4 - Moles Flashcards
What is an atomic unit?
The unit we use to show the mass of a substance or atom
What is an atomic unit equivalent to?
1/12 the mass of an atom of carbon-12
Define relative isotopic mass
The mass of an isotope compared with 1/12 of the mass of an atom of carbon-12
What is relative isotopic mass always the same as?
The mass number of the isotope
Define relative atomic mass (Ar)
The weighted mean mass of an atom compared with 1/12 of the mass of an atom of carbon-12
How do you calculate the Ar of an element?
You do (abundance * relative isotopic mass) for each isotope of the elements You add all of these values together
How could the abundances and isotopic masses be shown to us in an exam question?
A mass spectrum graph
Describe the mass spectrum
It is a stick graph with abundance on the y axis and isotopic mass on the x axis
Define relative molecular mass (Mr)
The sum of all of the relative atomic masses in a substance (could be a compound)
What is the Avogadro constant value and What does the it represent?
N~A
6.02 * 10^23
it represents the amount of atoms (or molecules if it’s polyatomic) per mole of a substance
How much of a substance is defined as a mole?
The amount of substance there is when there are 6.02 * 10^23 molecules of the substance
What is the molar mass and its unit?
The mass of one mole of a substance in grams.
units: g/mol
What is the molar mass equivalent to?
The relative molecular mass (Mr) in g/mol
what are the mole equaltions?
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)
What is the equation linking the number of atoms (or molecules) with the number of moles?
Number of atoms (or molecules) = N~A * n
How many moles are there in 12g of carbon-12?
n = m / Mr
= 12g / 12g/mol
= 1 mole
How many atoms are there in 2 moles of carbon-12?
No. of atoms = N~A * n
= (6.02 10^23) 2
= 1.204 * 10^24 atoms
Define empirical formula
The simplest whole number ratio of atoms of each element present in a compound
Define molecular formula
The actual number of atoms of each element in a molecule
When might the empirical formula be incorrect?
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
How do you calculate empirical formula?
- 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
define atom economy
Atom economy tells us the percentage of all of the products of a reaction which are useful products
If you calculate the empirical formula but the Mr is different than the one stated in the question, what do you do?
Multiply up the numbers of atoms in the substance so that the Mr is correct
Define atom economy
Atom economy tells us the percentage of all of the products of a reaction which are useful products
What is the formula for atom economy?
(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%
What are the 3 advantages of high atom economy?
Large amount of useful products produced
Few unwanted waste products
Makes the best use of natural resources (sustainable)
What are the 4 factors that make a reaction sustainable?
Atom economy
Readily available reactants
Low energy costs
Percentage yield
What is the easiest way to significantly increase atom economy?
Find a use for unwanted products
They are then desired products
What is a hydrated salt?
A salt that contains water of crystallisation inside its structure
How do you remove water of crystallisation from a salt?
Heat the salt until the water separates from the salt as a liquid
How do you show the water of crystallisation inside a salt in an equation?
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)
What is an anhydrous salt?
Anhydrous describes the salt after the water of crystallisation has been removed
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)
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
What is the rule to do with gases, moles and volume?
Every gas in existence takes 24dm^3 if there is 1 mole of the gas at RTP
What are the values for RTP?
25C temp
101kPa pressure
What is stoichiometry?
Using the ratio of moles in an equation to calculate masses
What is the difference between relative formula and relative molecular mass?
Both are Mr
Formula mass is for compounds (sodium chloride)
Molecular mass is for molecules (water)
What is molar volume?
The volume taken up by one mole of a substance
How do you translate between centigrade and kelvin?
Centigrade + 273 = Value in kelvin
What is the concentration?
The amount of solute per unit volume
mol dm^-3
or
g dm^-3
What is a standard solution?
A solution that has a precisely known concentration so that it can be used in equations after doing a reaction with it
Describe how you find the concentration of an acid or alkali after doing a titration
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
What is mass concentration?
Mass concentration of a solution is the MASS of solute dissolved in 1 dm^3 of solvent
How do you calculate mass concentration?
Mass concentration (g dm^-3) = concentration (mol dm^-3) * Mr of solute
Similar to m = n * Mr
What is a limiting reactant?
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
How do you deal with limiting reactants?
You calculate the number of moles for each reactant
You ensure the products have the number of moles per molecule of the limiting reactant
