Further Calculations

An Overview

• - The relative atomic mass is defined as follows : The weighted average of all the element's natural isotopes on a scale where the mass of Carbon-12 is exactly 12.
• - The relative atomic mass of an element or the relative formula mass of a compound is always equal to the mass of one mole of the substance in grams.
• - A mole of a substance is defined as being 6.02*10^23 atoms/molecules of that substance.
• - The molar volume of any gas is always the same and one mole of gas occupies 24dm^3 or 24000cm^3.
• - Empirical formulae are the most simple ratio of atoms of each element in a compound, while the molecular formula gives the actual numbers of each element in a compound. For instance, the molecular formula for Hydrogen Peroxide is H2-O2, while its empirical formula is H-O.
• - To work out the relative molecular mass of compund, simple add the relative atomic mass of its constituent elements. For instance, the Mr of H2-O is the 2 lots of the Ar of Hydrogen (1*2=2) plus 1 lot of the Ar of Oxygen (1*1=1).

Moles

• - To work out the number of moles of moles in a certain mass of a known substance, simply divide the mass of the substance by its Ar/Mr.
• - To work out the mass of a certain number of moles of a known substance, simply multiply the number of moles of the substance with the Ar/Mr.
• - To work out the Ar/Mr of an unknown mass of a substance, and knowing the moles, simply divide the mass of the substance with the number of moles of that substance.
• FOR THE FOLLOWING, THE VOLUME AND MOLAR MUST USE THE SAME MEASUREMENT!!!
• - Molar Volume = 24dm^3 or 24000cm^3 at 1atm and at 25C
• - To work out the number of moles in a certain volume of a known gas, simple divide the volume by the Molar Volume.
• - To work out the volume of a certain known gas, simply multiply the number of moles of the gas with the Molar Volume.
• - You don't need to work out Molar Volume as it is constant!

Percentage Yields

• - Percentage Yield is a measure of the efficiency of a chemical reaction.
• - Theoretical Yield is how much of a product you should get.*
• - Experimental Yield is how much of a product you do get.*
• *You can measure this in mass or moles, but you must be consistent.
• - To work out the Percentage Yield simply divide the Experimental Yield by the Theoretical Yield and then multiply the answer by 100, making the answer a percentage.
• - To work out the Experimental Yield simply multiply the Percentage Yield with the Theoretical Yield and then divide the answer by 100.
• - To work out the Theoretical Yield simply multiply the Experimental Yield by 100 then divide the answer by the % yield.