Chapter 2 - Amount of Substance

Question 1

define relative atomic mass

Answer 1

the average mass of an atom relative to one twelfth of the average mass of a carbon 12 atom
average mass of one atom of an element*12/mass of one carbon-12 atom

Question 2

what is relative molecular mass

Answer 2

the same as relative atomic mass but the average mass of one molecule
you can find this mass by adding up all the relative masses in a molecule

Question 3

what is relative formula mass

Answer 3

a way to measure the mass of one repeating unit in an ionic compound because they dont exist as molecules but rather a giant lattice

Question 4

what is the avogadro constant

Answer 4

6.022 x 10^23

Question 5

what is the significance of the avogadro constant

Answer 5

the number of atoms or molecules in one mole of a substance
one mole of a substance is equal to the Mr or Ar in grams
The Avogadro constant or Avogadro number is the number of atoms in 12 g of carbon-12.

Question 6

how can you find the mass of one atom of a substance in Kg

Answer 6

(mass/1000)/6.022*10^23

Question 7

what is a solute

Answer 7

the substance that is dissolved in a solvent

Question 8

what is a solvent

Answer 8

the substance that dissolves the solute

Question 9

what is a solution

Answer 9

a combination of a dissolved solute and a solvent

Question 10

what does concentration indicate

Answer 10

the amount of a substance in one dm^3 of said substance

Question 11

what is boyles law

Answer 11

as long as the temperature is constant, the product of the pressure and the volume is constant

Question 12

what is charles’ law

Answer 12

The volume is proportional to the temperature as long as the pressure
remains constant
pressure/temperature = constant

Question 13

what is the constant volume law

Answer 13

The pressure is proportional to the temperature as long as the volume
remains constant.
pressure/temperature = constant

Question 14

what is the ideal gas equation

Answer 14

pV=nRT

Question 15

what is back titration

Answer 15

a substance of an unknown concentration is titrated with a substance of a known concentration in excess.
the resulting mixture is titrated with another substance of known conc.
the moles reacted can be subtracted from the total moles used to find how many moles reacted in the initial reaction.
this can then be used to find the concentration of the unknown substance.

Question 16

when are back titratoins used

Answer 16

when the reaction is slow, the substance is not pure, the reaction is difficult to observe, the analyte is a non-soluble solid

Question 17

when do we use relative formula mass

Answer 17

when we have a giant covalent or ionic lattice

Question 18

define a mole

Answer 18

6.022 x 10^23 units of a substance

Question 19

define concentration

Answer 19

the amount of a solute (in g or mol) per unit volume (always dm^3)

Question 20

what assumptions are made for the motion of gas particles

Answer 20

Gas particles move rapidly and randomly.
The volume of the actual gas particles is negligible compared to empty space between them.
There are no intermolecular attractions or repulsions.
Collisions between particles are elastic - no energy is lost.
The average kinetic energy of particles depends only on their temperature.

Question 21

what is an ideal gas

Answer 21

a gas which obeys all the assumptions made in kinetic theory and can be exactrly represented using the ideal gas equation at room temp and pressure

Question 22

what is the difference between the empirical and molecular formula

Answer 22

empirical formula gives the lowest whole number ratio of elements present in a compound while the molecular formula shows the actual number of atoms of each element present in a compound

Question 23

how can you calculate empirical formula from percentage composition by mass

Answer 23

assume the total sample size is 100g
find the moles of each element present by dividing the percentage by the Ar
find the lowest whole number ratio of the moles

Question 24

how can you find the molecular formula from the empirical formula

Answer 24

calculate the empirical formula mass
then divide the molecular formula mass by the empirical formula mass
multiply the ratio of atoms in the empirical formula by the resulting number

Question 25

how do you write an ionic equation

Answer 25

write out each ion present (remember that only aqueous solutions will separate in to ions, keep solids, liquids and gases the same)
cancel out ions that appear on both sides of the equation
check the charges are balanced

Question 26

what is the limiting reactant and what is its significance

Answer 26

the reactant which runs out first, the one that doesnt is “in excess”
the limiting reactnace puts a “limit” on the amount of products that can be formed.

Question 27

what does atom economy measure

Answer 27

how efficient a chemical reaction is at incorporating the reactants into the desired product
atom economy measures the percentage of reactant atoms that end up in the final desired product

Question 28

equation for atom economy

Answer 28

mr of desired product / mr of all reactants including their mole ratios x 100

Question 29

what is the significance of atom econoimy

Answer 29

a high atom economy is more sustainable because it generates less waste and uses raw materials more efficiently. it also reduces costs on separating products and requires lower amounts of reactants.

Question 30

what is theoretical yield

Answer 30

the maximum mass of a product that could be made in ideal conditions

Question 31

why is the actual yield always less than the theoretical

Answer 31

some material may not react completely
some products may be lost if they are gaseous, or when transferring substances between containers
side reactions may occur, using up some reactants

Question 32

formula for percentage yield

Answer 32

actual yield / theoretical yield x 100

Question 33

what does percentage yield indicate

Answer 33

how efficiently a reaction is carried out
how close the actual mass produced is to the maximum mass produced