3 - Amount of Substance Flashcards
What does a mole measure?
The amount of an substance. (The number of particles)
What is a mole?
The amount of any substance contains as many elementary particles as there are carbon atoms in exactly 12g of the carbon-12 isotope (6.02 x 10^23)
What is the equation to find the number of moles from the number of atoms or molecules?
Number of Moles = Number of particles you have / Number of particles in a mole (Avogadro Constant 6.02 x 10^23)
I have 1.50 x 10^24 carbon atoms. How many moles of carbon is this?
Number of Moles = 1.50 x 10^24 / 6.02 x 10^23
= 2.49 moles
What is the molar mass, M?
The mass per mole of a substance, in units of g mol^-1. Mass of ONE mole of a substance
Same as Relative Molecular Mass.
Find the Molar Mass of CaCO3?
M = (40.1) + (12.0) + (3 x 16.0) = 100.1 g mol-1
I.e 1 mole of CaCO3 weighs 100.1 grams.
What is the equation with moles, Mass and Molar Mass?
Number of Moles = Mass of Substance / Molar Mass.
How many atoms are in 8.5 g of H2S?
Molar Mass = 1.0 + 1.0 + 32.1 = 34.1 g mol-1
Number moles = 8.5 / 34.1 = 0.249 mol
Number of Molecules of H2S = 0.249 x (6.02 x 10^23) = 1.50 x 10^23
3 atoms in one molecule of H2S so, total no. atoms = (1.50 x 10^23) x 3 = 4.5 x 10^23
What is the molar gas volume?
The volume per mole of gas molecules at a stated temperature and pressure.
Units of dm^3mol^-1
At room temperature and pressure what is the Molar Gas Volume?
24 dm3mol-1 (298 K and 101 kPa)
N = V / 24
If the temperature and pressure stays the same for one mole of any gas what is the effect on volume?
One mole of gas always has the same volume, if temperature and pressure stay the same.
What is the formula for working out the number of moles in a volume of gas?
Number moles = Volume in dm^3 / Molar Gas Volume (24 at r.t.p)
How many moles are there in 6.0 dm^3 of oxygen gas at r.t.p?
Number of moles = 6.0 / 24 = 0.25 moles of oxygen molecules
(Molecules not atoms as gaseous oxygens only exist as O2 not O).
What does the ideal gas equation let you find?
The number of moles in a certain volume at any temperature and pressure.
What is the ideal gas equation, and it’s UNITS for each?
pV = nRT
Where:
p = pressure (Pa)
V = volume (m^3)
n = number of moles
R = 8.314 J K^-1 mol^-1 (gas constant)
T = temperature (Kelvin (°C + 273))
How many dm in cm?
1dm in 10cm
How many dm in m
1m is 10dm
At a temperature of 60.0°C and a pressure of 250 kPa, a gas occupied a volume of 1100 cm^3 and a mass of 1.60 g. Find the relative molecular mass.
n = pV / RT = (250000) x (1.1 x 10^-3) / 8.314 x (273+60) = 0.0993 mol
Mr = M/n = 1.60 / 0.0993 = 16.1
So 1 mole is 16.1 g and the Mr is 16.1.
What is the empirical formula?
A formula that shows the simplest whole-number ratio of atoms of each element present in a compound.
What is the Molecular Formula?
A formula that shows the actual number of atoms of each type of element in a molecule.
A molecule has an empirical formula of C4H3O2, and a molecular mass of 166 g mol-1. Work out its molecular formula.
(4 x 12.0) + (3 x 1.0) + (2 x 16.0) = 83.0 g mol-1
Mr= 166 g mol-1
166 / 83.0 = 2 empirical units in the molecule.
Empirical formula x 2 = C8H6O4
When a hydrocarbon is burnt in excess oxygen, 4.4 g of carbon dioxide and 1.8 g of water are made. What is the empirical formula of the hydrocarbon?
No. of moles of CO2 = 4.4 / (12.0 + (2 x 16.0)) = 0.10 moles
1 mole of CO2 contains one mole of carbon atoms, so 0.10 moles of carbon.
No. of moles of H2O = 1.8 / ((2 x 1.0) + 16.0) = 0.10 moles
1 mole of H2O contain two moles of hydrogen atoms (H), so you must started with 0.20 moles of hydrogen atoms.
Ratio C:H = 0.10:0.20
0.10 / 0.10 = 1
0.02 / 0.10 = 2
So empirical formula must be CH2.
A compound is found to have percentage composition 56.5% potassium, 8.70% carbon and 34.8% oxygen by mass. Calculate its empirical formula.
56.5 / 39.1 = 1.45 moles of K
8.70 / 12.0 = 0.725 moles of C
34.8 / 16.0 = 2.18 moles of O
Divide each number of moles by the smallest number- this case 0.725
K - 1.45 / 0.725 = 2.00
C - 0.725 / 0.725 = 1.00
O - 2.18 / 0.725 = 3.01
Empirical Formula = K2CO3
When 4.6 g of an alcohol, with molar mass 46 g mol-1, is burnt in excess oxygen, it produces 8.8 g of carbon dioxide and 5.4 g of water. Calculate the empirical formula for the alcohol and then its molecular formula.
Remember Alcohols contain C,H and O.
No. of moles of CO2 = 8.8 / 44 = 0.2 moles
1 mole of CO2 contains 1 mole of C, so 0.2 moles of CO2 contain 0.2 moles of Carbon.
No. of moles of H2O = 5.4 / 18 = 0.3 moles
1 mole of H2O contains 2 moles of H, so 0.3 moles of H2O contain 0.6 moles of hydrogen.
Mass of C = 0.2 x 12 = 2.4 g
Mass of H = 0.6 x 1.0 = 0.6 g
Mass O = 4.6 - (2.4 + 0.6) = 1.6 g
Number of moles O = 1.6 / 16 = 0.1 moles
Moles ratio = 2:6:1 so empirical formula is C2H6O
Mr of C2H6O = 46 so empirical formula is also the molecular formula.