C3 - Quantitive chemistry
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What is a relative formula mass
• The sum of the relative attic masses of the atoms in the numbers shown in the formula
What is the rule for relative formula mass in balanced chemical equations
• The sum of masses of the reactants in the quantities shown equals the sum of the relative formula mass of the products
• How can you calculate the % mass of an element in the compound
Percentage mass of an element = Atomic number x number of atoms // relative formula mass
• What is the law of mass conservation
• No atoms are lost or made during a chemical reaction so the mass is always equal
What does the law of mass conservation mean
• Chemical equations can be represented by a symbol equation which are balanced
Why might there seem like there is an increase in mass
• One of reactants is gas that is in the air and products are non gases • Can’t measure before and its mass isn’t accounted for • When it reacts it becomes part of it and so the weight increases • E.g metal and oxygen- mass is metal plus the oxygen
Why may the mass may seem to decrease
• One of the products is a gas and products are not • All are weighed before • If its not enclosed it can escape and can’t be measured • e.g Metal carbonate- thermally decomposes to form metal oxide and co2 . Decrease as carbon is not measured
• What should any measurement be met with
• Uncertainty - Can make estimations by plotting
What are chemical amounts measured in
• Moles - Symbol is mol
What is the mole equal to
• The mass of one mole of a substance in grams is equal to its relative formula mass
How are moles equal
One mole of a substance contains the same number of stated particles, atoms, molecules or ions as any other substance - All have an equal number
The is the avogadro constant
• Number of atoms, molecules or ions in a mole - Always equals this
What are the steps in balancing formula for moles
• Work out relative atomic mass for each of the substances • Divide actual mass by relative atomic mass- this gives the number of moles • If not whole numbers-0 divide all by smallest mol value to get whole numbers • Can write as balanced
What are the steps in calculating the mass of reactants and products
• Relative atomic mass times by number of mols gives value in grams
What is a limiting reactant
• The reactant that used up before the rest- the rest are in excess • Directly proportional to amount of product
What is done to limit limiting reactions
• Common to use excess of one to ensure all of other reactant is used up
How can you calculate the mass of a product by using mass of limiting reactant
• Write out balanced chemical equation • Work out relative formula mass of reactant and product you want • Work out how many moles there are in reactant • Look at ratio between the two and calcite from there e.g if 5 moles of al react to produce 2.5 mole of aluminium oxide then there will be half the number of moles. So 2.5 x relative formula mass
What is concentration
• Measure of how much substance is is a mixed volume - Measured in grams/dm3
How can concentration be measured
• Concentration = mass of solute // volume of solvent