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

$72.08g{H}_{2}O\times \frac{1mol{H}_{2}O}{18.02g{H}_{2}O}\times \frac{4molCH}{2mol{H}_{2}O}\times \frac{13.02gCH}{1molCH}=104.16gCH$

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.

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

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

NOTE: THIS IS A POOR EXAMPLE OF HOW TO FIND THE MOLECULAR FORMULA OF A COMPOUND. I AM A COLLEGE STUDENT, AND MERELY CHANGED THE NUMBERS SO THAT THE MATH MADE SENSE, BUT THE WAY THE PROBLEM IS INTRODUCED IS RIDICULOUS. SEE YOUR TEACHER FOR A BETTER EXAMPLE, AND PERHAPS ASK THEM HOW THIS PAGE MAY BE EDITED PROPERLY TO GIVE AN ACCURATE EXAMPLE OF HOW YOU WOULD FIND THE MOLECULAR FORMULA OF A COMPOUND ON THE AP EXAM. IF YOU DO NOT KNOW HOW TO DO THE ACTUAL EDITING, PUT ANY SUGGESTIONS ON THE DISCUSSION PAGE FOR THIS ARTICLE AND I WILL DO MY BEST TO FOLLOW THEM. Template:AP Chemistry