1.2 Amounts of Substance Flashcards
What is the definition of empirical formula?
The SIMPLEST whole number ratio of atoms in a compound
What is the definition of molecular formula?
The actual number of atoms of each element in a compound
Why is the actual molecular mass not need to be known exactly in an empirical formula question?
Because the Mr is an average
How do you calculate empirical formula?
If given in percentage treat percentage as mass
Mass / Mr
Divide through by smallest number
How do you calculate moles of hydration?
If percentage treat as mass
What is the definition of relative atomic mass?
Average mass of an element compared to 1/12 mass of a carbon 12 atom
What is avogadros constant?
6.022 x 10 ^23The number of particles in a mole
How to calculate the number of particles?
Mass / Mr
Ans x avogadros constant
What is the definition of a mole?
The amount of a substance with the same number of molecules in 12 g of carbon 12
What is the mole equation with a mass in grams?
Moles = mass/mr
Equation to calculate number of moles of a gas at normal conditions?
Moles= volume/24000
How to calculate the number of moles in a solution?
Moles= concentration x volume
How do you work out how much of one product is needed to react with a known mass?
Balance equation
Work out ratio of moles
Work out mole of known substance
Deduce moles of unknown substance
Moles x Mr = answer
How do you find out the concentration from a titration?
- Balance equation
- Calculate number of moles of substance that’s got two parts known
- Using ratio of moles work out the moles of substance with unknowns
- Use equation moles = concentration x volume
What is the ideal gas equation?
PV = nRt
Pressure (Pa) x volume (m^3)= number of moles (mol) x gas constant (mol^-1k^-1) c temperature (k)
How do you turn Celsius in to kelvin?
You add 273
How do you turn dm3 into m3?
Divide by thousand
What is the definition of atom economy?
Percentage of reactants that become useful products. Shows how efficient a reaction is.
What is the atom economy formula?
Mr of desired product/sum of mr of all products x 100
What is the percentage yield equation?
Actual yield/ theoretical yield x 100
Why is actual yield never 100%?
- Mass lost in transfer operations
- Reversible reactions
- Unwanted side reactions
Why are less efficient reactions still used?
- One may be more environmentally friendly than the other
- less energy may be used
- waste product may be sellable
How do you do a reacting mass calculation?
Balance equation to find ratio of moles
Calculate moles of known mass
Deduce number of moles of unknown amount
Use moles = mass/mr
What equipment is used to measure a set volume of solution?
Bulb Pippette
What piece of equipment is used to prepare a standard solution?
Volumetric flask
What piece of equipment measures variable volumes of solution?
Burette
What piece of equipment has sides to reduce loss of solution through splashing?
Conical flask
How is percentage uncertainty calculated?
Uncertainty / volume of apparatus x 100
Why is limited amounts of indicator used in a titration?
Indicator is a weak acid
Make titre less accurate
Why is a funnel left in the top of a Burette during a titration incorrect?
Drops left in funnel could drop into Burette making the volume left in Burette incorrect
Why is the tip of the burette not being filled up incorrect in a titration?
Less than the stated amount in the burette so the volume is incorrect
Why is it not good if the burette he not been washed out with acid?
Water may still be in burette which will affect pH
What is the colour change of methyl orange?
Yellow to Pink
Why is a small titration problematic and what improvements can be done?
Percentage error is high
Increase concentration of reagent
Increase mass
What should all calculations be rounded to unless otherwise stated?
3 significant figures
What happens in a back titration?
The concentration of a reactant is determined by reacting it with an excess of a known amount of a reagent
When are back titrations used?
When the reactant is insoluble
What are the steps to find percentage mass of a substance by back titration?
Write out and balance equation
Find out number of moles of reagent reactant was titrated with
Deduce number of moles of unknown based on molar ratio
Calculate number of moles in entire flask before titration (e.g. if 25cm3 was titrated from 250cm3 x by 10)
Calculate mass using moles = mass/ mr
Divide mass of reactant over overall substance
