Quantitave Chemistry Flashcards
What is relative atomic mass?
The relative atomic mass of an element is the average mass of one atom relative to 1/12th mass of one atom of carbon 12.
It is a way of saying how heavy different atoms are.
What is relative formula mass?
All the relative atomic masses in a compound added together
What is the value of one mole?
6.02 x 10^23
Same as Avogadro’s Constant
How do you calculate the number of moles?
Number of moles = mass (g) / mass of one mole (g) (same as RFM)
How do you calculate the number of particles?
Number of particles = Avogadro’s Constant x Number of moles
What is empirical formula?
The simplest whole number ratio of elements in a compound e.g. Butane: molecular formula C4H10, empirical formula C2H5
How can you calculate molecular formula using empirical formula and RFM?
- Find RFM of empirical formula
- Divide RFM of molecular formula by RFM of empirical formula
- Multiply quantities in empirical formula by result
e.g. Empirical formula is CH2O and RFM is 90.
CH2O weight: 12+2+16 = 30
90/30 = 3
Molecular formula is 3*CH20 = C3H6O3
How can you calculate empirical formula from reacting masses?
- List the elements
- Write in the experimental masses
- Divide by the mass no of each element
- Divide each number by the smallest
- Find the simplest ratio
e.g.
Fe and O
44.8 and 19.2
0.8 and 1.2
1 and 1.5
2 and 3
Fe2O3
What is conservation of mass?
Mass is never gained or lost in chemical reactions. This is known as the Law of Conservation of Mass.
The same law applies to physical changes like dissolving or boiling.
When a reaction happens, the atoms in the chemicals are rearranged into different compounds. No new atoms are created and no atoms can disappear.
How do you calculate masses of reactants and products?
- Write out the balanced equation
- Work out the RFMs
- Calculate the number of moles of the thing you know
- Look at the ratio from the balanced symbol equation
- Calculate mass
e.g. Mass of magnesium oxide when 60g magnesium is burned in air?
2Mg + O2 –> 2MgO
24 and 40
60/24=2.5
2 moles Mg makes 2 moles MgO, so 1:1
2.5*40=100g MgO
What is limiting reagent?
Limiting reagent is all used up at the end of the reaction. The amount of product is directly proportional to the amount of limiting reagent. The reagent in excess is still there at the end, but in a smaller amount.
How do you calculate the amount of a product from all the reactants?
- Calculate number of moles
- Identify which substance is not in excess
- Multiply this many moles by the RFM of the product
e.g. 20g C2H4 reacts with 10g H2 to make C2H6. Mass?
20/28=0.71 10/2=5
C2H4 limiting reagent
30*0.71=21.43g
How do you calculate the amount of a product from all the reactants when quantities are different?
e.g. N2 + 3H2 –> 2NH3
56g N2 reacted with 16g H2. Maximum amount of ammonia?
56/28 = 2 moles 16/2 = 8 moles
Hydrogen is in excess as you need 3 times as much hydrogen, so you need 6 moles and have 8 moles
Moles of ammonia = 4 moles (22)
Mass of ammonia = 417 = 68g
What is stoichiometry?
The relationship between the relative quantities of substances taking part in a reaction or forming a compound, typically a ratio of whole integers.
Basically working out the balancing numbers in a reaction.
How do you balance an equation?
- Calculate number of moles
- Find ratio of moles, find the simplest whole number ratio
- Balance the whole equation
e.g. 10.8g aluminium reacts with 42.6g chlorine to produce AlCl3. What is the balanced equation?
10.8/27=0.4 42.6/71=0.6
0.4:0.6 = 1:1.5 = 2:3
Equation: 2Al + 3Cl2 –> AlCl3
Balanced: 2Al + 3Cl2 –> 2AlCl3
