Having trouble understanding what to do.

Question

I am trying to figure out the exact steps on how to solve this problem.

Given the following equation:

N₂ (g) + 3 H₂ (g) → 2 NH₃ (g)

How many grams of H₂ are required if 16.2 grams of N₂ reacts? 

Answer 1

Hi Secoya,

The goal to solving this problem is understanding what the numbers (coefficients) of each reactant or product in the reaction mean. In this reaction, if you react 1 molecule of N₂ with 3 molecules of H₂, then you will get 2 molecules of NH₃. You can also use moles instead of molecules because moles refer to a specific number molecules (more specifically, 1 mole is about 6.022 × 10²³ molecules). However, you cannot use mass (measured in grams in this problem) in the place of molecules as mass does not refer to how many molecules there are, but rather how much matter the object takes up. So, 14 grams of N₂ is a different number of moles than 14 grams of H₂.

To solve this problem, we must convert the 16.2 g of N₂ to moles. To do this, we have to use the molar mass. The molar mass of N (Nitrogen) is 14.007 g/mol, so the molar mass of N₂ (2 molecules of nitrogen) is twice this amount, or 28.014 g/mol. To find the number of moles of N₂, you divide mass by molar mass, and we get

16.2 g / (28.014 g/mol) = 0.578 mol of N₂

In this chemical reaction, we need 3 times more H₂ than N₂ for all of the moles of N₂ to react. This means we need 3 * 0.578 mol = 1.73 mol of H₂. To get the mass of H₂, we multiply the number of moles by the molar mass. Like how we found the molar mass of N₂, the molar mass of H₂ is 2 * 1.008 g/mol = 2.016 g/mol, so the mass of H₂ we need is

1.73 mol * 2.016 g/mol = 3.49 grams.

Hope this helps!

Akshat Y.