# AP Chemistry/Stoichometry

• Percent composition - the percent mass of an element in a compound. This only requires the empirical formula which is the ratio of elements in a compound. This is different from a molecular formula, which counts the total number of atoms of each element in the compound.
• Limiting reactants - In a reaction, if there is not enough of a specific reactant, it is called the limiting reactant. To find out which substance is the limiting reactant, first find out how many moles of each substance exist at first. Then, divide each by the coefficient of the respective substance in the balanced equation. The substance with the smallest number in the end is the limiting reactant.
• Percent yield - (actual yield / theoretical yield) * 100. If this isn't self explanatory, you shouldn't be taking the AP exam.

## Examples

A certain compound has the empirical formula CH. When 4 moles of it is burned with excess oxygen (combustion), it produces 72.08gH2O. What is the molecular formula of the compound?

Start by writing a balanced equation of the combustion of CH.

4CH + 5O2 → 4CO2+ 2H2O

Now use stoichometry to find the mass of the CH.

72.08gH2O

${\displaystyle 72.08gH_{2}O\times {\frac {1molH_{2}O}{18.02gH_{2}O}}\times {\frac {4molCH}{2molH_{2}O}}\times {\frac {13.02gCH}{1molCH}}={104.16g}CH}$

Remember that four moles were burned, so the mass you found is found in four moles. You can now use this to find the molecular formula.

${\displaystyle {\frac {104.16g}{4molCH}}\times {\frac {1molCH}{13.02gCH}}=2}$

This is number is the number you multiply the subscripts by, and so you find the molecular formula to be C2H2