Question

According to the reaction $3H_2+N_2\to 2N{H}_3$, how many grams of hydrogen gas and nitrogen gas are needed to make exactly 68 grams of ammonia?

• A. 2 grams of hydrogen gas and
  28 grams of nitrogen gas
• B. 3 grams of hydrogen gas and
  1 gram of nitrogen gas
• C. 12 grams of hydrogen gas
  and 56 grams of nitrogen gas
• D. 102 grams of hydrogen gas
  and 34 grams of nitrogen gas
• E. 6 grams of hydrogen gas and
  2 grams of nitrogen gas

Answer 1

The correct option is C 12 grams of hydrogen gas

and 56 grams of nitrogen gas
The molar mass of ammonia is 17 g/mol.
68 grams of ammonia corresponds to $\frac{68g}{17g/mol}=4moles$
4 moles of ammonia will be obtained from $\frac{4\times 1}{2}=2$ moles of nitrogen and $\frac{4\times 3}{2}=6$ moles of hydrogen.
The molar masses of nitrogen and hydrogen are 28 g/mol and 2 g/mol respectively.
2 moles of nitrogen corresponds to $2\times 28=56$ grams.
6 moles of hydrogen corresponds to $6\times 2=12$ grams.