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Chemistry Topic 3 Quantitative Chemistry, magnesium reaction,…
Chemistry Topic 3 Quantitative Chemistry
Relative Formula Mass
Mr = The sum of the relative atomic masses of the atoms in the numbers shown in the formula.
The sum of Mr in the products is equal to the reactants.
Mass changes when one of the reactants or products is a gas
Mass appears to
increase
during a reaction then one of the reactants is a gas.
Mass appears to
decrease
during a reaction then one of the products is a gas and has escaped.
Conservation of Mass
No atoms are destroyed or made in a reaction.
Mass of the products equals the mass of reactants.
Moles
Chemical amounts are measured in moles.
Mass of one mole of a substance in grams = relative formula mass:
One mole of H2O = 18g (1 + 1 + 16)
One mole of Mg = 24g
Avagadro Constant
One mole of any substance will contain the same number of particles, atoms, molecules or ions:
Example:
6.02 x 1023 per mole
One mole of H2O will contain 6.02 x 1023 molecules
One mole of NaCl will contain 6.02 x 1023 Na+ ions
Using moles to balance equations
To balance it with moles you must convert the them to their relative formula mass. Convert the masses in grams to moles and convert the moles to simple whole ratio numbers.
Concentration of solutions
Measured in mass per given volume of solution (g/dm3)
Concentration: mass/volume
Greater mass= higher concentration Greater volume = lower concentration
Chemical measurements
Whenever a measurement is taken there's always some uncertainty. Therefore you should:
Calculate the mean.
Calculate the range.
Estimate of uncertainty in mean would be half the range.
Limiting Reactants
It is the reactant that is completely used up. It limits the amount of product made. Less moles of product are made.
Atom Economy
A measurement of the amount of reactants that get used as a useful product.
Relative Formula Mass of Desired Product from Equation/ Sum of relative Atomic Mass of all Reactants X 100
Atom Economy Equation:
Percentage Yield
Yield is the amount of product obtained.
It's not always possible to obtain all of the calculated products. For example:
1.
Some of the product may not be obtained because the reaction is reversed.
2.
Some of the product is lost when it's separated from the reaction mixture.
3.
Some of the reactants my react differently to expected.
Percentage Yield Equation:
Mass of product made/ max. theoretical mass X 100
Use of amount of substance in relation to volumes of gases
Equal amounts of gases or moles occupy the same volume under the same conditions of temperature and pressure.
The volume of one mole of any gas at room temperature is 24dm3.
Number of moles of gas equation:
Volume of gas (dm3) / 24dm3
Using concentrations of solution in mol/dm3
Concentration of a solution is the amount of solute per volume of solution.
Concentration Equation
:
amount (mol)/ volume(dm3)
Titrations
If the volumes of two solutions that react completely are known, and the concentrations of one solution is known, the concentration of the other solution can be calculated.
