Quick questions on Stoichiometry of aqueous reactions: VCE Chemistry Unit 2

where $n$ is moles, $c$ is concentration in $mol\ L^{-1}$, and $V$ is volume in litres. If you have $V$ in millilitres, either convert to litres or use $c$ in $mol\ mL^{-1}$ (rare; convert).

1. Write the balanced equation with state symbols. 2. Calculate the moles of each reactant using $n = cV$ (or $n = m/M$ if a solid is being added to solution).

When two solutions are mixed, the reactants are rarely present in exact stoichiometric proportions. The limiting reagent runs out first; once it is gone, no more product can form. The other reactant is the excess reagent, and some of it remains in solution after the reaction.

The theoretical yield is the mass (or amount) of product predicted by stoichiometry from the limiting reagent, assuming the reaction goes to completion and nothing is lost.

When the product stays in solution, the answer is usually a molar concentration:

$25.0\ mL$ of $0.200\ mol\ L^{-1}\ HCl$ is added to $35.0\ mL$ of $0.100\ mol\ L^{-1}\ NaOH$. Calculate (a) the limiting reagent, (b) the concentration of the species in excess, and (c) the pH of the final solution. Assume volumes are additive.

Coefficient matters. If the reaction needs $2A$ per $1B$, then with $n(A) = 0.10$ and $n(B) = 0.10$, $A$ is limiting because you need $0.20\ mol$ of it for that much $B$.

Always read the coefficients off the balanced equation, not the formulas alone.

Theoretical yield is from the limiting reagent and the balanced equation.