Topic 1.41-1.53 - Calculations involving masses Flashcards
What are the limitations of dot and cross models?
In covalent molecular, dot-cross diagrams don’t indicate the relative sizes of the atom or express the relative attraction of shared electrons due to electronegativity
What are the limitations of ball and stick models?
-Don’t showcase the movement of electrons
-The gaps between atoms are bigger than they would be in real life
What are the limitations of 2 and 3 dimensional representations?
-2D diagrams don’t show the 3D arrangement of atoms
-3D diagrams don’t show the share or transfer of electrons
Describe the properties that most metals have
Most metals are shiny solids which have high melting points, high density and are good conductors of electricity
Describe the properties that must non-metals have
Most non-metals have low boiling points and are poor conductors of electricity
What is the calculation for relative formula mass based on relative atomic masses?
-Work out how many atoms of each element are in the chemical formula.
-Multiply how many atoms of an element there are by the Ar (RAM) value.
-Repeat for each element and add together
How do you calculate the empirical formula of simple compounds?
-Work out moles of each element using moles = mass/relative atomic mass
-Work out the ratio of moles
-Times the ratio so that you get the smallest whole numbers possible
-Find the formula by putting this result as a little number next to its element. Combine the elements into one formulae.
(e.g. Fe₂O₃)
How do you deduce the molecular formula of a compound from its empirical formula and its relative molecular mass?
-Find the molecular mass (relative formula mass) of the empirical formulae
-Divide the actual relative formula mass by the molecular mass of the empirical formula
-Multiply the numbers in the empirical formula by 3
(e.g. Empirical formula is CH₂ and its Mr is 42. The molecular formula is C₃H₆.)
How do you deduce the empirical formula of a compound from the formula of its molecule?
-If you have a common multiple, the empirical formula is the simplest whole number ratio e.g. Fe₂O₄ –> FeO₂
-If there is no common multiple, you already have the empirical formula
Describe the experiment to determine the empirical formula of a simple compound such as magnesium oxide
1.Measure and record the mass of an empty crucible with its lid
2.Put a length of magnesium ribbon into its crucible
3.Measure and record the total mass of the crucible, its lid and contents.
4.Place the crucible on a tripod with a pipe clay triangle.
Strongly heat the crucible for several minutes using a Bunsen burner
5.When the magnesium has stopped flowing, turn off the Bunsen burner and allow the crucible to cool down.
6.Measure and record the crucible with lid and contents once more.
-Calculate the mass of magnesium used by step 3 - step 1.
-Calculate the mass of oxygen gained during heating by step 6 - step 3
-Calculate the empirical formula
Explain the law of conservation of mass applied to a closed system including a precipitation reaction in a closed flask
Law of conservation of mass: no atoms are lost or made during a chemical reaction so the mass of the products = mass of reactants.
Precipitate that forms is insoluble and is solid, as all the reactants and products remain in the sealed reaction container it is easy to show that the total mass is unchanged.
Explain the law of conservation of mass applied to a non-enclosed system including a reaction in an open flask that takes in or gives out a gas
Law of conservation of mass: no atoms are lost or made during a chemical reaction so the mass of the products = mass of reactants.
It does not hold for a reaction in an open flask that takes in or gives out a gas, since mass will change from what it was at the start of the reaction as some mass is lost when the gas is given off.
How do you calculate masses of reactants and products from balanced equations, given the mass of one substance?
-Find moles of that one substance: moles = mass/molar mass
-Use balancing numbers to find the moles of desired reactant to product
-Mass = moles x molar mass(of the reactant/product) to find mass
How do you calculate the concentration of solutions in g dm⎺³?
-Concentration of a solution can be measured in mass per given volume of solution e.g. grams per dm3 (g/dm³)
-Concentration(g dm⎺³) = mass of solute (g)/volume (dm³)
Define one mole of particles of a substance
-The Avogadro constant (6.02 x 10²³ atoms, molecules, formulae or ions) of that substance
-A mass of ‘relative particle mass’ g
How would you calculate the mass of one mole of a substance?
The mass of one mole of a substance in grams is equal to its relative formula mass
What is the equation involving moles and mass?
moles = mass (g)/relative atomic mass
What is the calculation that involves Avogadro’s constant?
number of moles of a substance = moles x Avogadro’s constant
Explain why, in a reaction, the mass of product formed is controlled by the mass of the reactant which is not in excess
-In a chemical reaction with 2+ reactants you will often use one in excess to ensure that all of the other reactant is used.
-The reactant that is not in excess is called the limiting reactant since it limits the amount of product.
-If a limiting reactant is used, the amount of reactant in excess that actually reacts is limited to the exact amount that reacts with the amount of limiting reactant you have, so you need to use the moles/mass of the limiting reactant for any calculations.
What is stoichiometry?
Balancing numbers in front of compounds/elements in reaction equations
How would you deduce the stoichiometry of a reaction from the masses of the reactants and products?
-Convert the masses in grams to moles (Moles = mass/Mr)
-Convert the number of moles to simple integer ratios
