Moles Flashcards
RAM
Relative Atomic Mass: the average mass of one atom of an element compared to the mass of a carbon-12 atom
How do you calculate the RAM?
Calculate the weighted average of the isotopes:
- Multiply each isotope’s mass by its abundance
- Add all the totals together
- Divide this total by the total abundance number (may not be 100)
Isotopes
Atoms of the same element, with the same number of protons but a different number of neutrons
Calculate the RAM of boron, given that a sample of born is found to be composed:
20% has an isotopic mass of 10.0
80% has an isotopic mass of 11.0
RAM = 10 * 0.2 + 11 * 0.8 RAM = 10.8 g/mol
RFM
Relative Formula Mass: the sum of all the RAMs for each of the elements in the formula
Calculate the RFM of water
*H = 1, O = 16
RFM = 2 * 1 + 1 * 16 RFM = 18 g/mol
Calculate the RFM of ammonia
*N = 14
RFM = 1 * 14 + 3 * 1 RFM = 17 g/mol
How would you calculate the percentage composition of element A in compound B?
% A = (RAM of A * number of A atoms in B) / RFM of B * 100
Mole
The amount of a substance that contains Avogadro’s Number’s worth of particles (atoms, molecules, ions, electrons, etc.)
Avogadro’s Number
6.02 * 10^23
How many particles are there in?
- 1 mole
- 10 moles
- 5 moles
- 6.02 * 10^23
- 6.02 * 10^24
- 3.01 * 10^24
What is the mass of one mole of a substance?
The substance’s RFM, measured in grams
What mass would 5 moles of oxygen (RFM = 32) have?
Mass = 32 * 5 Mass = 160g
What equation links mass, RFM and moles?
Mass = RFM * # of moles
How many moles are there in 54g of water?
RFM = 18 g/mol
54 / 18 = 3 moles
0.02 moles of a compound is found to have a mass of 1.64g. Find the formula mass of the compound.
RFM = mass / # of moles RFM = 1.64 / 0.02 RFM = 82 g/mol
Steps of experiment to measure the change in mass when magnesium burns
- Clean the magnesium ribbon and loosely coil
- Weigh a clean crucible and lid. Place the magnesium inside and reweigh
- Heat the crucible for 5–10 minutes, lifting the lid a bit from time to time using tongs. Ensure that as little product escapes as possible
- Continue heating until glowing ceases (reaction complete)
- Cool the crucible and reweigh
How would you find the masses needed to calculate the empirical formula of magnesium oxide, using the following values from the change in mass experiment?
- Mass of crucible
- Mass of crucible and magnesium
- Mass of crucible and magnesium oxide
- Find mass of magnesium by taking value #1 away from #2
2. Find mass of oxygen by taking #2 away from #3
Empirical formula
The simplest ratio of atoms in a compound
What is the empirical formula of H₂O₂?
HO
Steps to calculate the empirical formula
- Determine mass or percentage of elements
- Divide the given masses or percentages by mass by their respective RAMs
- Divide the results by the smallest value
- Convert into the simplest whole number ratio
What is the empirical formula of a compound made up of 20% calcium and 80% bromine by mass?
*Ca = 40, Br = 80
Ca: 20 / 40 = 1/2 Br: 80 / 80 = 1
Ca: 0.5 / 0.5 = 1 Br: 1 / 0.5 = 2
(Both already integers)
1 : 2 OR CaBr₂
The empirical formula of sodium chloride is NaCl. It does not have a molecular formula. Why is this the case?
Sodium chloride does not exist as a molecule. It is an example of a giant ionic lattice
Steps to find mass of A where the mass of B is known
- Calculate RAM of B (the chemical we have the most info about)
- Calculate # of moles of B
- Calculate # of moles of A using the molar ratio between A and B
- Calculate RAM of A
- Calculate mass of A
If 25g of calcium carbonate thermally decomposes, how much calcium oxide is produced?
CaCO₃ → CaO + CO₂
*Ca = 40, O = 16
CaCO₃ CaO Mass 25g 14g RFM 100 56 Moles 0.25 0.25 Ratio 1 1
14g of calcium oxide is produced
Avogadro’s Law
Equal volumes of gases at the same temperature and pressure contain the same # of molecules
How much space does one mole of a gas occupy at rtp?
24dm³
What is rtp?
Room temperature and pressure
How much is 1 litre in:
- dm³?
- cm³?
- 1dm³
2. 1000cm³
Gas moles equation in:
- dm³
- cm³
- Volume = moles * 24
2. Volume = moles * 24,000
Calculate the volume of 0.01g of hydrogen gas at rtp in cm³
Moles = 0.01 / 2 Moles = 1/200 Volume = moles * 24,000 Volume = 1/200 * 24 = 120cm³
What volume of CO₂ is produced in the thermal decomposition of 8g of CaCO₃?
CaCO₃ → CO₂ + CaO
*C = 12, O = 16, Ca = 40
CaCO₃ CO₂ Mass 8g RFM 100g Moles 0.08 0.08 Ratio 1 1
Volume = 0.08 * 24,000 Volume = 1920cm³
Percentage yield
The efficiency of a chemical reaction
Why might some reactions not react fully?
- Loss of solid or liquid when transferring container
- Side reactions occurring
- Reversible reactions
Theoretical yield
The quantity of a product that would be obtained from a complete reaction
Actual yield
The amount of product actually obtained in a chemical reaction
Equation to calculate percentage yield
Percentage yield = actual yield / theoretical yield * 100
In a reaction where the theoretical yield of magnesium oxide is 20g, only 14g is produced. What is the percentage yield of this reaction?
Percentage yield = 14/20 * 100
Percentage yield = 70%
Atom economy
A measure of the amount of starting materials that become useful products
Equation for atom economy percentage
Atom economy % = mass of useful products / total mass of reactants * 100
One mole of lead (II) iodide is formed in a reaction. Given that the RFM of lead (II) iodide is 461 g/mol and the total mass of reactants is 463g, what is the atom economy of the reaction?
Atom economy % = 461/463 * 100
Atom economy % = 99.6%
N₂ + 3H₂ → 2NH₃
Work out the atom economy for the above reaction, given that ammonia is the useful product.
Atom economy % = 2 * (14 + 3 * 1) / (2 * 14 + 3 * 2 * 1) * 100
Atom economy % = 17/17 * 100
Atom economy % = 100%
