4 - Moles

Question 1

What is an atomic unit?

Answer 1

The unit we use to show the mass of a substance or atom

Question 2

What is an atomic unit equivalent to?

Answer 2

1/12 the mass of an atom of carbon-12

Question 3

Define relative isotopic mass

Answer 3

The mass of an isotope compared with 1/12 of the mass of an atom of carbon-12

Question 4

What is relative isotopic mass always the same as?

Answer 4

The mass number of the isotope

Question 5

Define relative atomic mass (Ar)

Answer 5

The weighted mean mass of an atom compared with 1/12 of the mass of an atom of carbon-12

Question 6

How do you calculate the Ar of an element?

Answer 6

You do (abundance * relative isotopic mass) for each isotope of the elements
You add all of these values together

Question 7

How could the abundances and isotopic masses be shown to us in an exam question?

Answer 7

A mass spectrum graph

Question 8

Describe the mass spectrum

Answer 8

It is a stick graph with abundance on the y axis and isotopic mass on the x axis

Question 9

Define relative molecular mass (Mr)

Answer 9

The sum of all of the relative atomic masses in a substance (could be a compound)

Question 10

What is the Avogadro constant value and What does the it represent?
N~A

Answer 10

6.02 * 10^23

it represents the amount of atoms (or molecules if it’s polyatomic) per mole of a substance

Question 11

How much of a substance is defined as a mole?

Answer 11

The amount of substance there is when there are 6.02 * 10^23 molecules of the substance

Question 12

What is the molar mass and its unit?

Answer 12

The mass of one mole of a substance in grams.

units: g/mol

Question 13

What is the molar mass equivalent to?

Answer 13

The relative molecular mass (Mr) in g/mol

Question 14

what are the mole equaltions?

Answer 14

n = m/Mr (m=grams)

n=cv (v=dm^-3)

pV=nRT (v=m^3, R=8.31, T= kelvin)

n= v/24dm (gasses at RTP)

Question 15

What is the equation linking the number of atoms (or molecules) with the number of moles?

Answer 15

Number of atoms (or molecules) = N~A * n

Question 16

How many moles are there in 12g of carbon-12?

Answer 16

n = m / Mr

= 12g / 12g/mol

= 1 mole

Question 17

How many atoms are there in 2 moles of carbon-12?

Answer 17

No. of atoms = N~A * n

= (6.02 10^23) 2

= 1.204 * 10^24 atoms

Question 18

Define empirical formula

Answer 18

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

Question 19

Define molecular formula

Answer 19

The actual number of atoms of each element in a molecule

Question 20

When might the empirical formula be incorrect?

Answer 20

In instances like butane where the molecular formula is C~4 H~10

The empirical formula would be the simplest ratio which would be C~2 H~5

Question 21

How do you calculate empirical formula?

Answer 21

• line elements on top row
• percentage/amount of atoms/moles of atoms
• Mr of atoms
• %/Mr
• divide by smallest number to get whole number ratio

Question 22

define atom economy

Answer 22

Atom economy tells us the percentage of all of the products of a reaction which are useful products

Question 23

If you calculate the empirical formula but the Mr is different than the one stated in the question, what do you do?

Answer 23

Multiply up the numbers of atoms in the substance so that the Mr is correct

Question 24

Define atom economy

Answer 24

Atom economy tells us the percentage of all of the products of a reaction which are useful products

Question 25

What is the formula for atom economy?

Answer 25

(Mr of useful products * 100%) / Mr of all products e.g. if 2H~2 and CO~2 are products and hydrogen is useful, what is the atom economy? Mr of all = 48 Mr of 2H~2 = 4 Atom economy = (4 * 100%) / 48 = 8.33% Atom economy is 8.33%

Question 26

What are the 3 advantages of high atom economy?

Answer 26

Large amount of useful products produced Few unwanted waste products Makes the best use of natural resources (sustainable)

Question 27

What are the 4 factors that make a reaction sustainable?

Answer 27

Atom economy Readily available reactants Low energy costs Percentage yield

Question 28

What is the easiest way to significantly increase atom economy?

Answer 28

Find a use for unwanted products They are then desired products

Question 29

What is a hydrated salt?

Answer 29

A salt that contains water of crystallisation inside its structure

Question 30

How do you remove water of crystallisation from a salt?

Answer 30

Heat the salt until the water separates from the salt as a liquid

Question 31

How do you show the water of crystallisation inside a salt in an equation?

Answer 31

Write the salt Put a full stop Write the water of crystallisation after the full stop E.g. MgCl~2 . 7H~2 O (s) --> MgCl~2 (s) + 7H~2 O (l)

Question 32

What is an anhydrous salt?

Answer 32

Anhydrous describes the salt after the water of crystallisation has been removed

Question 33

The mass of CuSO~4.xH~2O = 6.8g and the mass of anhydrous CuSO~4 = 4.35g. (when there is a 'x' in front of a molecule, you have to find it)

Answer 33

xH~2O = 6.8g - 4.35g = 2.45g N = m / Mr is used for the CuSO~4 and the H~2O Water = 0.1361 mol Anhydrous salt = 0.027 mol Ratio of anhydrous salt to water = 1:5 CuSO~4 + 5H~2O x = 5

Question 34

What is the rule to do with gases, moles and volume?

Answer 34

Every gas in existence takes 24dm^3 if there is 1 mole of the gas at RTP

Question 35

What are the values for RTP?

Answer 35

25C temp 101kPa pressure

Question 36

What is stoichiometry?

Answer 36

Using the ratio of moles in an equation to calculate masses

Question 37

What is the difference between relative formula and relative molecular mass?

Answer 37

Both are Mr Formula mass is for compounds (sodium chloride) Molecular mass is for molecules (water)

Question 38

What is molar volume?

Answer 38

The volume taken up by one mole of a substance

Question 39

How do you translate between centigrade and kelvin?

Answer 39

Centigrade + 273 = Value in kelvin

Question 40

What is the concentration?

Answer 40

The amount of solute per unit volume mol dm^-3 or g dm^-3

Question 41

What is a standard solution?

Answer 41

A solution that has a precisely known concentration so that it can be used in equations after doing a reaction with it

Question 42

Describe how you find the concentration of an acid or alkali after doing a titration

Answer 42

Do the molecular formula for the acid + base You already have the volume and concentration of the alkali Calculate the number of moles of the alkali using n = c * V Find the mole ratio by looking at the molecules of the acid and the alkali (if 3Alkali + 2Acid) the mole ratio is 3:2 Use the ratio to find the moles of the acid (with 3:2, if alkali is 3moles, there are 2 moles of acid) Use the volume of acid used (result of titration) and the calculated moles of the acid in the formula c = n / V to calculate the concentration of the acid

Question 43

What is mass concentration?

Answer 43

Mass concentration of a solution is the MASS of solute dissolved in 1 dm^3 of solvent

Question 44

How do you calculate mass concentration?

Answer 44

Mass concentration (g dm^-3) = concentration (mol dm^-3) * Mr of solute Similar to m = n * Mr

Question 45

What is a limiting reactant?

Answer 45

The reactant in a reaction with the least amount of moles per molecule Only that amount of product will be made Any reactants which have more moles per molecule than the limiting reactant will have some left after the reaction

Question 46

How do you deal with limiting reactants?

Answer 46

You calculate the number of moles for each reactant You ensure the products have the number of moles per molecule of the limiting reactant