Equations Flashcards
Calculating the number of moles using avogadros constant
Number of moles = number of particles you have / number of particles in a mole (6.02x10^23).
number of particles = moles x 6.02x1023
Equation to work out the number of atoms in something
Amount of substance in moles = mass in grams / molar mass
N = m/M
Number of moles, volume and concentration
Include units
Number of moles = volume x concentration
Volume must be in dm3 and concentration must be in mol/dm3
Mass of substance, volume and concentration
Include units
Mass of substance = concentration x volume
Volume is in dm3 and concentration is in g/dm3
Process for calculating empirical formula from experimental results or percentages
- Divide the mass or percentage composition by mass, of each element by its relative atomic mass.
- if necessary divide the answers by the smallest of the numbers.
- this gives answers that should be in an obvious whole number ratio.
- these are used to write the empirical formula
Working out empirical formula with one percentage or mass missing
Do the same steps as before but at the start minus the other percentages from 100 or minus other masses from total mass
Working out empirical formula from combustion
The masses of H2O and CO2 will be given
You take the mass of CO2 or H2O then times the Mr of the element you want to find by the mass and divide by CO2 or H2Os overall Mr.
Calculating molecular formula from empirical formula
Take the relative atomic mass and divide it by the Ar to get the individual ratio of the molecular formula.
Using balanced equations to work out the mass of a compound.
Equation should be balanced meaning the larger number is the number of moles.
You can calculate the moles of another compound if one is given for you along with its mass or Mr.
you use the equation Moles = mass / Mr
Or moles = mass / molecular mass
Volume in dm3, number of moles and molar volume
What is the molar volume at room temperature and pressure
Number of moles = volume in dm / molar volume
24dm3/mol
R, T, p, V, n - arrange into equation and give what each mean along with units.
Write the conversation from degrees C to K
pV = nRT
p = pressure (Pa) V = volume (m3) n = number of moles R = 8.31 J/K/mol T = temperature (K)
C + 273 = K
Give the equation used for titration calculations
Number of moles = concentration x volume
Calculating reacting masses from equations
Calculate the Mr or molar mass of selected reactant and products formed.
Then calculate the number of moles of the products.
Then use this to calculate the mass of the selected reactant.
How to work out an equation from reacting masses
Calculate the molar mass or mr of both of the reactants or products given.
Then work out the number of moles of reactants or products given.
From these work out the ratio between the reactants or the products.
Use this to calculate the balanced number of moles for the products or reactants which are said in the question.
Explain how to carry out a back titration
How could it be complicated further and how do you solve this?
- You are given one equation A+B -> stuff. You are told B is in excess and know how much is used (known conc & vol) and you know all of A reacts.
- Some of B must therefore still be present but how much? you are given a second reaction where B+C -> stuff.
- you now know how many moles are now present.
- This is the excess from the first reaction and can be added to what was reacted to find the total of B.
- B’s concentration and volume can now be used to find the concentration or volume of A.
- This could be complicated further by saying a sample of B e.g 25 cm3 is taken from the 100cm3 in the first reaction. to solve this you then times the results from reaction 2 by 4.
