Chemistry - amount of a substance

what is the ‘amount of substance’?

a quantity to count the number of particles in a substance

what is amount of substance measured in?

moles

one mole =?

the amount of substance that contains 6.02×10²³ particles

what is the avogadro constant?

6.02×10²³ mol-1, the umber of particles in each mole of carbon-12

what is 6.02×10²³ particles per mole linked to?

the mass of carbon-12. 12g of carbon-12 contains 6.02×10²³ atoms

how do you find the mass of one mole of atoms of any element?

its its relative atomic mass in grams

why does formula matter for moles?

eg.

1 mol of H; 1 mol of hydrogen atoms

1 mol of H2; 1 mol of hydrogen molecules

what is molar mass, M?

the mass per mole of a substance

what are the units of molar mass?

gmol-1

moles =?

mass / molar mass

eg. M(NO2) =?

14 + (16 ×2) = 46gmol-1

whats a molecule?

2 or more atoms held together by covalent bonds

what is molecular formula?

the number of atoms of each element in a molecule

what is empirical formula?

the simplest whole number ratio of atoms of each element in a compound

what is empirical formula important for?

substances that dont exist as molecules, like metals, non metals and ionic compounds, as they form giant structures that would be impossible to formulate

what is relative molecular mass, Mr?

compares the mass of a molecule with the mass of an atom of carbon-12

how do you calculate relative molecular mass?

add together the relative atomic masses of the elements making up a molecule

what is relative molecular mass used for?

small molecules, eg. water

what is relative formula mass?

compares the mass of a formula unit with the mass of an atom of carbon-12

how do you calculate relative formula mass?

add together the relative atomic masses of the elements in the empirical formula

what is relative formula mass used for?

giant structures, eg. NaCl

what is analysis?

investigating the chemical composition of a substance

how do you work out empirical formula from mass with example?

how to work out molecular formula with example?

why are many coloured crystals hydrated?

water molecules are part of their crystalline structure

what is this water known as?

water of crystallisation

eg. what happens when blue crystals of hydrated copper 2 sulfate are heated?

bonds holding the water within the crystal are broken and the water is driven off, leaving behind white anhydrous copper 2 sulfate

equation for this?

method for how to carry out an experiment to determine the waterof crystallisation in hydrated crystals?

how accurate is an experimental formula?

not very accurate as some assumptions could be made:

1. all the water has been lost

2. no further decomposition

1. all the water has been lost explained

you may think all the water has been removed if the colour changes when heated but it actually hasnt fully, and water could still be there. a good solution is to continue heating until constant mass, suggesting all the water is removed

2. no further decomposition explained

many salts decompose further when heated, so might change formula. eg. when heated strongly copper sulfate decomposes to form black copper oxide

how are liquids and gases measured?

by volume

how many cm³ are in 1ml?

1cm³

how many cm³ are in 1dm³ and 1l?

1000cm³

what is a solute?

a dissolved compound

what is the concentration of a solution?

the amount of solute in moles dissolved in each 1dm³ of solution

what does a 1moldm-3 solution contain?

1 mol of solute dissolved in each 1dm³ of solution

concentration =?

moles / volume

examples using this equation?

what is a standard solution?

a solution of known concentration, prepared by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume

example of working out the mass required to prepare a standard solution?

what are some other units that can be used for concentration?

gdm-3

how do you convert moldm-3 to gdm-3?

convert between moles and grams using the equation : moles =mass/Mr

at the same temp and pressure…

equal volumes of different gases contain the same number of molecules

what is molar gas volume? (Vm)

the volume per mole of gas molecules at a stated temp and pressure 

what does the volume of a gas depend on?

the pressure and temp, but many experiments are carried out at RTP

what is RTP?

20 degrees C and 101kPa (1atm)

at RTP…

1 mole of gas molecules has a volume of 24dm³

at RTP, molar gas volume =?

24dmmol-1

volume of gas =?

moles x molar gas volume (24dm³)

worked example for this equation?

what is the ideal gas equation?

an equation when you are using diff temps and pressures of gases

what assumptions will you come across for the molecules making up an ideal gas?

• random motion

• elastic collisions

• negligible size

• no intermolecular forces

what is the ideal gas equation?

pV = nRT

what is R?

constant

what value is R always?

8.314Jmol-1K-1

what is temp measured in?

Kelvin

how do you convert degrees C to kelvin?

+273

each rise in 1K is…

the same as 1degree rise

units for all of the gas equation?

how to convert cm³ to m³

x 10-6

how to convert from dm³ to m³

x10-3

how to convert from kPa to Pa

x 1000

worked example using equation?

what is the incorrect assumption?

that 25 degrees C is room temp

how to do the method to get the results needed to find relative molecular mass?

how to use the results to find relative molecular mass?

what 2 assumptions does the ideal gas equation rely on?

• forces between molecules are negligible

• gas molecules have negligible size compared to the size of their container

when are these assumptions right?

when there are low pressures and high temps when the gas molecules are far apart and moving fast

what is the case when gas molecules are close together?

the volume of the molecules compared with the volume of the container start to become significant

what is the case if the gas molecules are moving slower?

they has less energy and intermolecular forces may become significant

what has then happened?

there have been improvements of the ideal gas equation for real gases

what is this equation?

what is stoichiometry?

the ratio of the amount, in moles, of each substance in a balanced equation

what do chemists use balanced equations to find?

• the quantities of reactants required to prepare a required quantity of a product

• the quantities of products that should be formed from certain quantities of reactants

what is the basic method for working out unknown information about a substance?

1. work out the amount in moles of whatever possible

2. use the equation to work out the amount in moles of the unknown chemical

3. work out the unknown info required

worked example with this method: reacting masses

worked example with this method: reacting mass, gas volumes, and concentration

steps for an experiment that can be carried out to identify an unknown metal (group 2 metal X)

1. set up apparatus as shown

2. weigh a sample of the metal and add to the flask

3. using a measuring cylinder, add 25cm³ of 1moldm-3 HCl to flask and replace bung

4. measure the max volume of gas in the syringe

how to identify unknown metal from results?

what is the maximum possible amount of product called?

the theoretical yield

why is the theoretical yield difficult to achieve?

• the reaction may not have gone to completion

• other reactions may have taken place alongside the main reaction

• purification of the product may result in loss of some product

what is the actual yield?

what you actually obtain from a reaction

percentage yield =?

(actual yield/ theoretical yield) x 100

worked example to calculate percentage yield?

what is the limiting reagent?

the reactant that isnt in excess and will be completely used up first and stop the reaction

how can you find out which reaction is in excess?

work out the amount in moles of each reactant and compare with the equation

example using limiting reagents

what is atom economy?

a measure of how well atoms have been utilised

reactions with high atom economies:

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

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

atom economy = ?

(sum of the molar masses of desired products/ sum of molar masses of all products) x 100

what has atom economy been developed from?

awareness of finite resources and environmental concerns

what is good about improving atom economy?

makes industrial processes more efficient, preserves raw materials, and reduces waste

what would ideal atom economy be?

100% (all products are desired)

worked example with atom economy?

what makes a reaction more sustainable?

using reactants that are readily available. energy may be needed to produce some, but costs for obtaining starting materials are low

what does efficiency and sustainability depend on?

both atom economy and % yield

can one be high and one low?

yes