MCAT Chemistry > Stoichiometry
Law of Constant Composition
Law of Constant Composition says that any pure sample will contain the same elements in an identical mass ratio (water is water)
See Molecular Weight
Molecular Weight (g/mol) is the sum of all the atomic weights of all atoms in a molecule, whose units are AMU
See Formula Weight
Formula Weight (ionic compounds) is the sum of the atomic weights of constituents
Moles are the quantity of molecules equal to number of particles found in 12g of carbon-12.
One mole is equal to 6.022 x 1023 particles (Avogadro's number).
Equivalent Weight occurs in acid/base, redox and precipitation reactions. It represents how many moles of the "thing" we need (H+, Hydroxide ions, electrons) that the number of moles of present compound will produce
Gram Equivalent Weight (GEW) = (molar mass)/n
Equivalents = (mass of compound (g))/GEW
For example, one mole of HCl will donate one mole equivalent of H ions. 49g of H2SO4 (98 g/mol) is needed to produce 1 mol equivalents. One equivalent of protons will neutralize one hydroxide (H30-) atom.
Normality is a measure of concentration equivalents per liter. For example, a 1N solution has [H] = 1 mol / liter, a 2N solution has [H] = 2 mol/liter.
Molarity = Normality/n
Net Ionic Equations have spectator ions which do not participate in the reaction
Empirical Formulas give the simplest whole number ratio of elements in a compound
Molecular Formulas give the exact number of atoms of each element in the compound
Percent Composition = (mass of x) / (formula weight of compound) x 100
- 1) Balance all carbon
- 2) Balance all hydrogen
- 3) Balance all oxygen
- 4) Balance the rest of the atoms and charge if present
Limiting Reactants limit the amount of product that can be formed. Opposite = excess
All comparisons must be done in terms of moles
Rate determines the limiting reactant
Yields represent the theoretical amount of a product produced and the experimentally determined amount. The percent yield correlates the two in a ratio.
0 yield = (actual yield)/(theoretical yield) x 100