Stoichiometry Flashcards
(26 cards)
What is the relative atomic mass (Ar)?
the average mass of the isotopes of an element compared to 1/12th the mass of an atom of Carbon-12.
What is the relative molecular mass (Mr)?
the sum of the relative atomic masses.
exp:
Mr of H2O = 18
Ar of O = 16
16 + 1 + 1 = 18
What is the formula of Mr Mole carries a Mass?
Mass = Mr x Mol
Mol = Mass / Mr
Mr = Mass / Mol
Calculate the mass of magnesium needed to form 12g of magnesium oxide:
2Mg(s) + O2(g) → 2MgO(s)
- Find Mr of Mg: 24
- Find Mr of MgO: 24 + 16 (Ar of O = 16)
- Find mol of MgO: Mass of MgO + Mr of MgO ( 12/40 = 0.3 )
- Moles of Mg = 0.3
- Mass of Mg: Mr of Mg x mol of Mg
= 24 x 0.3
= 7.2 g
What is the molecular formula?
the actual number of atoms of each element present in a compound.
exp: H2O
What is the empirical formula?
the simplest whole number ratio of the different atoms/ ions in a compound.
exp:
C2H6 = CH3
How can you deduce the formula of an ionic compound from the relative numbers of the ions present in a molecule?
- Identify the ions of each element and note their charges.
- Ionic compounds have overall charges of 0.
How can concentration be measured?
mol/dm³ or g/dm³
What is mol? (Avogadro Constant)
the unit of amount of substance that one mole contains 6.02 x 10²³ particles
How can you calculate the mol?
exp: Calculate the mol in 426g of Na2SO4.
1. Calculate Mr of Na2SO4: (23x2) + 32 + (16x4) = 142
2. Mass / Mr:
426g / 142 = 3 moles of Na2SO4
How do you calculate mass?
exp: Calculate the mass of 0.5 moles of Na.
1. Find Mr of Na = 23
2. Mr x mol:
23 x 0.5 = 11.5g
How do you calculate molar mass? (Mr / Ar)
exp: Calculate the molar mass of AlCl3
1. Find it organically through the Periodic Table or
2. Mass / Mol
534g / 4 mol = 133.5
How do you calculate the number of particles? (Avogadro Constant)
Avogadro Constant = 6.02 x 10²³ particles
Number of moles
exp: Calculate the number of CO2 molecules in 1.5 moles of CO2.
6.02 x 10²³ x 1.5
= 9.03 x 10²³ particles
What is the volume of 1 mol of any gas at room temperature and pressure? (20℃ and 1 atm)
24 dm³
What is a limiting reagent/ reactant?
when one of the reactants are used up and determines how much product is formed.
The leftover is in excess.
How can you calculate limiting reactants?
exp: 0.96g of Mg reacts with 2.19g of HCl.
Mg + 2HCl → MgCl2 + H2
What mass of MgCl2 is formed?
1. Work out moles for both reactants.
Moles of Mg = 0.96g / 24 = 0.04
Moles of HCl = 2.19 / 36.5 = 0.06
2. Use the molar ratio of the reactants to determine which is the limiting reactant.
Mg : HCl
1 2
1 mole of Mg : 2 Moles of HCl
= 0.04 mol of Mg : 0.08 mol of HCl
3. HCl is the limiting reagent because there is only 0.06 moles.
4. Use the molar ratio of the limiting reactant and product to find the moles of product formed.
HCl : MgCl2 = 1:2
0.06 : 0.03
5. Mass of MgCl2:
0.03 mol x Mr of MgCl2
24 + (35.5 x 2) = 95
Mass of MgCl2:
0.03 x 95 = 2.85g
How can you convert between cm³ and dm³?
1dm = 10cm
1dm³ = 1000cm³
How do you calculate the volume of gases at RTP? (room temperature)
exp: Calculate the volume of 0.75 mol of O2 at room temperature and pressure.
Volume of gas in dm³ = Moles x 24 dm³
Volume of O2: 0.75 x 24 = 18dm³
exp: Calculate the volume of 0.5 mol of H2 at room temperature and pressure.
Volume of gas in cm³ = Moles x 24000 cm³
Volume of H2: 0.5 x 24000 = 12000 cm³
How can you calculate the volume and concentration of solutions in g/dm³ and mol/cm³?
Mol = Moles
C = Concentration
V = Volume
—————————
Mol = C x V
C = Mol / V
V = Mol / C
exp: 200cm³ of a hydrochloric acid contains 0.25 mol of dissolved hydrogen chloride. Calculate the concentration in g/dm³ and mol/dm³.
C = Mol / V
1. Convert volume in cm³ to dm³.
200cm³ / 1000 = 0.2dm³
2. Calculate the concentration in mol/dm³: Mole / V
0.25 / 0.2 = 1.25 mol/dm³
3. Convert to g/dm³
Calculate the Mr of HCl:
1 + 35.5 = 36.5
mol/dm³ → g/dm³
1.25 x 36.5 = 45.625 g/dm³
How can you calculate the empirical formula?
exp: Find the empirical formula of the following compounds using their percentage masses:
Al = 20.2% Cl = 79.8%
- Write the relative atomic masses (Ar) of each element
Al = 27 Cl = 35.5 - Divide percentage masses by the Ar of each element
Al = 20.2 / 27 = 0.748
Cl = 79.8% / 35.5 = 2.248 - Divide by the smallest answer from Step 2 to find the ratio of atoms of each element
Al = 0.748 / 0.748 = 1
Cl 2.248 / 0.748 = 3 - Find the empirical formula
AlCl3
How can you calculate the molecular formula?
exp: Find the molecular formula of the compound with the empirical formula CH2O and with an Mr of 180
- Find the Mr of the empirical formula
12 + (2 x 1) + 16 = 30 - Divide the Mr of the molecular formula by the Mr of the empirical formula
180 / 30 = 6 - Multiply each element in the empirical formula by the answer in Step 2
1 atom of C x 6 = 6 C Atoms
2 Atoms of H x 6 = 12 H Atoms
1 atom of O x 6 = 6 O Atoms - Find the molecular formula.
C6H12O6
How do you calculate percentage yield?
( actual amount of a product produced / theoretical amount of product possible ) x 100
Why is it not always possible to obtain the theoretical amount of a product?
- reaction may not go to completion because its reversible
- some product may be lost when separated from the reaction mixture
- some reactants may react in different ways than expected to
How do you find the theoretical yield of a substance?
Mr x Moles
exp: 32g of sodium hydroxide reacts with hydrochloric acid. 16.2g of sodium chloride was produced. Calculate the percentage yield of sodium chloride.
NaOH + HCl → NaCl + H2O
- Calculate the the theoretical yield of NaCl
a. Find the Mr of NaOH and NaCl
NaOH = 23 + 16 + 1 = 40
NaCl = 23 + 35.5 = 58.5
b. Calculate the moles of NaOH
32 / 40 = 0.8 mol of NaOH
Ratio of NaOH:NaCl = 1:1
0.8 mol of NaCl
c. Calculate the theoretical mass of NaCl
0.8 x 58.5 = 46.8g - Calculate the percentage yield of NaCl
( 16.2g / 46.8g ) x 100 = 34.6%
