Stoichiometry Basics Notes

Stoichiometry Basics Stoichiometry ●​ Stoichiometry is a calculation of the amount of substances in a chemical reaction from a balanced equation ○​ A balanced chemical equation provides a great deal of Information ○​ Involves the quantitative relationships between the amounts of reactants and products Real World Stoichiometry Problem ●​ Kim looks in the refrigerator and finds she has 8 slices of ham. She wants to make sandwiches that have 2 slices of ham (H), 1 slice of cheese (C), 1 slice of tomato (T) , 5 pickles (P), and 2 slices of bread (B). In order to make as many sandwiches as possible, how many pickles does she need? Stoichiometry Problem Basics 1.​ Stoichiometry problems can be characterized by 2 things: ○​ The info given in a problem (known) ○​ The info that is to be solved for (unknown) 2.​ The amounts of substances can be expressed in moles or mass (grams) ●​ The amount of a gas may be expressed by its volume ●​ We will focus on problems where they are expressed in moles Coefficients ●​ The coefficients of a balanced equation can represent the number of molecules or moles of each substance ●​ The most useful quantity for counting particles is the mole. ○​ If each coefficient in the example is multiplied by a mole, 1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia ○​ Converting moles to grams using molar mass will show that the law of conservation of mass is followed ■​ Mass an # of atoms must be conserved, but the # of molecules is not necessarily conserved Mole Ratio ●​ Mole ratio is a conversion factor that relates the amount of moles of any 2 substances in a reaction ○​ Numbers for this conversion factor come from the coefficients in the balanced equation ●​ In a mole ratio problem, the known substance is written first (expressed in moles). ●​ The appropriate conversion factor will convert from moles of the known to moles of the unknown Mole Ratio Example Mass-Mole Stoichiometry ●​ While mole ratios are present in all stoichiometry calculations, amounts of substances in reality are measured using mass ○​ Need to use mole-mass calculations in combination with mole ratios to solve these problems ○​ Mass of known → moles of known → moles of unknown ○​ Conversions use first molar mass to go from grams to moles, then the mole ratio from the balanced equation Mass-Mole Stoichiometry Problem ●​ Mass-mass calculations are the most practical but require three steps (combination of the 2 previous types) ●​ Mass of known → moles of known → moles of unknown→ mass of unknown ●​ Mass of the known is converted to moles. Then the molar ratio is used as a conversion factor. Finally the moles of the unknown are converted to mass Volume-Volume Stoichiometry ●​ As a reminder, at STP, 1 mole of any gas occupies a volume of 22.4 L ●​ The mole ratios between substances in a gas phase reaction are also volume ratios ●​ Volume ratios can easily be used when the volume of one gas is known and you need to determine the volume of the second gas as long as STP is maintained Mass-Volume Stoichiometry: ●​ Chemical reactions can involve solids as well as gases in the same reaction. ○​ Mass of known → moles of known → moles of unknown → volume of unknown ○​ Volume of known → moles of known → moles of unknown→ mass of unknown ●​ Both types involve a conversion from moles to volume or vice versa using the molar volume of 22.4 L/mol at STP