Quantitative Chemistry Flashcards
What does the formula H2SO tell you?
There are 3 elements: hydrogen, sulfur and oxygen. The number of atoms of each are: 2 hydrogen atoms, 1 sulfur atom, 4 oxygen atoms.
What is the atomic mass (Ar)?
Top number on the periodic table - sum of protons and neutrons.
What is the relative formula mass (Mr)?
Sum of the relative atomic masses of the atoms in the formula (must look at little numbers).
e.g. H2O 2+16 = 18
What is the law of conservation of mass?
No atoms are lost or made during a chemical reaction so the mass of the products equals the mass as the reactants.
How do the relative formula masses of the reactants compare to the products?
They are equal.
Why could the mass of products be less than the mass of the reactants?
A gas is made which can escape e.g. A metal carbonate and acid makes carbon dioxide or during thermal decomposition.
HINT look for (g) state symbols after the arrow.
Why could the mass of products be more than the mass of the reactants?
When a gas is added to a reactant e.g. When magnesium reacts with air oxygen is added.
HINT look for (g) state symbols after the arrow.
How do you calculate % element in a compound?
% element = (Atomic mass of the element x number of atoms x 100) / Relative formula mass of the compound.
e.g. What is the % of oxygen in H2SO4: % element = (1 + 1 + 32 + 16 + 16 + 16 + 16) * 100 = 65%
What is concentration measured in?
Grams per decimetre (g/dm)
How do you calculate concentration?
concentration = mass (g) ÷ volume (dm³)
What units do mass and volume have to be in when calculating concentration?
Mass must be in grams (g), volume must be in decimetres (dm)
How do you convert cm³ to dm³?
Divide by 1000
How do you convert kg to g?
Multiply by 1000
What does resolution mean?
Smallest change in the equipment (e.g. On a ruler it is 1mm or 0.1cm)
What does uncertainty mean?
The interval within which the true value of a quantity can be expected to lie.
How do you calculate the uncertainty of results?
Uncertainty = range
What are chemical amounts measured in?
Moles, the unit is mol
How do the number of particles in one mole compare to one mole of water?
They are the same, a mole is a fixed number of particles.
What is the mass of one mole?
The relative formula mass in grams.
How many moles are reacted in Mg + 2HCl → MgCl₂ + H₂?
1 mole of magnesium with two moles of hydrochloric acid.
What unit is used for mass in chemistry?
Grams, g
How can you calculate number of moles?
moles = mass (g) ÷ Relative formula mass, Mr
How many moles are there in 500g of NaOH?
Mr (NaOH) = 23 + 16 + 1 = 40
moles = 500 ÷ 40 = 12.5 mol
How can you calculate mass?
mass = moles x relative formula mass
What is the Mr of a compound when 3 moles has a mass of 126g?
Mr = mass ÷ moles
Mr = 126 ÷ 3 = 42
How to balance an equation using moles?
Step 1: work out any unknown masses (use conservation of mass rule)
Step 2: work out the moles of all substances (you will be given their masses)
Step 3: Convert the numbers of moles into whole numbers (these are ratios so must multiply all numbers by same amount)
Balance an equation example: MgCO₃ → MgO + CO₂
Step 1: work out any unknown masses
Step 2: work out the moles of MgO
Step 3: work out the moles of MgCO₃
Step 4: work out the moles of CO₂
Step 5: convert the number of moles into whole number ratios
What is the limiting reactant?
The reactant that is completely used up in a reaction.
What does it mean if the reactant is said to be in excess?
More than enough present to allow the limiting reactant to get fully used up.
How to work out the limiting reactant?
Step 1: Calculate the number of moles for each reactant using the grams in the question.
Step 2: Pick ONE of the reactants and using the ratios in the balanced equation, work out what the moles you would need for the OTHER reactant.
Step 3: Compare the worked out moles in step 2 for the reactant against actual moles worked out in Step 1.
How to work out the reactant mass?
Step 1: Balance the symbol equation.
Step 2: Work out the moles for the substance you have the mass of.
Step 3: Use the ratio to work out the number of moles of the unknown.
Step 4: Calculate the mass of the unknown using moles just calculated.
What is Avogadro’s number?
Avogadro’s constant is 6.02 × 10²³.
What is the number of particles in one mole called?
Avogadro’s number.
How do you calculate the mass of one atom?
Mass of one atom = mass of one mole (Ar) / Avogadro’s number.
How do you calculate the number of atoms/molecules/particles?
Number of atoms = moles x Avogadro’s number.
What is percentage yield?
The amount of product obtained from a reaction compared to the theoretical amount.
How to calculate percentage yield?
% yield = (mass of product actually made / theoretical mass) x 100.
What value is % yield normally between?
0 to 100%.
Why is yield below 100%?
Reaction may not go to completion, product may be lost, some reactants may react differently, or side reactions may occur.
Why would industry want a high yield?
Reduces waste and cost.
What is atom economy?
A measure of the amount of starting materials that end up as useful products.
Why is atom economy important?
Sustainable development and economic reasons.
Why would industry want a high atom economy?
Less waste, saves resources, increased profit.
How to calculate atom economy?
Atom economy = (relative formula mass of desired product / sum of relative formula masses of all reactants) x 100.
How can a company increase its atom economy?
By selling off any by-products.
What are the units for concentration?
mol/dm³
How do you calculate concentration?
Concentration (c) = moles / volume
What is concentration mol/dm³ related to?
It relates to the amount of solute in a given volume of solution.
How do the volumes of gases compare?
Equal moles occupy the same volume at room temperature and pressure (rtp).
What is the volume of one mole of any gas at room temperature and pressure?
24 dm³
What is the formula for calculating the volume of a gas from its mass?
Volume of gas = (mass of gas / Mr of gas) x 24
What volume of gas does 1.6kg of Bromine gas occupy?
Volume = 1600 ÷ 160 x 24 = 240 dm³
Mr of Br₂ = 160 (2 x 80), Convert 1.60Kg to g = 1600g.
