Topic 5 - Formulae, Equations and Amounts of Substance

• The empirical formula of a compound shows the smallest whole number ratio of atoms of each element within the compound. In order to calculate this, divide the mass/percentage composition of each element by the relative atomic mass (mass/Ar). Next, divide all these numbers by the smallest number to give whole numbers, which form the empirical ratio.
• The molecular formula of a compound shows the actual numbers of the atoms of each element in the compound. An example is hydrogen peroxide, which has a molecular formula of H2O2, however it has an empirical formula of HO, which is the smallest ratio of atoms in the compound.
• The mole is a unit of measurement for substances, and one mole of any substance always contains the same number of particles within in. This is known as the Avogadro's constant, which states that one mole of any substance will contain 6.022 X 10^23 particles within it. Therefore, the number of particles within a substance can be calculated by multiplying the moles by Avogadro's constant (moles X 6.022 X 10^23).
• The moles of a substance can be calculated using the equation moles = mass/Mr, where Mr is relative molecular mass, and mass is measured in grams. Relative molecular mass (Mr) is the mean mass of a compound, divided by 1/12 of the mean mass of an atom of the carbon-12 isotope.
• The molar mass of a substance is the mass in grams per mole, and has the units g mol^-1. The relative atomic mass (Ar) is the mean mass of an atom of an element, divided by 1/12 of the mean mass of an atom of the carbon-12 isotope.
• When discussing an ionic compound, we refer to the relative molecular mass (Mr) as relative formula mass instead.
• An alternative equation to calculate the moles of a substance involves multiplying the volume (dm^3) by the concentration (mol dm^-3). If the volume is given in cm^3, a conversion is required. 1000cm^3 = 1dm^3.
• The ideal gas equation is pV = nRT. In this equation, p represents pressure in pascals (Pa), V represents volume in metres cubed (m^3), T represents temperature in Kelvin (K), n represents moles, m represents mass in grams (g), and R represents the ideal gas constant (8.31 J K^-1 mol^-1). To convert from Celsius to Kelvin, you need to add 273 to the Celsius temperature. To convert from cm^3 to dm^3 to m^3, you divide by 1000 each time.
• The formula for sodium hydroxide is NaOH. The formula for nitric acid is HNO3. The formula for iron (II) sulfate is FeSO4. The formula for iron (III) oxide is Fe2O3. The formula for calcium carbonate is CaCO3.
• In order to write a chemical equation, firstly write down a word equation for the reaction (the reactants and products). Then, write in the correct formulae for each reactant/product. Next, you need to balance the coefficients of the symbol equation to ensure that there are equal numbers of atoms of each element on either side. You can use whole numbers or decimals (nearest 0.5) as the coefficients.
• State symbols are used in symbol chemical equations. (s) is the state symbol for a solid, (aq) is the state symbol for an aqueous solution (dissolved in water), (l) is the state symbol for a liquid, and (g) is the state symbol for a gas.
• Balanced chemical equations can be used to calculate reacting masses, percentage yield and atom economy. Ionic equations only show particles that are undergoing a change during a reaction and not the spectator species. Ionic equations must also be balanced. Spectator ions are known as ions which don't take part in a reaction, and so are left out of ionic equations.
• In order to achieve ionic equations from full equations, firstly write out the equation in full. Next, replace the formulae of ionic compounds with their individual ions. Finally, cross out any ions that appear the same on both sides.
• The percentage yield of a reaction indicates how much of the maximum amount of product you obtained during an experiment. A low percentage yield could indicate an incomplete reaction (i.e. if the reaction is reversible), or the loss of product during purification. It could also be due to other side-reactions that lead to unwanted products being produced.
• The equation for percentage yield of a reaction is (actual yield/theoretical yield) X 100. The theoretical yield of the reaction is the maximum amount of the desired product that could be produced if the reaction was fully complete and had no losses (i.e. 100% yield). The actual yield is the weighed amount of product actually produced in an experiment.
• The atom economy of a reaction is a measure of the efficiency, since it measures the proportion of reactant atoms that are converted into the desired product. The equation for percentage atom economy is (Mr of desired product/total Mr of reactants) X 100.
• In industrial chemical processes, it is desirable to have a high atom economy (as close to 100% as possible), since this means that there is less waste and more desired product. This means that reaction is more economical. Other factors must also be considered, such as the availability of raw materials, the cost of raw materials and the quantity of energy needed.
• You can also calculate moles from the molar mass by using the equation moles = mass/molar mass, where mass is measured in grams (g) and molar mass is measured in (g/mol^-1).
• Reacting masses show how much of one reactant reacts with another in an experiment. A hydrate is a compound that contains molecules of water of crystallisation.
• Avogadro's Law states that 'provided that the conditions of temperature and pressure are constant, equal volumes of gas will contain the same number of molecules'. Therefore, one mole of any gas will always occupy the same volume.
• Molar volume is the volume occupied by one mole of any gas. The molar volume is constant for any gas unless the temperature or pressure changes. Room temperature/pressure (RTP) is given as 25 degrees Celsius and 1 atm pressure. Molar volume at RTP is 24.0 dm^3 mol^-1. 1 atm pressure is equivalent to 101 325 Pa. The equation for molar volume is (24.0 = volume/moles), where volume is measured in dm^3.
• Mass concentration is the mass of solute per volume of solution. You can calculate the mass concentration using the equation mass concentration = mass of solute/volume of solution, where mass concentration is measured in g dm^-3, mass is measured in grams (g), and volume is measured in dm^3.
• Molar concentration is the moles of solute per volume of solution. You can calculate the molar concentration using two equations. Firstly, use moles = mass/Mr. Next, use the equation (molar) concentration = moles/volume, where volume is measured in dm^3.
• A solute is a substance that is dissolved. A solvent is a substance that dissolves a solute. A solution is a solute dissolved in a solvent.