Calculations Involving Masses Flashcards
(24 cards)
What is relative formula mass (Mr) and how do you calculate it?
Relative formula mass is the sum of the relative atomic masses (Ar) of all the atoms in a formula. You calculate it by adding together the Ar of each element in the compound, using the periodic table.
How do you calculate the empirical formula of a compound from the molecular formula?
Find the highest common factor of the number of atoms in the molecular formula. Divide each number of atoms by that factor to find the simplest whole-number ratio.
How do you calculate the molecular formula from the empirical formula and Mr?
Calculate the Mr of the empirical formula. Divide the compound’s actual Mr by the empirical Mr. Multiply each atom in the empirical formula by the result.
What is the law of conservation of mass?
Mass is neither created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.
Why does mass appear to increase in a reaction?
A gas from the air reacts to form a product, increasing the total mass. This happens in an unsealed container where the gas is not initially measured.
Why does mass appear to decrease in a reaction?
A gas is produced and escapes into the air. This reduces the mass of measured substances if not done in a sealed container.
How do you calculate the mass of substances in a reaction using balanced symbol equations?
Use the ratio of substances from the balanced equation. Convert between mass and moles using Mr, then scale accordingly.
How do you calculate the empirical formula from masses of elements in a compound?
Divide the mass of each element by its Ar to find moles. Divide each mole value by the smallest one to get a whole-number ratio.
How do you calculate the empirical formula from percentage composition?
Treat percentages as masses out of 100g. Follow the same steps as using actual masses to find mole ratio.
(6 marker) How do you carry out an experiment to determine empirical formula using a metal in a crucible?
Weigh a crucible with lid, then add a metal like magnesium and reweigh. Heat the crucible strongly with lid slightly ajar to allow oxygen in. Cool and reweigh the crucible and contents. Calculate the mass of metal and oxygen to find mole ratio and deduce empirical formula.
How do you calculate the concentration of a solution in g/dm³?
Use the formula: concentration = mass of solute ÷ volume of solution. Make sure volume is in dm³ (1000 cm³ = 1 dm³).
How do you calculate the mass of solute in a volume of solution using concentration?
Use the formula: mass = concentration × volume. Ensure volume is in dm³ before multiplying.
How do you calculate the mass of product formed from a balanced symbol equation?
Calculate the moles of the known reactant. Use the mole ratio to find moles of the product. Multiply moles by Mr to get mass of product.
How do you calculate the mass of a reactant needed to make a given mass of product?
Calculate the moles of product using mass ÷ Mr. Use the balanced equation to find moles of reactant. Multiply by Mr of the reactant to find its mass.
What is a limiting reactant?
The reactant that is used up first in a chemical reaction. It limits the amount of product that can be formed.
How does the limiting reactant control how much product is formed?
The amount of product formed is directly proportional to the amount of limiting reactant. Once it is used up, the reaction stops regardless of excess other reactants.
What is the Avogadro constant?
The number of particles (atoms, ions or molecules) in one mole of a substance. It is equal to 6.02 × 10²³.
What does one mole of a substance represent?
One mole is the relative formula mass in grams. It contains 6.02 × 10²³ particles.
How do you convert mass to moles?
Use the formula: moles = mass ÷ Mr. Ensure mass is in grams and Mr is the relative formula mass.
How do you convert moles to mass?
Use the formula: mass = moles × Mr. This gives you the mass in grams.
How do you convert number of particles to moles?
Divide number of particles by Avogadro’s number (6.02 × 10²³). Use: moles = particles ÷ 6.02 × 10²³.
How do you convert moles to number of particles?
Multiply moles by Avogadro’s number. Use: particles = moles × 6.02 × 10²³.
What does a balanced chemical equation show in terms of moles?
The coefficients show the ratio of the number of moles of each substance. These ratios are used to scale masses and predict quantities.
How do you balance a chemical equation using masses?
Convert given masses into moles using Mr. Use mole ratios to balance the equation with smallest whole numbers.
