Chapter 3: Quantitative Chemistry Flashcards
What is an isotope?
A version of an atom with a different number of neutrons
What is relative atomic mass?
The ratio of the average mass of all of the isotopes of an element to one twelfth of the mass of an atom of carbon-12. So, the relative atomic mass of oxygen would be 16
What is relative formula mass?
The total of the relative atomic masses of each element in a formula. For example, H2O is (2x1) + (1x16) = 18
What is the law of conservation of mass?
Mass is never lost or gained in any chemical reaction
Why can it sometimes seem as though mass has been lost?
Because some of the reactants could become gasses
What is Avogadro’s constant
6.02 x 10^23, the amount of particles in a mole of anything
How much would 1 mole of carbon atoms weigh?
12 grams (the atomic weight)
What is a mole?
The relative formula mass of a substance, given in grams, is 1 mole of a substance. 12g of carbon is a mole
What is the formula triangle that links mass, formula mass, and moles?
mass (g) = Mr x moles
What is a balanced chemical equation?
An equation where the number of atoms of each element on each side of the equation is the same
Why is the mass of the product greater than that of the reactants when metals react with oxygen?
The extra mass comes from the oxygen in the air
What is a thermal decomposition reaction?
A reaction where heat causes a substance to break down into simpler substances. Some gas may escape into the air
What do metal carbonates decompose into when heated?
Metal oxide and carbon dioxide
How do you do reacting mass calculations?
Remember, the balancing number means that many moles of the substance. Work out the relative formula masses of both elements in the question. Calculate the moles of the substance whose mass is given. You can then use molar ratios to work out the rest, before converting back into mass
What can these ratios be used for?
To calculate how many moles would react and be produced in reactions
How do you deduce the balancing numbers from the masses?
Calculate the moles of each substance, then find the simplest whole number ratio by dividing by the smallest mole value
What is an excess?
When the amount of a reactant is greater than the amount that can react
What is a limiting reactant?
The reactant we don’t have an unlimited supply of, which determines the amount of products formed
Why are excesses used?
To ensure that all the reactant is used up, especially when it’s expensive
What is percentage yield?
The mass of the product formed compared to the mass of the reactants (the largest theoretical mass)
List 3 reasons a reaction’s yield might not be 100%
Some reactions do not finish completely, Some may be left on the apparatus when it’s removed, The reactants might also take part in other reactions
What is the atom economy?
A way of measuring what percentage of the mass of the reactants ends up in the desired product
How do you calculate the atom economy?
100 x (sum of relative formula mass of desired product) / (sum of relative formula mass of all reactants)
How does the volume of a gas vary with the temperature?
Higher temperature = Greater volume. If the temperature and pressure are the same, equale numbers of moles of gases will have the same volume
What is the volume of 1 mole of a gas at room temperature?
24 dm^3
What equation links the volume of a gas to the number of moles?
volume (dm^3) = 24 x moles
What equation links mass, concentration, and volume?
mass (g) = concentration (g/dm^3) x volume (dm^3)
How many cm^3 are in 1dm^3?
1000
How do you measure how concentrated a solution is in mol/dm^3?
moles = concentration (mol/dm^3) x volume (dm^3)
How do you convert mol/dm^3 to g/dm^3?
mol/dm^3 x relative formula mass
What are titrations?
Techniques that can be used to find the concentration of a reaction by reacting it with a solution of known concentration
Describe the 6 steps of titration
- A known volume of solution of an acid is measured out with a pipette and placed in a conical flask
- A few drops of indicator (phenolphthalein) are added
- The other solution is added from the burette until the indicator changes colour
- The volume added from the burette is recorded, and the experiment optionally repeated
How can the concentration of a solution be found using the results of a titration?
- Work out the moles of the solution with the known concentration
- Use the molar ratios of the balanced equation to work out the moles of the unknown solution
- Use c=m/v to work out the concentration
