PHYSICAL CHEM - AMOUNT OF SUBSTANCE Flashcards
What is the equation to calculate moles?
moles (mols) = mass (g) / RFM (n = m/Mr)
What is Avagadro’s constant?
6.02 x 10^23
What is relative atomic mass?
the avg mass of 1 atom of an element compared to 1/12th the mass of an atom of carbon-12
What is relative molecular mass?
the avg mass of 1 molecule of a substance compared to 1/12th the mass of an atom of carbon-12
What is theoretical yield?
what you could make in theory
What is actual yield?
what you actually make
How do you calculate % yield?
(actual/theoretical) x 100
What are 3 reasons why the actual yield can never be 100%?
- reaction is reversible
- other side reactions
- some product lost
What is the equation for density?
density (kg/m^3) = mass (kg) / volume (m^3) (ρ = m/V)
What is the equation for atom economy?
(Mr of wanted product/Mr of total reactants) x 100
What is the empirical formula of a substance?
the simplest ratio of atoms of each element in a substance
What is the ideal gas equation?
pressure (Pa) x volume (m^3) = moles (mol) x temp (K) x gas constant (J mol^-1 K^-1) (pV = nRT)
How do you convert from °C to K?
+ 273
How do you convert from mols to g?
x Mr
How do you convert from mol dm^-3 to g dm^-3?
x Mr
How do you convert from g to mols?
÷ Mr
How do you convert from g dm^-3 to mol dm^-3?
÷ Mr
RP - Titrations: method
- using pipette and pipette filler measure 25 cm^3 of alkali into the conical flask
- add a few (5-10) drops of indicator (could be methyl orange/phenolphthalein) to the flask
- place a white tile under the conical flask
- use a funnel to fill burette with acid of known conc. remove funnel and record initial volume
- add acid from the burette slowly and swirl the flask constantly
- add acid drop wise towards the end
- as soon as there is a colour change stop adding acid
- calculate volume of acid added
- repeat titration and calculate mean
Why is it important to remove the funnel before doing the titration?
so no additional drops fall into the burette whilst you’re doing the titration as this would affect the volume
What should you rinse the burette and pipette with? Why?
- pipette and burette should be rinsed with the solution you will be using in each one
- this is because rinsing them with water would affect the conc. of the solutions
Why should you only add a few drops of indicator?
so it doesn’t react with the solution in the flask which would affect the end point
What are concordant results?
results within 0.1 of each other
When calculating mean titre which results should you use?
only use concordant results
What is the equation used to calculate % error?
% error = (uncertainty in measurement / measurement) x 100
How can % error be reduced?
- taking a larger measurement
- use equipment with a smaller resolution
How do you form an ionic equation?
remove any ions that don’t react
eg: original: HCl (aq) + NaOH (aq) → NaCl (aq) + H2O(l)
ionic equation: H^+ (aq) + OH^- (aq) → H2O (l)
What is a standard solution?
a solution containing a precisely known conc of an element in a substance
How do you make a standard solution (of Na2CO3)?
- measure mass of Na2CO3 in a weighing boat using a mass balance
- pour Na2CO3 into beaker
- re-weigh empty weighing boat and subtract its mass from the mass of the weighing boat with NaCO3 and record this value
- dissolve Na2CO3 in ~100cm^3 of distilled water and pour into volumetric flask
- rinse beaker with distilled water and pour rinses into volumetric flask (repeat at least 3 times)
- use pipette to add distilled water until meniscus reaches 250cm^3
- invert flask many times
