Stoichiometry basics

mole conversionsmole mapgrams to molesstoichiometry formulaspercent yield

The conversions every mass-to-mass calculation runs through — moles from grams, particles from moles, gas volume, composition and yield.

Moles from Mass (n = m/M)

n = \frac{m}{M}

Converts a measured mass into an amount of substance by dividing by the molar mass.

Particles from Moles (Avogadro's Number)

N = n\,N_A

Converts an amount in moles into an actual particle count using the Avogadro constant NA = 6.02214076 × 10²³ per mole.

Gas Volume at STP

V = n\,V_m

Converts between moles of an ideal gas and its volume at STP using the molar volume Vm = 22.414 L/mol.

Percent Composition of an Element

\%X = \frac{a\,M_X}{M_{\text{compound}}} \times 100\%

Gives the mass percent an element contributes to a compound from the formula subscript and molar masses.

Percent Yield

\%\,\text{yield} = \frac{m_{\text{actual}}}{m_{\text{theoretical}}} \times 100\%

Compares the mass actually isolated from a reaction to the maximum mass stoichiometry predicts.

How they fit together

Every one of these is a bridge to or from the mole, and that is the point: you cannot convert grams of one substance into grams of another directly, because the balanced equation counts particles, not mass. Molar mass gets you into moles, the coefficients move you across the equation, and molar mass gets you back out. Avogadro's number handles counts and the 22.4 L molar volume handles gases at STP, but they are the same bridge in different currency.

Pick by what you are given and what you want, and always convert to moles first — the mistake that ruins more stoichiometry problems than any other is applying the balanced coefficients to grams. Doubling the coefficient doubles the moles, not the mass, unless the two substances happen to share a molar mass. Percent yield is the separate question asked at the end: everything above predicts the theoretical yield, and only a measurement tells you what the reaction actually gave up.