Fundamental Concepts of Chemistry: MDCAT Notes and MCQs
Mole concept
- 1 mole = 6.022 × 1023 particles (Avogadro's number, NA) = gram atomic/molecular/formula mass.
- Moles = mass / molar mass = particles / NA = volume of gas at STP / 22.414 dm3.
- 1 mole of H2O = 18 g = 6.022 × 1023 molecules = 3 × 6.022 × 1023 atoms.
Stoichiometry
Coefficients of a balanced equation give mole ratios. E.g. 2H2 + O2 → 2H2O: 2 mol H2 ≡ 1 mol O2 ≡ 2 mol H2O. Steps: mass → moles → use mole ratio → moles of product → mass/volume.
Limiting reactant
- The reactant consumed first; it controls the maximum amount of product. The other is in excess.
- Method: convert each reactant to moles, divide by its coefficient — the smallest value is limiting (or calculate product from each; the smaller amount wins).
Yield
% yield = (actual yield / theoretical yield) × 100
Theoretical yield is calculated from the limiting reactant; actual yield is less due to side reactions, reversible reactions, loss during handling.
PMDC learning outcomes for this unit
- Moles and Avogadro's Number: 1.1 Construct mole ratios from balanced equations for use as conversion factors in stoichiometric problems.
- Moles and Avogadro's Number: 1.2 Perform stoichiometric calculations with balanced equations using moles, representative particles, masses and volumes of gases (at STP).
- Limiting and Excess Reactants: 1.3 Explain the limiting reagent in reaction
- Limiting and Excess Reactants: 1.4 Calculate the maximum number of products produced and the amount of any un-reacted excess reagent
- Yield: 1.5 Given information from which any two of the following may be determined, calculate the third: theoretical yield, actual yield, percentage yield.
- Yield: 1.6 Calculate the theoretical yield and the percent yield when given the balanced equation, the amount of reactants and the actual yield.