MCAT Chemistry


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 Molecules

Molecular Weight

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

Formula Weight

See Formula Weight

Formula Weight (ionic compounds) is the sum of the atomic weights of constituents


See Compounds


See Moles

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

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

Chemical Reaction Types

See Chemical Reaction Types


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

Balancing Equations

Balancing Equations

Limiting Reactants limit the amount of product that can be formed. Opposite = excess

Stoichiometry - Limiting Reactange

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

MCAT Chemistry

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