Stoich Test Flashcards
How to find the formula or molecular mass?
Add up all the individual masses of the elements `
Units for formula/molecular mass
amu
What does S.T.P stand for
Standard temperature and pressure
Mass spectrometer
used to compare the masses of atoms
the amount of deflection by a magnet is measured
the most massive are deflected the smallest amount
what are the amu masses on the periodic table?
averages of all the isotopes
What does the mole techinically equal?
the number of carbon atoms in exactly 12g of carbon
What else can particles be called?
Formula units
molecules (not MOLES)
atoms
Does it matter the element for how many moles there are?
No
No matter what elements there are an equal number of atoms in each mole
Like saying a dozen of something (could be eggs or cookies)
Difference between formula/molecular mass and molar mass?
Molar mass is in g/mol
Molecular mass is in amu
Difference between 1 and 2 step stoich problems and 3 step stoich problems
3 step you need to cross the mole bridge and involve more types of molecules than just one
How would I solve:
how many grams of copper chloride is needed to react with 160 grams of aluminium?
3 step mole problem
cross mole bridge
do NOT do limiting and excess reagents since you are only given 160 grams
How would I solve:
What volume of ammonia at STP is produced when 25 g of nitrogen reacts with hydrogen?
3 step mole problem
cross mole bridge
do NOT do limiting and excess reagents since you were only given 1 number
Stoichiometry
the study of the quantitate relationships that can be derived from chemical equations
Why use stoichiometry?
Allows scientists to accurately predict amounts of reactants and product before a reaction occurs
Limiting reactant
the reactant in a chemical reaction that limits the amount of product that can be produced
When does a reaction stop?
When all the limiting reactant is consumed
Excess reactant
the reactant in a chemical reaction that remains when a reaction stops
always has leftovers because the limiting reactant ran out
How would I solve:
Identify the limiting reactant when 3.0g of H2 and 32g of O2 react to produce water
Get the equation
solve a limiting reactant problem
can tell since it gives two gram measurements and asks for limiting reactant
% Yield
actual/theortical *100
% Accuracy
experimental-theortical/theoritical *100
How would I solve:
What is the concentration of the solution if you dissolve 0.25 moles HCl of 0.5 L of solution?
Molarity problem
Take moles/liters
Can tell through keyword “concentration”
What is the molarity formula?
Moles/liters
How would I solve:
How many moles of 5 liters of a 3M NaOH solution?
5L is flying solo by itself and has to be multiplied by 3moles/L (which is the molarity)
This will cancel out the liters and then your good
Remember: in molarity conversion volume flys solo
How would I solve:
How many grams of sodium hydroxide are necessary to make 500mL of a 0.5M concentrated solution?
Molarity conversion
1) Convert mL to L
2) L flys solo
3) Multiply L by moles/L (M)
4) Get to grams by multiplying by grams/moles
What do you always need to solve a limiting reactant problem?
Need the BALANCED formula!!!
Dilution
the process of using a stock solution to create one that is less concentrated (more dilute)
What helps to know about dilution?
Most of the time we are using water.
This might not be explicitly stated in the problem and we need to infer this
How would I solve:
How would you make 100mL of a 2M HCl from a 3M solution?
Dilution problem
1) Use MaVa=MbVb to solve for Ma
2) Can leave in mL cause it makes it easier than a whole bunch of decimals
3) Subtract found Ma from 100mL. This solves for how much water you use
4) Add water to found Ma
What formula do we use in dilution?
How would you make problems…
MaVa=MbVb
How do you determine Ma vs. Mb in formula?
Look at what chemical they give you and see if the given is an acid or a base
Titration
a method of determining concentration by reacting with a standard solution
Phenolphtalein
pH indicator that tells when the acid and base are neutral
will turn a light pink
Empirichal formula
Represents the smallest ratio of atoms present in a compound
Molecular formula
Gives the total number of atoms of each element present
Is molecular or empirichal the “true” formula?
Molecular
Empirical is just the smallest ratio
Connection:
- Empirical=general anti-derivitative +C
- Molecular=specific anti-derivative
How to find the molecular formula?
1) Have to find the empirichal formula
2) Then find the formula mass of the empirichal formula
3) Divide the given mass with the found formula mass
4) Multiply each subscript in the empirichal formula by the ratio that was just found
How would I solve:
A student fills 1.0 L flask with CO2 at STP. How many molecules in the flask?
Two step stoich
Can tell because you only have one molecule (no need for a bridge)
Just go from liters to moles to particals
How would I solve:
0.09 moles of sodium sulfate in 12 mL of solution
Simple molarity problem
1) Change mL to L
2) Divide moles/L
How would I solve:
Find the mass of sodium required to produce 8.65L of hydrogen gas
2Na+2H2O=2NaOH+1H2
3 step stoich
Cross the mole bridge
How would I solve:
How many grams of Al2S3 are formed when 5.00g of Al is heated with 10gs of S
Limiting reactant problem
Can tell because you are given two masses
1) Need to get the formula
2) Find the limiting
3) Use limiting to find how many grams of product are produced
What is normally your theoretical value?
The value you solve for in the math
The math never lies!
How would I solve:
What volume of a 0.75 M solution can be made from 90.0g of NH4Cl
Molarity conversion
1) Grams fly solo
2) Find grams of NH4Cl and cancel them out to get moles
3) Set up an equation that relates found moles to the molarity
4) Solve for liters under the found moles
Algebra requires cross multiplication
