What is Stoichiometry?
In chemical equations, unless stated otherwise, the reactants and products will theoretically always remain in stoichiometric ratios.
The stoichiometry of a reaction is the relationship between the relative quantities of products and reactants, typically a ratio of whole integers.
Consider the following chemical reaction: aA + bB ⇒ cC + dD.
The stoichiometry of reactants to products in this reaction is the ratio of the coefficients of each species: a : b : c : d.
Now let's apply this knowledge to the question to be attempted:
first, we can start by writing out a balanced chemical equation, with states.
NH₃(g) + HCl(g) ⇒ NH₄Cl(s). This is an example of an acid/base decomposition reaction.
Hence, the stoichiometry of this reaction is 1 : 1 : 1
Now we can calculate the number of moles of each reactant that we have, by dividing the mass present (g; symbol = m), by the molar mass (g/m; symbol = M).
- n(NH₃) = m/M = 19.049/(14.01+3×1.008) = 1.118 mol
- n(HCl) = m/M = 31.109/(1.008+35.45) = 0.8533 mol
Next, we need to determine if, in the reaction, the substances ARE present in stoichiometric ratios. If they are not, then we need to identify the limiting reagent (the reactant which reacts completely), and the excess reagent (the reactant which is not completely used up). We can do this by inputting the mole values in the question into the ratios, until we figure out which doesn't match up.
Limiting reagent = HCl; Excess reagent = NH₃
[note: in this situation, the stoichiometry is 1 : 1, so it's very easy to determine the limiting reagent)
(i) which of the reactants was in excess and by how much
NH₃ is in excess of 0.2647 mol
(ii) how much ammonium chloride was formed?
Now we use the limiting reagent, HCl, to calculate moles of NH₄Cl formed, using our stoichiometric ratios.
Moles of NH₄Cl = moles of HCl = 0.8533 mol
(iii) how much more of the insufficient reactant would be needed to completely react with the excess of the other reactant?
Since we use our stoichiometric ratios, if NH₃ is limiting reagent, then we require 0.2647 extra mol of HCl to react.