Chapter 3- Amount of substance

what is a mole?

unit of measurement for substances

• it always contains the same number of particles

amount of substance meaning?

the number of particles in a substance and is measured in moles

avogrado constant?

6.02 × 10^23

what is the avogrado constant?

the number of particles per mole

• it allows the number of particles present in a sample of a substance with know mass to be found

how to calculate number of particles using the avogrado’s constant?

number of particles = moles x avogrado constant

2 ways to calculate moles?

• moles = mass/ mr

• moles = volume (dm³) x concentration (mol/dm³)

what is molar mass?

mass per mole, and has the units g/mol

what is molar gas volume?

gas volume per mole and has the units dm³ /mol

what is empirical formula?

the simplest whole number ration of atoms of each element present in a compound

what is molecular formula?

the number and type of atoms of each element in a molecule.

• the true number of each atom in the molecule

how to find the molecular formula of a molecule?

using the Mr of the empirical formula and the true Mr of the molecule.

• this gives a multiplier value, which can be used to scale up the empirical formula

how to find the multiplier?

Mr of molecule / empirical Mr

what is water of crystallisation?

the water that is part of the crystalline structure.

• the molecules are stoichiometrically chemically bonded into the crystal structure

what is the limiting reagent in a reaction?

the reactant that is not in excess.

• it will be used up first, causing the reaction to stop

what is the ideal gas equation?

pV = nRT

• p = pressure in Pascals

• V = volume in m³

• T = temperature in Kelvin

• n = moles

• R = ideal gas constant (which is 8.31JK^-1 mol^-1

what are the values for room temp and pressure?

• room temp- 20C

• pressure- 101kPa (1atm)

• how to convert from cm³ to m³?

• how to convert from dm³ to m³?

• how to convert from kPa to Pa

• x10^-6

• x10^-3

• x10³

how to calculate percentage yield?

(real yield/ theoretical yield) x100

• most experiments have a percentage yield of less than 100%

why do most experiments have a percentage yield of less than 100%?

• incomplete reaction

• loss of products on equipment

• unwanted side reaction

what is the atom economy of a chemical reaction?

a measure of how well atoms have been utiilised

how to calculate atom economy?

(Mr of desired product / Mr of reactants) x100

characteristics of reactions with high atom economies?

• they produce a large proportion of desired products and few unwanted waste products

• important for sustainability as they make the best use of natural resources

why is it desirable to have a high atom economy?

• because it means there is little to no waste product

• therefore, process is more economically viable for industrial scale manufacture

• helps to preserve raw materials

what is stoichiometry?

the ratio of amount, in moles, of each substance

how to do back titrations?

• calculate moles of the base (e.g- sodium hydroxide)

• work out moles of the acid (e.g- hcl) that reacted with the base

• then find moles in the bigger volume

• work out moles of acid that reacted with the substance at beginning

  • moles of original acid - moles in volumetric flask

• use equation to give moles of the substance reacting with the acid

• work out mass of the substance

• divide this mass by mass given at beginning

• multiply by 100

example of back titration?