Stoichiometry Flashcards
What is an excess reagent?
The reagent that is present in larger quantities than necessary
What is a limiting reagent?
The reactant that determines how much product can be formed
What is predicted or theoretical yield?
The expected amount of product
What is experimental or actual yields?
The quantity of the product actually obtained
How likely is it for the predicted and experimental yields to be the same?
Extremely rare
How do you do calculations for limited and excess reagents?
- Calculate the moles of each reactant
- Divide the moles of that reactant by its coefficient
- The lower number is the limiting reagent
- Use this as moles given
What factors affect experimental yields?
- Competing reactions
- Reaction is very slow
- Collection and transfer methods
- Reactant or product purity
- Reaction doesn’t go to completion
What are competing reactions?
When two chemicals react to give different products
What is an ideal percent yield?
As close to 100% as possible
How is percent yield calculated?
%yield = (actual yield/predicted yield) x 100
When the percent error is closer to zero, is it better or worse?
Better
How do you calculate percent error?
%error=(actual-predicted)/predicted x 100
What is a titration?
A procedure used to find the volume of substances so you can calculate the concentration
What is standardization?
When a solution of known concentration, a standard solution, is reacted with a solution of unknown concentration
What types of chemicals need to be standardized and why?
Strong acids and strong bases because their concentrations will change over time
What is a titrant?
A solution that is transferred from a precisely marked tube called a burette to a flask containing the sample and an indicator
Why is an indicator used?
Because a sudden change in color indicates the completion of the reaction
What is the endpoint?
The point where the titrant reacts completely with the sample
What is the equivalence point?
The volume needed to reach the endpoint
What is a pH titration curve?
A plot of the pH versus the volume of titrant added
How are titration curves shaped?
They are S-shaped
What happens when a strong monoprotic acid is titrated with a strong monoprotic base?
The equivalence point will always have a pH of seven at 25°C
What does the first point on the curve indicate?
The pH of the sample
What is the buffer region
The pH changes very gradually at first
What happens to the rate of change as the point is approached?
The pH changes very rapidly
What is stoichiometry?
The study of the relative quantities of reactants and products in a chemical reaction
How do you find the moles of a substance when given the moles of another substance in the chemical reaction
NW = NG x W/G
What does gravimetric mean?
Mass measurements
How do you find moles using mass and molar mass?
n = m/M
When can you use the law of combining volumes?
When the pressure and temperature conditions are constant for both the reactants and products
What is the ideal gas law?
PV = nRT
