Calculations Flashcards
Formula for mass when given number of moles
mass = molar mass x moles
Formula for number of moles given mass of sample
moles = (mass) / (molar mass)
Units for mass, moles and molar mass
grams (g), moles (mol), gram/moles (g/mol)
What is RAM?
Relative atomic mass - the mass of one mole of an element relative to 1/12th to mass of one mole of carbon-12.
What does Avogadro’s number represent?
The number of particles in one mole.
Formula for number of particles when given number of moles
Number of particles = Number of moles x Avogadro’s number
Numerical value of Avogadro’s number
6.02x10^23 mol^(-1)
Mass percentage equation
((total elative mass of element in molecule)/(Relative molecular mass))x100
Mass percentage example
Step 1. write the word equation
Step 2. what information has been provided?
Step 3, find the difference between reactants and known product
Step 4. use the percentage equation
Purity percentage
Purity percentage = ((mass of pure substance)/(mass of impure substance)) x 100
Empirical formula
Shows the simplest whole number ratio of elements present in a compound.
Molecular formula
Shows the actual number of atoms in a molecule.
How to work out empirical formula
Step 1. note element and mass
Step 2. divide each by atomic mass
Step 3. divide each answer by the smallest number
Step 4. Not a whole number? Multiple the ratio by 2, 3 or 4
Step 5. This is the ratio for the formula - write formula
How to work out molecular formula
Step 1. find the molar mass of the empirical formula
Step 2. divide the molecular formula molar mass by the empirical formula molar mass
Step 3. use this number to multiply the empirical formula ratio
Definition of molar volume of gas
1 mole of any gas at room temperature (25oC) and room pressure (1 atm) occupies a volume of 24.8L
Formula for volume of gas given number of moles
Volume = molar volume of gas x number of moles
Volume
Space filled, unit of L (liters)
Molar volume of gas at room temperature and pressure (SLC)
24.8 L/mol
Theoretical yield
Maximum amount of product we could get from a given amount of reactants if we had no side reactions and all reactants collided successfully.
Actual yield
Amount of product we get in a reaction.
Percentage yield
Percentage yield = ((actual yield)/(theoretical yield))x100
Molar concentration definition
Number of moles of solute per volume solution
Equation for molarity (molar concentration) given number of moles of solute and volume of solution.
Molar concentration = (number of moles of solute)/(volume of solution)
Titration
Titration is a technique to find the concentration of a solution.
How to titrate accurately
Rinse to flush out potential contaminants.
The burette scale includes the volume of liquid in and below the tap.
Use clean flask to reduce contamination.
Pipette scale accounts for “last drop” to be left in pipette.
Need enough indicator to see a colour change, but not so much as to change the volume of the solution substantially.
White tile allows the colour change to be seen more clearly.
We can see where the level is between markings but cannot justify a precision more that 0.05 mL.
The trial run will give us an idea of when to go drop-by-drop. We go drop-by-drop to avoid over-shooting.
We repeat to get concordant results for reliability and to calculate the mean average for greater accuracy.
Standard solution
Solution of accurately known concentration.
Aliquot
Accurately known volume of the reagent solution with unknown concentration.
Indicator
Gives visual indication of when reaction is complete.
Equivalence point
The equivalence point marks the stage at which the reaction is complete, when neither reagent is in excess.
End point
The end point is not the same as the equivalence point, but if care is taken the end point is the same as the equivalent point to the precision of the volume of one drop.
