Module 2: Atoms and reactions

Question 1

What is relative atomic mass?

Answer 1

Relative atomic mass is the average mass per atom of the naturally occurring form of an element relative to 1/12th of the mass of a carbon-12 atom.

Question 2

What is relative isotopic mass?

Answer 2

Relative isotopic mass is the mass of an atom of an isotope relative to 1/12th of the mass of a carbon-12 atom.

Question 3

What is relative molecular mass?

Answer 3

Relative molecular mass is the sum of the relative atomic masses of the constituent atoms of a molecule relative to 1/12th of the mass of a carbon-12 atom.

Question 4

What is the equation for calculating the relative atomic mass (Ar) of an isotope?

Answer 4

Ar = { ( abundance of isotope x isotopic mass of isotope ) /100

Question 5

What is the equation linking molar mass, mass and number of moles?

Answer 5

number of moles = mass (g) / Mr (g/mol)

Question 6

What is meant by the molar mass?

Answer 6

The mass per mole of a substance

Question 7

What number represents Avogadro’s constant?

Answer 7

6.02 x 10*23

Question 8

What is the empirical formula?

Answer 8

The empirical formula is the formula showing the simplest whole number ratio of atoms of each element present in a compound.

Question 9

What is the molecular formula?

Answer 9

The molecular formula is the formula showing the actual number of atoms of each element in a molecule.

Question 10

What is stated by Avogadro’s law?

Answer 10

At the same temperature and pressure, 1 mole of any gas will occupy the same volume.

Question 11

What is standard temperature and pressure?

Answer 11

Pressure = 1 Atmosphere = 100,000Pa
Temperature = 25*C = 298 K

Question 12

What is the equation for number of moles using volume (in dm cubed and cm cubed) at RTP?

Answer 12

moles = volume/molar volume

number of moles = volume (dm cubed) / 24

number of moles = volume (cm cubed) / 24000

Question 13

What is the ideal gas equation?

Answer 13

PV = nRT

P = pressure in Pa
V = volume in m cubed
dm cubed / 103 = m cubed
cm cubed /106 = m cubed
n = number of moles
R = universal gas constant = 8.314 Jmol-1K-1
T = temperature in K
T(K) = T(*C) + 273

Question 14

What does the % yield of a chemical reaction show?

Answer 14

The % yield of a chemical reaction shows how much product was actually made compared with the amount of product the was expected

Question 15

What is the equation for % yield?

Answer 15

% yield = actual yield (g) / theoretical yield (g) x 100

Question 16

What are some reasons as to why yield is not often 100%?

Answer 16

• the reaction may be incomplete
• side reactions may occur, leading to by-products
• reactants may not be pure
• some reactants/products may be left behind in the apparatus

Question 17

What is atom economy?

Answer 17

Atom economy is the % of reactants that end up as useful products

Question 18

What is the equation for atom economy?

Answer 18

Atom economy = mass of wanted product (s) / total mass of all products x 100

Question 19

What is the equation linking concentration, moles and volume?

Answer 19

Conc. (moles.dm-3) = moles / volume (dm3)

Question 20

What is concentration?

Answer 20

A measure of how much solute is dissolved per solvent.

Question 21

What is meant by hydrated?

Answer 21

Hydrated refers to a crystalline compound containing water molecules.

Question 22

What is meant by anhydrous?

Answer 22

Anhydrous refers to a substance that contains no water molecules.

Question 23

What is water of crystallisation?

Answer 23

Water molecules that are within the crystalline structure of the compound.

Question 24

Why do different isotopes of the same element react in the same way?

Answer 24

• chemical reactions involve electrons, and isotopes have the same number and arrangement of electrons
• neutrons make no difference to chemical reactivity

Question 25

What are the... 1. atomic numbers 2. mass numbers 3. no. of protons 4. no. of neutrons 5. no. of electrons in... a) 23 Na + 11 b) 35 Cl- 17

Answer 25

a) 1. 11 2. 23 3. 11 4. 12 5. 10 b) 1. 17 2. 35 3. 17 4. 18 5. 18

Question 26

What does the contribution made by an isotope to the overall mass depend on?

Answer 26

- the percentage abundance of the isotope - the relative mass of the isotope

Question 27

Why is the term relative formula mass used for some compounds, instead of relative molecular mass?

Answer 27

Compounds with giant structures don't exist as simple molecules - includes ionic compounds and covalent compounds

Question 28

What can mass spectrometry be used for?

Answer 28

- identifying unknown compounds - finding the relative abundance of each isotope of an element - determining structural info. about molecules

Question 29

How is a mass spectrometer used to determine the mass of a molecule/isotope?

Answer 29

It measures the mass-to-charge ratio of ions 1. it causes substances to become positive ions 2. positive ions are passed through the apparatus and separated depending on their mass and charge 3. a computer analyses the data on the ions present and produces a mass spectrum

Question 30

Why do atoms of Be, B, C and Si not usually form ions?

Answer 30

It requires too much energy to transfer the outer shell electrons to form ions

Question 31

What is the equation for converting moles to no. of particles?

Answer 31

No. of particles = n (moles) x Avogadro's constant (6.02x10^23)

Question 32

Define Avogadro's constant

Answer 32

The number of atoms per mole of the carbon-12 isotope (6.02x10^23)

Question 33

Define molar gas volume

Answer 33

The volume per mole of a gas - units are dm3/mol

Question 34

What is the molar gas volume at RTP?

Answer 34

24.0 dm3/mol

Question 35

What do chemists often use to measure gas volumes?

Answer 35

Gas syringe Alternatively could use a gas delivery tube leading into an upturned measuring cylinder in a water bath

Question 36

What does the concentration of a solution tell us? What is the unit for concentration?

Answer 36

How much solute is dissolved in a given amount of solvent. The concentration of a solution is the amount of solute, in mol, dissolved per 1dm3 of solution Moldm-3

Question 37

What is the equation for calculating the amount, in mol, of any volume of a solution of a known concentration?

Answer 37

n = cV

Question 38

What is a standard solution and what are they often used for?

Answer 38

A standard solution is a solution of known concentration. Often used in titrations to determine unknown info about another substance

Question 39

What piece of equipment is used to make up a standard solution?

Answer 39

A volumetric flask

Question 40

How would you make up a standard solution?

Answer 40

1. weigh out the sol

Question 41

Find the mass of potassium hydroxide required to prepare 250cm3 of a 0.200 moldm-3 solution

Answer 41

1. Find amount of KOH in mol required in the solution n = cV n = 0.200 x (250/1000) = 0.0500 mol 2. Convert moles into grams n = mass/Mr mass = n x Mr mass = 0.0500 x 56.1 = 2.805g

Question 42

What is meant by… 1. Concentrated 2. Dilute

Answer 42

1. A large amount of solute per dm3 2. A small amount of solute per dm3

Question 43

What do concentrated acids have a concentration greater than?

Answer 43

10 moldm-3

Question 44

What is a species in chemistry?

Answer 44

A type of particle that takes part in a reaction A species could be an atom, ion, molecule, or empirical formula or electron

Question 45

What are the 3 ways to work out an amount in moles?

Answer 45

From mass : n = mass/Mr From gas volumes : n = V (in dm3) / 24.0 n = V (in cm3) / 24000 From solutions : n = cV (V in dm3)

Question 46

The empirical formula for hydrated magnesium chloride is MgCl2H10O5 Determine the dot formula

Answer 46

MgCl2.5H2O