Chemical Calculations Flashcards
What does RAM stand for?
Relative Atomic Mass
Why is RAM used instead of the actual atomic mass for calculations?
It simplifies calculations by providing a standardized mass for elements, due to the small size of individual atomic masses.
What is the definition of Relative Atomic Mass?
The weighted average mass of the isotopes in an element.
Which isotope of carbon is the most common?
C-12
How is the Relative Molecular Mass (Mr) calculated?
Add together the Ar’s of its constituent elements
Calculate the Mr of CH4.
16
Calculation: Mr = 12 + (4x1)
Calculate the Mr of Ca(OH)2.
74
Calculation: Mr = 40 + 2(16 + 1)
What is a mole in terms of its atoms?
1 mole of hydrogen would be 1g, 1 mole of carbon would be 12g.
True or False: The Relative Atomic Mass of an element is a fixed number.
False, it varies based on the isotope it is.
What is the RAM of carbon?
12.011
How is the percentage mass of an element in a compound calculated?
Percentage mass = (Ar of element / Mr of compound) × 100
What is the formula for calculating the molar mass (Mr) of a compound?
Mr = sum of the Ar of all elements in the compound
Calculate the molar mass (Mr) of FeSO₄ given the atomic masses: Fe = 56, S = 32, O = 16.
Mr = 56 + 32 + 4(16) = 152
How do you calculate the percentage mass?
Percentage mass = (Ar of element / Mr of compound) x 100%
What is the percentage mass of Fe in FeSO₄?
Percentage mass of Fe = (Ar of Fe / Mr of FeSO₄) × 100 = (56 / 152) × 100 = 36.8%
What does the term ‘mole’ refer to in chemistry?
A mole is a unit that represents 6.02 x 10²³ particles.
What is Avogadro’s number?
Avogadro’s number is 6.02 x 10²³ particles. It is equal to 1 mole of that element.
What does the molar mass (Mr) represent?
The molar mass is equal to the mass of 1 mole of that substance.
Fill in the blank: 1 mole of an element is its ______ in grams.
Ar
What is the unit for molar mass?
g/mol
What does ‘Mr’ stand for in chemistry?
Molar mass
What is the relationship between mass and moles?
mass = moles x molar mass
True or False: The mass of products is equal to the mass of reactants in a chemical reaction.
True
What is the significance of the molar ratio in reacting masses calculations?
To calculate or deduce moles of the other species
Fill in the blank: No atoms are made or lost during a chemical reaction so the mass of _______ = mass of reactants.
products
How can you do reacting masses calculations?
- Identify the two relevant species (mass given and mass to find)
- Calculate moles of mass given
- Use molar ratio to find moles of other species
- Calculate mass (moles x mr)
In the equation ‘I Mg + 2 HCl → MgCl2 + H2’, what is the mass of H2 if the mass of Mg is 192g?
The Ar of Mg is 24, molar ratio is 1:1, and the Mr of H2 is 2
mass = 16
If you know how many moles are in a substance, how can you calculate the mass?
mass = moles x Mr
What does the symbol ‘Ar’ represent in chemistry?
Relative atomic mass
What does ‘molar ratio’ refer to?
The ratio of moles of reactants to moles of products in a reaction.
What is the definition of the empirical formula?
The empirical formula gives the simplest proportion of elements in a compound, but not its actual numbers.
What is every alkene’s empirical formula?
CH2
How can you calculate the empirical formula?
- Calculate moles of each element
- Work out the simplest molar ratio by dividing by the smallest number (eg 1:2:1)
- Write the empirical formula (eg CH2O) to work out molecular formula (eg C6H12O6)
- Calculate MF / EF and scale up your EF accordingly
When calculating the empirical formula, if you are given a percentage of an element, what should you do?
Assume the percentage is the mass in grams.
What is the formula for percentage yield?
percentage yield = (amount of product produced / max. amount of product possible) × 100%
What are some reasons why percentage yield may not be 100%?
- Incomplete reaction
- Loss in transfer (some reactants left in apparatus)
- Reaction may be reversible (products react to produce reactants again)
- Hard to separate some products which are formed as a mixture
- Gaseous products may escape
What do you try to find in solution calculations?
The amount of solute dissolved in a volume of solvent.
What units can be used for volume in solution calculations?
- dm³
- cm³
What units can be used for mass in solution calculations?
- grams
- moles
What units can be used for concentration in solution calculations?
- g/dm³
- g/cm³
- mol/dm³
What is the relationship between concentration, mass, and volume?
concentration = mass / volume
What is 1 dm³ in cm³?
1 dm³ = 1000 cm³
What is 1 dm in cm?
1 dm = 10 cm
What is a reagent and what do we need them for?
A substance added to a mixture to make a reaction occur.
What is a limiting reagent?
The reactant which is totally consumed during a chemical reaction.
How is the product formed limited by the limiting reagent?
No more product can be formed as the reaction cannot continue without this reagent.
In the reaction with 3 mol of A and 5 mol of B, which is the limiting reagent?
A is the limiting reagent.
This is because 3 mol of A reacts with 3 mol of B to create 3 mol of C, leaving 2 mol of unreacted B.
What is the molar ratio in the reaction 2B + 1A → IC?
1:2:1
This indicates that for every 1 mol of A, 2 mol of B are required to create 1 mol of C.
If you have 8 mol of A and 10 mol of B, how many mol of B are needed for a complete reaction, and so which is the limiting reagent.?
In a ratio of 1:2:1 (A:B:C)
16 mol B, so B is limiting.
The calculation is based on the ratio, where 8 mol A requires 16 mol B to fully react to make 8 mol C.
What is the complete reaction equation for 3 mol A and 6 mol B, giving your answer in terms of C?
In a 1:2:2 ratio
3 mol A + 6 mol B → 6 mol C.
What is the molar volume of a gas?
Not the actual volume, the definition
The volume occupied by 1 mole of any gas at the same temperature and pressure is called the molar volume.
How many moles of any gas occupy the same volume at the same temperature and pressure?
1 mole of any gas occupies the same volume as and contains the same number of molecules as 1 mole of any other gas.
Fill in the blank: The molar volume of a gas is always _______ at room temperature and pressure.
24 dm³ or 24,000 cm³
True or False: The molar volume of a gas varies with the type of gas.
False
What is the volume occupied by 1 mole of gas at room temperature and pressure in cm³?
24,000 cm³
What is water of crystallisation?
Water that is chemically attached to a crystal structure.
It is found in hydrated salts like hydrated copper sulphate.
What is the formula for hydrated magnesium chloride?
MgCl2 • 6H2O
This indicates that magnesium chloride is attached to six molecules of water.
What do we call a substance with no water attached?
Anhydrous
How do you find the ‘x’ in a hydrated salt?
eg. MgCl2 * xH2O
Find the ratio of moles of the salt to the moles of water
This involves calculations based on mass and molar mass.
Fill in the blank: The term for the simplest molar ratio in a compound is _______.
Empirical formula
It represents the simplest whole-number ratio of atoms in the compound.
