Chemical Calculations with Mass Flashcards
What does the conservation of mass mean?
Atoms cannot be created or destroyed in a chemical reaction
What happens to atoms in a chemical reaction?
They are rearranged into new substances
Why can the mass decrease in a reaction?
- Produced in reaction
- Lost to the surroundings
Why can the mass increase in a reaction?
One of the reactants is a gas - atoms from the gas added to another reactant
What is Mᵣ?
Formula mass = sum of (relative atomic mass of all atoms in the formula)
What is Avogadro’s constant?
The amount of atoms, ions or molecules contained in one mole of a substance:
6.02 x 10²³
How do you calculate moles?
Moles = mass / Mᵣ
Remember: molar ratios, balanced equations!
How can you calculate:
Mass of product formed from a given mass of reactant?
- Balance symbol equation
- Calculate moles (moles = mass/Mᵣ)
- Using symbol equation, work out the number of moles of the substance
- Calculate the mass of the substance using mass = moles x Mᵣ
How can you balance and equation?
- Calculate Mᵣ of all substances
- Calculate number of moles of each substance (moles = mass/Mᵣ)
- Convert to a whole number ratio
- Balance the equation
How do you work out the Mᵣ from the periodic table?
Sum of relative atomic mass
E.g:
H₂. H has an atomic mass of 1 so 1 + 1 = 2.
When is a reactant in excess?
When there is more of it in a reaction than the other reactant(s)
What is the limiting reactant?
The reactant that runs out first in a reaction
How do you work out which reactant is in excess or is a limiting factor?
- Write balanced symbol equation
- Pick a reactant and quantity given in question
- Use ratio of reactants in equation to see how much of other reactant is needed
- Compare value to quantity in question
- Determine which one is bigger (excess) and smaller (limiting)
How do you calculate concentration?
Concentration = mass/volume
