Calculations Involving Masses Flashcards
All 20 cards in this deck
What is the relative formula mass () of a compound?
The sum of the relative atomic masses of all the atoms shown in its formula.
What is the equation for the percentage by mass of an element in a compound?
percentage by mass = (relative atomic mass × number of atoms of that element in the formula) ÷ relative formula mass × 100
True or false? Relative formula mass is measured in grams.
False. Relative formula mass is a comparison of masses, so it has no units.
What is an empirical formula?
The formula showing the simplest whole-number ratio of atoms of each element in a compound.
What is the empirical formula of butene, ?
— divide both numbers by 4 to get the simplest ratio.
How is the molecular formula related to the empirical formula and the relative molecular mass?
Multiply every number in the empirical formula by (relative molecular mass ÷ mass of the empirical formula unit).
True or false? For some compounds the empirical formula and the molecular formula are the same.
True. If the formula is already the simplest ratio they are the same, e.g. and .
How do you find the ratio of atoms for an empirical formula from reacting masses?
Divide the mass of each element by its relative atomic mass, then turn the answers into the simplest whole-number ratio.
In the magnesium oxide experiment, why is the crucible lid lifted only briefly during heating?
To let oxygen in while stopping white magnesium oxide smoke from escaping.
True or false? If some white smoke escapes from the crucible, the measured mass of oxygen gained is too low.
True. Magnesium oxide is lost, so the final mass is too small.
State the law of conservation of mass.
No atoms are lost or made in a chemical reaction, so the total mass of the products equals the total mass of the reactants.
What happens to the balance reading when a precipitation reaction is carried out in a closed flask?
It stays the same — nothing is lost and nothing is gained.
Why does the mass increase when magnesium is burned in an open crucible?
Oxygen from the air combines with the magnesium and is added to its mass.
True or false? When a reaction in an open flask gives off a gas, mass has been destroyed.
False. The gas escapes into the air, so the flask weighs less, but the total mass is unchanged.
How is the mass of a product related to the mass of a reactant in a balanced equation?
mass of product = mass of reactant × (total relative formula mass of the product ÷ total relative formula mass of the reactant), using the balancing numbers from the equation
In a reacting mass calculation, what do you multiply a relative formula mass by?
e.g. in a balanced equation
The balancing number in front of the formula, e.g. .
Can a reacting mass calculation be done in tonnes or kilograms instead of grams?
Yes — the mass ratio is the same in any unit, as long as both masses use the same unit.
True or false? In , 2 g of magnesium forms 2 g of magnesium oxide.
False. The oxide also contains the oxygen that reacted, so its mass is greater than the mass of magnesium.
What is the equation for the concentration of a solution in g dm⁻³?
concentration (g dm⁻³) = mass of solute (g) ÷ volume of solution (dm³)
How many cm³ are there in 1 dm³?
1000 cm³ (1 dm³ is the same as 1 litre).