Chemical Formulae, Equations And Calculations Flashcards
What is the relative formula mass?
This is found by adding the relative atomic masses of the atoms or ions in that compound.
What is a mole?
A unit to describe a certain number (Avogadro’s number: 600 000 000 000 000 000 000 000 or 6 x 10 to the power of 23) or particles of a certain substance. It was chosen so that one mole of carbon-12 would weigh exactly 12g.
What is the equation to calculate mass of an atom or compound (solids and liquids)?
For atoms: mass (in grams) = moles x Ar For compounds: mass (g) = moles x Mr
How do you calculate percentage yield?
IPY = actual mass of product mad in experiment —————————————————— X 100% Maximum theoretical yield of product
Why is it not always possible to obtain the calculated amount of a product? What can go wrong?
- The reaction may not go to completion because it is reversible (year 11). - Some of the product may be lost when it is separated from the reaction mixture. - Some of the reactants may react in ways different from the expected reaction.
How does one carry out an experiment for water of crystallisation (dehydration)?
- Weigh an empty evaporating basin. 2. Add five heaped spatula measures of hydrated copper (II) sulphate and weigh again. 3. Gently heat and stir continually with a glass rod (it is very important not to overheat the copper (II) sulphate). 4. When the colour change appears to be complete, stop heating, allow the basin to cool and weigh again. 5. If you have time, heat the basin for further 2 minute periods until the mass is constant.
How can the formulae of simple compounds then be obtained experimentally, including metal oxides, water and salts containing water of crystallisation? (How do you calculate the ratio of number of salt molecules to water molecules?).
Eg. Copper sulphate. - Calculate the number of moles (mass/Mr) of the CuSO4 (using the mass after heating: anhydrous) and H20 (mass of hydrated CuSO4 - mass of anhydrous CuSO4). - Divide both by the smaller number of moles to find the ratio. - Put the ratio to the nearest whole number. - Replace the ‘x’ (unknown number of water molecules to salt molecules) with the ratio.
Define empirical formula and molecular formula.
Empirical formula: The smallest whole number ratio of atoms of each element in a compound. Molecular formula: The actual whole number of atoms of each element in a compound.
How does one calculate empirical formulae from experimental data?
- Prepare a table with each component of the molecule as the columns and the mass, Ar, moles and ratio as the rows. - Fill in the mass of each atom that makes up the molecule (should be given in the question, either as a mass in a specific unit (eg. g, kg etc.) or a percentage, with which you assume that mass of the molecule is 100). - Find the Ar of the atoms using the periodic table (the top number in each box). - Calculate the number of moles of each atom by dividing the mass by the Ar. - Calculate the nearest whole number ratio by dividing each of them by the smallest number of moles.
How does one calculate molecular formulae from experimental data?
- Calculate the empirical formula.
- Calculate the Mr of the EMPIRICAL formula using the periodic table.
- From this, and the actual Mr of the formula (given in the question), work out the molecular formula so that it corresponds to the real Mr.
How does one calculate gas volumes?
Volume = 24 x moles (24 because 1 mole of gas occupies 24 000cm3 or 24dm3 at room temperature and pressure (rtp)).
How does one calculate the mass of a specific substance in an equation?
Strategy:
- Identify the ‘!’ substance (mass of this substance already given in the question) and the ‘?’ substance (the actual question).
- Find the mole connection (ratio) between ‘!’ and ‘?’.
- Find the moles of ‘!’ using the equation: moles = mass/Mr
- Use the mole connection to find the moles of ‘?’.
- Use the equation triangle to find the answer (the mass).
NB, when finding the Mr, ignore the coefficient (‘big number’) eg. 3 H20: ignore the ‘3’.
How do you carry out a combustion of magnesium experiment?
- Weigh a crucible and lid.
- Clean the magnesium ribbon by rubbing its surface with sandpaper.
- Add about 5cm3 of clean magnesium ribbon and weigh again.
- Place the crucible on a pipe clay triangle.
- Heat the crucible strongly, lifting the lid every so often to allow more air into the crucible (allowing the reaction to continue, and ensuring that all the Mg reacts and oxidises as more oxygen is allowed to enter).
- Do not allow and gas to escape- this is fine particles of magnesium oxide, and its escape would affect the final mass.
- When the reaction appears to be over, remove the lid and heat strongly for another two-three minutes.
How do you carry out a reduction of copper oxide experiment?
- Weigh an empty reduction tube.
- Add two heaped spatula measures of copper oxide in the position shown in the diagram (you will need to keep the tube horizontal).
- Set up the apparatus.
- Allow methane to flow through the tube for a minute before lighting the gas (this is to ‘flush out’ the air- specifically oxygen).
- Heat strongly.
- When the colour change appears to be complete, stop heating but keep the flame at the end of the tube burning.
- When the tube is cool turn off the flame, then weigh the tube.
- Heat for a further two minutes.
- Repeat the last two steps until the mass remains constant.
How do you determine the formula of magnesium oxide by combustion?
- Find the mass of the magnesium and oxygen.
- Find the Ar of the magnesium and oxygen.
- Find the moles of the magnesium and oxygen (moles = mass/Ar).
- Find the mole ratio of the magnesium and oxygen.
- Use this to find the formula.
