# How Much?

• Created by: DeanJowl
• Created on: 06-11-12 20:18
• How Much?
• The mass of atoms
• The relative mass of protons and neutrons is 1.
• The atomic number of an atom is its number of protons.
• The mass number of an atom is the total number of protons and neutrons in the necleus.
• Isotopes are atoms of  the same element with different numbers  of neutrons.
• Masses of atoms and Moles
• We compare the masses of atoms by measuring them relative to atoms of Carbon-12
• We worl out the relative formula mass of a compound by adding up the relative atomic masses of the elements in it, in the ratio shown by its formula.
• One mole of any substance is its relative formula mass, in grams.
• Percentages and formulae
• The relative atomic masses of the elements in a compound and its formula can be used to work out its percentage composition.
• We can calculate empirical formulae given the masses or percentage composition of elements present.
• Equations and calculations.
• Balanced symbol equations tell us the number of moles of substance involved in a chemical reaction.
• We can use balanced symbol equations to calculate the masses of reactants and products in a chemical reaction.
• The yield of a chemical reaction.
• The yield of a chemical reaction describes how much product is made.
• The percentage yield of a chemical reaction tells us how much product is made compared with the maximum (100%).
• Factors affecting the yield of a chemical reaction include product being left behind in the apparatus and difficulty separating the products from the reaction mixture.
• It's important to get the highest yield and minimise energy wasted to benefit the environment.
• Reversible reactions
• In a reversible reaction  the products of the reaction can react to make the original reactants.
• The reaction can be shown to be reversible by using a
• Analysing substances
• Although simpler to use than bench chemistry methods, instrumental methods still need trained technicians to operate.
• Additives may be added to food in order to improve its appearance , taste and its 'shelf life'.
• Food scientists can anaylse foods to identify additives.
• Modern instrumental techniques provide fast, accurate and sensitive ways of analysing chemical substances.
• Instrumental analysis.
• Compounds in a mixture can be seperated using gas chromatography.
• Once separated , compounds can be identified using a mass spectrometer.
• The mass spectrometer can be used to find the relative molecular mass of a compound from its molecular ion peak.

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