Atoms and moles

Question 1

What is empirical formula?

Answer 1

The simplest whole number ratio of atoms of each element in a compound.

Question 2

What is molecular formula?

Answer 2

The actual number of atoms in each type of element in a molecule.
It is made up of a whole number of empirical units.

Question 3

How do you calculate molecular formula?

Answer 3

Find the empirical mass.
Divide the molecular mass by the empirical mass.
Multiply the empirical formula by that number to find the molecular formula.

Question 4

How can you calculate the mass of reactants when there is only one?

Answer 4

Mass of reactant A = mass of product - mass of reactant B.
Then you can find out the moles of each reactant used.

Question 5

How do you calculate empirical formula from percentage compositions?

Answer 5

Turn the percentages into masses.
The masses to mole.
Divide each number of moles by the smallest number to give the ratio of elements.

Question 6

How do you calculate molecular formula from empirical formula?

Answer 6

Molar mass / mass of empirical formula
This tells the factor you need to scale the empirical formula up by to get the molecular formula.

Question 7

How do you calculate mass of product from reactant?

Answer 7

Write out the balanced equation for the reaction.
Use the molar ratio from the balanced equation to work out the number of moles of product that will be formed from this much reactant.
Calculate the mass of that many moles of product.

Question 8

What is relative molecular mass?

Answer 8

Mr compares the mass of a molecule with the mass of an atom of carbon-12.
Add up the relative atomic masses of the elements making up the molecule.
Relative formula mass is the mass of a formula unit compared to an atom.

Question 9

What do all salts have?

Answer 9

All solid salts consist of a lattice of positive and negative ions.
In some salts water molecules are incorporated in the lattice too.

Question 10

What are hydrated salts?

Answer 10

A solid salt containing water of crystallisation.

Question 11

What are anhydrous salts?

Answer 11

The salt doesn’t contain water of crystallisation in the lattice.

Question 12

How do you calculate the formula of hydrated salts?

Answer 12

Find the mass of water lost by taking the mass of the anhydrous salt from the mass of hydrated salt.
Find the number of moles of water lost.
Find the number of moles of anhydrous salt produced.
Work out the ratio of anydrous salt to moles of water.
Scale so the ratio is in the form of 1:n and round the answer.

Question 13

What assumptions are made for experimental formula of hydrated salts?

Answer 13

• All of the water has been lost, when in reality some might remain inside the crystals. Heating until the mass does not change suggests no water remains.
• No further decomposition, many salts decompose further when heated, which can be difficult to judge if there is no colour change.

Question 14

What is reaction stoichiometry?

Answer 14

This tells you the ratios of reactants to products, i.e. how many moles of a product are formed from a certain number of moles of reactant.

Question 15

What is percentage error?

Answer 15

Percentage error = uncertainty / reading x 100

Question 16

What is theoretical mass?

Answer 16

The mass of product that should be formed in a chemical reaction.
It assumes no chemicals are lost in the process.

Question 17

How do you calculate theoretical mass?

Answer 17

Work out how many moles of the limiting reagent you have.
Use the equation to work out how many moles of product you would expect that much reactant to make.
Calculate the mass of that many moles of product.

Question 18

Why is actual yield always less than theoretical yield?

Answer 18

Sometimes not all the starting chemicals react fully.
Some chemicals are lost e.g. left on the filter paper or lost during transfers between containers.

Question 19

How do you calculate percentage yield?

Answer 19

Percentage yield = actual yield / theoretical yield x 100

Question 20

What is atom economy?

Answer 20

The proportion of reactant atoms that become part of the desired product in the balanced chemical equation.

Question 21

How is atom economy calculated?

Answer 21

% atom economy = molecular mass of desired product / sum of molecular masses of all product x 100

Question 22

What are addition reactions?

Answer 22

The reactants combine to form a single product.
The atom economy is 100% since no atoms are wasted.

Question 23

What are substitution reactions?

Answer 23

Some atoms from one reactant are swapped with atoms from another reactant.
This results in at least 2 products - the desired and by-products.
So atom economy is less than 100%.

Question 24

What are the environmental benefits of high atom economy reactions?

Answer 24

Reactions with low atom economies are less sustainable.
Many raw materials are in limited supply, so should be used efficiently to make them last.
It is better for the environment if less waste is produced.

Question 25

What are the economical benefits of high atom economy reactions?

Answer 25

A low atom economy means lots of waste produced.
It costs money to separate the desired product from the waste produced and more money to dispose of the waste products safely.
It’s a waste of money if a high proportion of expensive chemicals reactants end up useless product.

Question 26

What is a mole?

Answer 26

The amount of substance, number of particles.
It is given the symbol n.
One mole is 6.022 x 10^23 particles, this is Avogadro’s Constant NA

Question 27

What is the equation for finding the number of moles from particles?

Answer 27

Number of moles = number of particles / number of particles in mole (6.02x10^23).

Question 28

What is the difference in particles for moles?

Answer 28

1 mol of H = 1 mol of hydrogen atoms
1 mol of H2 = 1 mol of hydrogen molecules

Question 29

What is molar mass?

Answer 29

M, the mass of one mole of a substance.
Molar mass has the same numerical value as the relative molecular mass.
It is measured gmol^-1

Question 30

What is the equation for moles from mass?

Answer 30

Number of moles = mass of substance / molar mass.

Question 31

How do you convert between volume measurements?

Answer 31

Cubic centimetre (cm^3)
Cubic millilitre (mm^3) 1cm^3 = 1ml
Cubic decimetre (dm^3) or litre
1 dm^3 = 1000cm^3 = 1000ml = 1 litre.

Question 32

What is gas volume?

Answer 32

At room temperature and pressure (298k and 100kPa) molar gas volume is 24dm^3 mol^-1 or 24000cm^3 mol^-1.

Question 33

What is the gas volume equation?

Answer 33

Number of moles = volume in dm^3 / 24
or n = volume in cm^3 / 24000

Question 34

What is the ideal gas equation?

Answer 34

pV = nRT
p - pressure (Pa)
V - volume (m^3)
n - moles
R - 8.314 JK^-1 mol^-1 - gas constant
T - temperature (k) room temp is 298k

Question 35

How do you convert ideal gas values?

Answer 35

kPa x1000 = Pa
C° + 273 = K
cm^3 x10^-6 = m^3
dm^3 x 10^-3 = m^3

Question 36

What is the concentration of a solution?

Answer 36

How many moles of something are dissolved per 1 dm^3 of solution.
Units are mol dm^-3
A solution that has more moles per dm^3 than another is more concentrated.

Question 37

What is the equation for moles from concentration?

Answer 37

number of moles = concentration x volume cm^3
/ 1000
or n = concentration x volume dm^3

Question 38

What factors should be taken care of when using a gas syringe?

Answer 38

The plunger should move smoothly along the syringe, without getting stuck.
No gas can escape from the syringe.
The syringe is the correct size for the experiment - too large and it will be hard to read how much gas is produced, too small the plunger could be blown out the end.
This could release gas into the atmosphere.