NH3: 14g/mol + 3 g/mol = 17 g/mol

4) Conversion of the data in grams to mol

#mol = mass / molar mass

# mol H2 = 15 g / 2 g/mol = 7.5 mol

# mol N2 = 25 g / 28 g/mol = 0.89 mol

5) Compare the ratio of the reagents with the theoretical ratio:

Reagents: 0.89 mol N2 : 7.5 mol H2

Theoretical: 1mol N2 : 3 mol H2

You conclude that the H2 is in excess and need to calculate how many moles react with 0.89 mol of N2.

6) moles of N2 that react:

x mol H2 / 0.89 mol N2 = 3 mol H2 / 1 mol N2 => x = 0.89*3/1 = 2.67 mol H2

7) Calculate the excess as Initial # mol H2 - # mol that reacted = 7.5 mol - 2.67 mol = 4.83 mol H2 in excess

8) Convert mol of H2 to grams of H2

mass = # mol  * molar mass

mass = 4.83 mol * 2 g/mol = 9.66 grams in excess

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