amount of substance Flashcards
how do you convert from cm^3 to dm^3?
divide by 1000
how do you convert from dm3 to cm3?
multiply by 1000
what is relative atomic mass?
● mass of each atom
● always to 1 d.p
what is the definition of relative atomic mass?
average mass of an atom relative to C−12 on a scale in which an atom of C−12 has a mass of 12
what is the definition of relative molecular (formula) mass?
average mass of the molecule relative to 1/12th the relative atomic mass of carbon-12
how do you calculate relative atomic mass?
(% abundance 1 x mass of isotope 1) + (abundance 2 x mass of isotope 2)
/ 100 (or total abundance)
what is avogadro’s constant?
6.022 x 10^23
what is the sig fig rule?
● generally 3 sig figs
● give answer the same amount of sig figs as the sig figs of the numbers in the question
what is molar mass?
numerically the same as Mr but in g / mol
what is the calculation for moles?
m = m / mr
what is the calculation for number of molecules?
number of molecules = moles x avogadro’s number
what is the calculation for number of atoms?
number of atoms = moles x avogadro’s number x number of atoms in molecule
what is the calculation for concentration?
c = n / v
what is a solute?
solid that dissolves in solvent to form a solution
what is a solution?
mixture formed when solute dissolves in solvent
what is a solvent?
the liquid in which a solute dissolves
what is the calculation for concentration using gdm^-3?
c = mass / v
how do you go from cm^3 to dm^3?
divide by 1000
how do you convert from moldm^-3 to gdm^-3?
x Mr
how do you convert from gdm^-3 to moldm^-3?
/ Mr
what is the dilution factor?
● initial conc / target conc
● sum of the parts
what is whatever you are diluting equal to?
one part
what is the calculation for % uncertainty?
% uncertainty = (instrument uncertainty / quantity measured) x 100
how do you improve accuracy?
larger volume, larger mass as it decreases % uncertainty
what is the ideal gas equation?
PV = nRT
P - pressure (Pa) (N/m^2)
V - volume (m^3)
n - number of moles
R - 8.31JK^-1mol^-1
T - temp (K)
how do you convert from degrees celsius to kelvin?
+273
how do you convert from kelvin to degrees celsius?
-273
how do you convert from kPa to Pa?
x 10^3
how do you convert from dm^3 to m^3?
x 10^-3
how to convert from cm^3 to m^3?
x 10^-6
how do you write ionic equations?
● make sure equation is balanced
● only write chemicals that are (aq) as ions
● make sure both sides are balanced
● cancel out any species that are identical either side of the arrow
what are spectator ions?
ions that are chemically unchanged at the end of a reaction
what is the calculation for percentage yield?
actual / theoretical x 100
what is theoretical yield?
number of moles or mass obtained by calculation
what is the actual yield?
number of moles or mass of product obtained experimentally
what is percentage yield?
shows how much of a particular product you get from the reactants compared to the maximum theoretical amount that you can get
what is atom economy?
shows how many atoms used in reaction become desired product
what is the calculation for atom economy?
mass (or mr) of desired product / total mass (or mr) of reactant (or product) x 100
what does atom economy take into account?
the big number
when is atom economy 100%?
when there’s 1 product
what is the method for making a volumetric solution and carrying out a simple acid-base titration?
● weigh sample out in boat
● transfer to beaker
● reweigh boat, calculate mass added (weighing by difference - to ensure that we know what solid has actually been transferred)
● dissolve sample in distilled water (must be less than 250cm3)
● add into volumetric flask with washings of glass rod, funnel, beaker (washings to ensure all of solid is in solution)
● make up to 250cm3 in volumetric flask, near 250cm3 graduation mark use pipette, ensure bottom of meniscus on 250cm3 mark
● invert (to give a homogenous solution rather than to dissolve)
what are the steps of titration?
● fill burette with acid of known conc after rinsing with NO air bubbles
● accurately measure amount of alkali using calibrated pipette and pipette filler after rinsing
● add alkali to conical flask with few drops of suitable indicator
● run in acid from burette and swirl flask until colour just changes showing solution in conical flask is neutral - rough titre
● repeat procedure until 3 values for volume of acid used at neutralisation are same, within experimental error
● concordant results are within 0.1cm^3 of each other
how do you prepare and use a burette?
● washing with water 3 times - removes any soluble impurities
● washing with solution the used to fill burette - ensures conc not changed by water
*applies for pipette
● use funnel to fill burette - don’t leave in funnel - drops could affect volume
● if air bubble left in burette, space will be filled during titration causing volume added to be too high
how do you read from a burette?
● from bottom of meniscus
● readings taken from 2 d.p.
how do you use indicator?
● 1-3 drops
● if too much pH may change as its acidic
why do we use a conical flask?
● less chance of losing liquid when swirling
● easier to swirl contents
how do you rinse a conical flask?
● near end point wash sides of conical flask with distilled water - returns reagents to reaction mixture
● does not affect accuracy as water does not change moles of each reagent
how do you see colour change?
use white tile
how to go from milligrams to grams?
divide by 1000
what is the empirical formula?
simplest whole number ratio of atoms of each element in a compound
what is the molecular formula?
actual number of atoms of each element in one molecule of the compound
why is the actual yield much less than the theoretical yield?
● reactants may be impure
● reaction may not go to completion
● some of the product may be left in the container
● it may be difficult to purify the product
how do you reduce error from burette?
● use larger volumes of solution
● must decrease concs of solution you are using in burette to ensure you are using larger volumes
why do you swirl the conical flask during a titration?
to ensure the complete mixing of reactants
what is the rough titre?
● the first titre you perform
● to get an approximate titre
● as single titre could be flawed / anomalous
● never include rough titre in an average
what is the general method for a back titration?
● react a known mass of solid to be analysed with an excess (but known) amount of acid
● make up excess acid to a specific volume and titrate against a standard base
● calculate the amount of acid remaining (the excess)
● calculate amount of acid used up in original reaction by subtraction from initial number of moles
● calculate number of moles present the original solid by consideration of the stoichometry of reaction
what are the uses of back titrations?
● finding relative formula mass of an unknown carbonate
● identifying metal in an unknown metal oxide
● finding the percentage of metal in an alloy
● finding purity of an impure carbonate or oxide
